3.4 Muscolo-skeletal System and Muscle Physiology

The human body consists of 11 primary systems that function interdependently, each playing a crucial role in maintaining overall health and well-being. As a personal trainer, understanding the intricacies of the musculoskeletal and cardiovascular systems is paramount.

This chapter will delve into the fascinating world of the musculoskeletal system, which comprises both the skeletal and muscular systems. These two systems work together to facilitate movement and maintain proper body posture. The skeletal system provides structure, support, and stability, while the muscular system enables movement and generates force.

The skeletal system consists of bones, cartilage, and ligaments, forming a robust framework for the body. It protects vital organs and serves as an anchor for the muscular system. The muscular system, on the other hand, is composed of muscles and tendons. Muscles generate force and movement, while tendons connect muscles to bones, facilitating motion.

The collaboration between these two systems is essential for the body’s ability to move efficiently and in a coordinated manner. As we explore the musculoskeletal system further, we will examine the various types of bones and muscles, their functions, and the mechanics of their interactions. Additionally, we will discuss the critical role of the cardiovascular system in delivering oxygen and nutrients to the muscles, ensuring optimal performance during physical activity.

By the end of this chapter, you will have gained valuable insights into the complex interplay between the skeletal and muscular systems, equipping you with the knowledge to help clients achieve their fitness goals while maintaining proper form and function.

3.4.1 Anatomical terminology

Anatomical terminology is a comprehensive and standardized language system that enables clear and precise communication about the structure of the human body. It plays an essential role in medicine, biology, kinesiology, and physiology by providing a consistent framework for describing various bodily structures, regions, and relative positions.

Utilizing specific anatomical terms allows for an accurate description of body parts and areas and the planes and lines used to delineate the body. This precision is vital for medical professionals when diagnosing, treating, and discussing the human body. As important as it is for the medical professional, it is for the exercise professionals to describe movements accurately.

There are several key components to anatomical terminology:

Directional terms: These terms are used to describe the relative position of one body part to another. Examples include anterior (front), posterior (back), medial (toward the midline), lateral (away from the midline), superior (above), and inferior (below).

Regional terms: These terms are used to identify specific areas or regions of the body. For instance, the abdominal region refers to the area of the torso between the diaphragm and the pelvis, while the thoracic region is the area of the chest encompassed by the ribcage.

Body planes: Imaginary flat surfaces that divide the body into sections are referred to as body planes. These include the sagittal plane (which divides the body into left and right), the coronal or frontal plane (which separates the body into front and back), and the transverse or horizontal plane (which divides the body into upper and lower sections).

Body cavities: Anatomical language also defines the various internal spaces within the body. Examples of body cavities include the cranial cavity (housing the brain), the thoracic cavity (containing the heart and lungs), and the abdominal cavity (encompassing organs such as the stomach, liver, and intestines).

Structural organization: Anatomical terminology is also applied to the different levels of structural organization within the body, from the molecular level to organs and organ systems.

The most commonly used terms are:

Anterior (or ventral) describes the front or direction toward the front of the body. The toes are anterior to the foot.

Posterior (or dorsal) describes the back or direction toward the back of the body. The popliteus is posterior to the patella.

Superior (or cranial) describes a position above or higher than another part of the body proper. The orbits are superior to the oris.

Inferior (or caudal) describes a position below or lower than another part of the body proper, near or toward the tail (in humans, the coccyx or lowest part of the spinal column). The pelvis is inferior to the abdomen.

Lateral describes the side or direction toward the side of the body. The thumb (pollex) is lateral to the digits.

Medial describes the middle or direction toward the middle of the body. The hallux is the medial toe.

Proximal describes a position in a limb that is nearer to the point of attachment or the trunk of the body. The brachium is proximal to the antebrachium.

Prone describes face down orientation. Anatomical terminology is also applied to the different levels of structural organization within the body, from the molecular level to organs and organ systems.

Distal describes a position in a limb that is farther from the point of attachment or the trunk of the body. The crus is distal to the femur.

Supine describes face up orientation.

In summary, anatomical language provides a systematic and precise method for describing the human body’s structure. This detailed terminology facilitates clear communication and understanding among medical and other professionals, allowing them to diagnose and treat patients or clients more effectively.

You may not see the need to know these terms immediately. Still, they become very useful in expanding your knowledge through continuing education and reading scientific literature to investigate possible solutions to newly presented problems.

In Chapter 5, we will delve extensively into the topic of movement, providing comprehensive details and analysis.

3.4.2. The skeletal system

The human adult skeleton, on average, comprises 206 bones connected by ligaments and tendons, forming a protective and supportive framework for the underlying soft tissues and muscles. This complex network of bones, cartilage, ligaments, and joints are intricately designed to work harmoniously, making the human skeleton a masterpiece of engineering and an essential component of our overall well-being.

The human skeletal system consists of 206 bones.

The skeletal system fulfills several crucial functions in the body:

  1. Bones function as levers, transmitting muscular forces.
  2. The skeletal system safeguards our organs.
  3. The skeletal system provides a structural framework for other tissues and organs.
  4. Bones serve as storage and release sites for essential minerals, such as calcium and phosphorus.

The skeleton is divided into axial and appendicular sections. The axial skeleton comprises 80 bones, including the skull, spine, ribs, and sternum, and the appendicular skeleton comprises 126 bones.

As we progress through this chapter, we will examine the different types of bones that make up the human skeleton, including long, short, flat, and irregular bones. We will discuss their unique characteristics, functions, and critical role in our body’s biomechanics. Additionally, we will delve into the fascinating world of joints, exploring their various types and the movements they facilitate. This comprehensive exploration will provide a deeper understanding of the skeletal system’s significance in maintaining our body’s strength, stability, and overall well-being.

3.4.3 The Human Skeleton and the Functions of the Skeletal System

The human adult skeleton is a vital protective and supportive framework for the body’s soft tissues and muscles.

Bone tissue, a hard and dense connective tissue, constitutes the majority of the adult skeleton and forms the internal support structure of the body. In areas where bones move against each other, such as joints like the shoulder or between spinal bones, cartilage—a semi-rigid type of connective tissue—provides flexibility and smooth surfaces for movement. Moreover, ligaments of dense connective tissue surround these joints and connect bones to one another, ensuring skeletal stability.

The skeletal system performs several essential functions, including:

  1. Protection of internal organs: By covering or surrounding them, the skeletal system shields vital organs from injury. For instance, the rib cage safeguards the lungs and heart, while the cranium (skull) encases and protects the brain.
  2. Facilitation of movement: Bones serve as points of attachment for muscles, enabling the body to move efficiently and effectively.
  3. Body support: The bones and cartilage of the skeletal system create a scaffold that upholds the rest of the body. Without the skeletal system, the body would be reduced to a shapeless mass of organs, muscles, and skin.
  4. Storage and release of fat: The skeletal system, specifically bone marrow, functions as a storage site for fat, which can be utilized as an energy source when needed.
  5. Storage and release of minerals: Bone tissue is a reservoir for various essential minerals, particularly calcium, and phosphorus. When integrated into bone tissue, these minerals can be released back into the bloodstream to maintain levels necessary for supporting physiological processes.

The skeleton consists of the axial and appendicular skeleton.

As mentioned earlier, the axial skeleton forms the vertical, central axis of the body. It includes all bones of the head, neck, chest, and back. It protects the brain, spinal cord, heart, and lungs. It also serves as the attachment site for muscles that move the head, neck, and back and for muscles that act across the shoulder and hip joints to move their corresponding limbs.

The axial skeleton of the adult consists of 80 bones, comprising the skull, the vertebral column, and the thoracic cage. 22 bones form the skull. Also associated with the head are seven additional bones, including the hyoid bone (found in the upper neck) and the ear ossicles (three small bones found in each middle ear). The vertebral column consists of 24 bones, each called a vertebra, plus the fused vertebrae of the sacrum and coccyx. Finally, the thoracic cage includes 12 pairs of ribs, and the sternum, the flattened bone of the anterior chest.

The appendicular skeleton includes all bones of the upper and lower limbs, plus the bones of the pectoral and pelvic girdles that attach each limb to the axial skeleton. The lower portion of the appendicular skeleton is specialized for stability during walking or running. In contrast, the upper portion of the appendicular skeleton has greater mobility and range of motion, allowing you to lift and carry objects.

As also mentioned earlier, there are 126 bones in the appendicular skeleton: 60 in the upper extremities, 60 in the lower extremities, 2 in the pelvic girdle, and 4 in the shoulder girdle.

A fundamental understanding of the muscular and skeletal systems and their components can aid personal trainers in targeting specific muscle groups and movements related to particular sports or exercises. This knowledge also serves as the foundation for comprehending and preventing injuries.

Bones in the human skeleton you should remember:

3.4.4 Bone Classification

The categorization of adult skeletal bones is based on their shape, with five distinct classifications identified.

The human skeletal system consists of 206 bones.

Each bone in the body serves a specific purpose, resulting in varying sizes, shapes, and strengths tailored to their respective functions. For instance, the bones in the lower back and lower limbs are designed to be thick and robust to effectively support the body’s weight. This interplay between form and function is a key aspect of the body’s skeletal system, ensuring optimal performance and support.

Long bones

Long bones are characterized by their cylindrical shape, with their length being greater than their width. It is important to note that the term refers to the shape of the bone rather than its size. Long bones can be found in the upper limbs (humerus, ulna, radius) and lower limbs (femur, tibia, fibula), as well as in the hands (metacarpals, phalanges) and feet (metatarsals, phalanges). These bones function as rigid bars that facilitate movement when muscles contract.

Short Bones

Short bones possess a cube-like shape, with their length, width, and thickness being approximately equal. In the human skeleton, short bones are found exclusively in the wrists’ carpals and the ankles’ tarsals. These bones provide stability and support while allowing for a limited range of motion.

Flat Bones

The term “flat bone” can be somewhat misleading because these bones are generally thin but often exhibit a curved structure. Flat bones, which include cranial (skull) bones, scapulae (shoulder blades), sternum (breastbone), and ribs, serve as attachment points for muscles and frequently provide protection for internal organs.

Irregular Bones

Irregular bones possess complex shapes that do not fit into any other classification category. For instance, the vertebrae that support the spinal cord and shield it from compressive forces are considered irregular bones. Many facial bones, particularly the jawbones housing the teeth, also fall into this category.

Sesamoid Bones

Sesamoid bones are small, round bones that develop within tendons. They not only protect tendons by helping them withstand excessive forces but also enhance the effectiveness of the tendons and their connected muscles. This unique structure-function relationship ensures optimal performance across various types of bones, catering to their specific roles within the skeletal system.

3.4.5 Bone Structure

Bone tissue, or osseous tissue, is distinct from other tissues in the body due to its characteristic hardness, which is essential for many of its functions.

Bones have three layers: bone marrow, compact bone, and periosteum. Inside long bones is a central marrow cavity, known as bone marrow, where red marrow produces red blood cells (carrying oxygen), white blood cells, and platelets. Yellow bone marrow is primarily composed of fat cells.

Bone marrow produces stem cells that develop into red and white blood cells and platelets, which are essential for immunity, oxygen transport, and blood clotting.

Encircling the marrow is a dense, rigid layer called compact bone, filled with small canals forming an interconnected inner bone network. This network and an extensive capillary system nourish every bone cell. However, bones are rich in blood vessels, so they bleed when fractured.

The periosteum, a specialized connective tissue layer, covers each bone. The bone’s surface is supplied with sensory nerves, which is why bone injuries can be extremely painful.

Understanding that bone is a living, dynamic tissue that continuously adapts to mechanical changes is crucial. While it is evident that muscles develop in response to exercise, our bones also respond by subtly altering their architecture beneath the skin.

Bones are dynamic organs capable of adjusting their density and thickness in response to external forces and changes in body chemistry. As a result, muscle attachment sites on bones will thicken if you start a workout program that increases muscle strength. Similarly, the walls of weight-bearing bones will thicken if you gain body weight or begin a new running routine. Conversely, decreased muscle strength or body weight can lead to thinner bones.

In response to external stimuli, the bones also subtly alter their architecture. This process is known as bone remodeling, and it helps the bones become stronger and better suited to handle the applied stress.

3.4.6 Major bones and joints

The upper limb is divided into three regions. These consist of the arm, located between the shoulder and elbow joints; the forearm, between the elbow and wrist joints; and the hand, located distal to the wrist. 

There are 30 bones in each upper limb. The humerus is the single bone of the arm, and the ulna (medially) and the radius (laterally) are the paired bones of the forearm. The base of the hand contains eight carpal bones, and five metacarpal bones form the palm of the hand. The fingers and thumb contain a total of 14 phalanges.

These segments are composed of the following bones and joints (Branco et al., 2015):

  • The shoulder is composed of the scapula and clavicle. The scapula connects to the upper arm through the glenohumeral joint.
  • The upper arm is composed of a single bone, the humerus, which connects to the lower arm through the humeroradial and humeroulnar joints.
  • The lower arm of the forearms is composed of the ulna and radius, which connect to the wrist through the radiocarpal and ulnocarpal joints.
  • The wrist comprises eight carpals connected by carpal joints, and the hand by carpophalangeal joints.
  • The hand comprises five metacarpals and 14 phalanges connected by interphalangeal joint.

The lower limb is divided into three regions. The thigh or upper leg is that portion of the lower limb between the hip and knee joints. The lower leg is the region between the knee and ankle joint. Distal to the ankle is the foot. 

The lower limb contains 30 bones. These are the femur, patella, tibia, fibula, tarsal bones, metatarsal bones, and phalanges. The femur is the single bone of the thigh. The patella is the kneecap and articulates with the distal femur. The tibia is the larger, weight-bearing bone located on the medial side of the leg, and the fibula is the thin bone of the lateral leg.

The bones of the foot are divided into three groups. The posterior portion of the foot is formed by a group of seven tarsal bones. In contrast, the midfoot contains five elongated metatarsal bones. The toes contain 14 small phalanges.

The lower limb comprises the hip, upper leg, lower leg, and foot. These segments are composed of the following bones and joints (Branco et al., 2015):

  • The hip is composed of innominate bones, which are the fusion of three bones – the ilium, ischium, and pubis. These bones connect to the upper leg through the hip joint.
  • The upper leg or thigh is composed of the femur and patella. The connection between them is the femoropatellar joint, and the femur connects to the tibia through the femorotibial joint.
  • The lower leg or shank is composed of the tibia and the fibula. These bones connect to the foot through the tibiotalar and fibulotalar joints.
  • The foot comprises seven tarsals, five metatarsals, and 14 phalanges connected by intertarsal, metatarsal, and interphalangeal joints, respectively.

The spine

The vertebral column, also known as the spinal column, is a crucial component of the human skeletal system, composed of a series of individual vertebrae (singular = vertebra) that are connected and separated by flexible, cartilaginous structures called intervertebral discs (Standring, 2020). Together, these vertebrae and intervertebral discs create a strong yet flexible column that provides support to the head, neck, and body, allowing for a wide range of movements. comprises seven tarsals, five metatarsals, and 14 phalanges connected by intertarsal, metatarsal, and interphalangeal joints, respectively.

The spine's flexibility, despite having 33 vertebrae, allows us to perform various physical activities like walking, running, jumping, and dancing.

One of the primary functions of the vertebral column is to protect the spinal cord, which is a bundle of nerves that extends from the brainstem and passes through openings in the vertebrae known as the vertebral foramen. In addition to safeguarding the spinal cord, the vertebral column also encloses and shields the spinal nerves, which branch out from the spinal cord to various parts of the body, playing a vital role in the nervous system’s communication with the rest of the body (Marieb & Hoehn, 2019).

Furthermore, the vertebral column serves as a solid anchor point for numerous muscles, including those responsible for controlling the pectoral-shoulder girdle and the latissimus dorsi, as well as other muscles in the back that facilitate arm movement. The strong and stable support provided by the vertebral column enables these muscles to function effectively and efficiently, contributing to the overall strength and flexibility of the upper body (Tortora & Derrickson, 2018).

The vertebral column is an essential structural component of the human body, providing support, protection, and flexibility. It plays a critical role in safeguarding the spinal cord and spinal nerves while also serving as a vital anchor point for various muscles involved in upper body movement.

Regions of the spine

Cervical region: 7 vertebrae (C1-C7)

Thoracic region: 12 vertebrae (T1-T12)

Lumbar region: 5 vertebrae (L1-L5)

Sacral region – Sacrum: 5 fused vertebrae

Coccyx – 4 fused vertebrae

The vertebral column originally develops as a series of 33 vertebrae, but this number is eventually reduced to 24 vertebrae, plus the fused vertebrae comprising the sacrum and coccyx.

The vertebral column is subdivided into five regions, with the vertebrae in each area named for that region and numbered in descending order.

In the neck are seven cervical vertebrae, each designated with the letter “C” followed by its number. Superiorly, the C1 vertebra articulates (forms a joint) with the occipital condyles of the skull. Inferiorly, C1 articulates with the C2 vertebra, and so on.

Below these are the 12 thoracic vertebrae, designated T1–T12.

The lower back contains the L1–L5 lumbar vertebrae.
The single sacrum, which is also part of the pelvis, is during growth formed by the fusion of five sacral vertebrae and the coccyx, or tailbone, which results from the growth dependant fusion of four (or, in some cases 3 or 5) small coccygeal vertebrae.

However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age.

We will discuss the disorders associated with the curvature of the spine in Chapter 8. They include kyphosis (an excessive posterior curvature of the thoracic region), lordosis (an excessive anterior curvature of the lumbar region), and scoliosis (an abnormal lateral curvature accompanied by twisting of the vertebral column).

Joints

All, except for the hyoid bone in the neck, bones in the human body are connected to at least one other bone. Joints are the location where bones come together.

There are two major classifications of joints: synarthrodial (a joint with no separation or articular cavity, such as the skull) and diarthrodial (a freely movable joint with an articular cavity). At these joints, the articulating surfaces of the adjacent bones can move smoothly against each other. However, the bones of other joints may be joined by connective tissue or cartilage. These joints are designed for stability and provide little or no movement.

There are two major classifications of joints: synarthrodial and diarthrodial.

Most of the joints between the bones of the appendicular skeleton are diarthrodial types of joints, also called freely movable or synovial joints. These joints allow the muscles of the body to pull on a bone, thereby producing movement of that body region. For example, the ability to kick a soccer ball, pick up a fork, and dance the tango depends on mobility at these types of joints.

In diarthrodial joints, the joint cavity provides a space for movement. It is lined by a synovial membrane which forms a sac or bursa. Smooth elastic hyaline cartilage is found at the ends of the adjoining bones. The synovial membrane contains nerve-ending receptors and secretes synovial fluid, which lubricates and nourishes the cartilage. The synovial membrane is surrounded by a fibrous joint capsule interlaced with ligaments and tendons.

These joints can be classified into six categories: 

Pivot joint (uniaxial) – this joint allows rotation around an axis, such as between the first and second cervical vertebrae, which allows for side-to-side rotation of the head

Hinge joint (uniaxial) – in this joint, the C-shaped surface of one bone swings about the rounded surface of another. Movements consist of flexion and extension, and the levers only move on one plane. For example, the hinge joint of the elbow works like a door hinge;

Saddle joint (biaxial) – the saddle joint allows the concave surfaces of two bones to articulate with one another. Movement is possible in two planes. For example carpometacarpal joint of the thumb;

Plane or gliding joints (nonaxial) allows for limited gliding movements between bones. For example, joints between the tarsal bones of the foot;

The condyloid joint (biaxial) – is a partial ball-and-socket configuration. It allows movement in two planes. Example: radiocarpal (wrist) joint;

Ball-and-socket joint (triaxial) – in the ball-and-socket joint, the ball-like head of one bone fits into the concave surface of another. Movement is possible in all three planes. The hip and shoulder joints are the only ball-and-socket joints of the body.

3.4.7 Connective tissue

As previously addressed in Section 3.2.2, connective tissues play a significant role in various physiological processes within the body. Their primary function involves providing support and connection between different tissues, ranging from the connective tissue sheath encapsulating a muscle to tendons that secure muscles to bones and ultimately to the skeletal framework that maintains the body’s overall posture.

Connective tissues encompass diverse structures, including bones, cartilage, tendons, and ligaments. Each component contributes to the body’s musculoskeletal system’s structural integrity and functional capacity, ensuring proper stability and movement.

Cartilage

Cartilage forms the foundation of bone tissue. It is found at bone ends (ends of ribs), in spinal discs, at joint surfaces, and in the nose and ears.

Cartilage provides shock absorption and structure as a smooth surface between adjacent bones. Cartilage also lubricates the working parts of a joint.

Cartilaginous tissue is avascular (it has no blood supply on its own). Thus, all nutrients must diffuse through the matrix to reach the chondrocytes. The only way for cartilage to receive oxygen and nutrients is through diffusion. This is why damaged cartilage heals very slowly.

Tendons

Tendons are extensions of the muscle fibers that attach muscle to the bone. They are slightly more elastic than ligaments but cannot shorten like muscles.

They are not as vascular as the bone but can repair themselves over time.

Tendons assist in maintaining joint stability.

Ligaments

Ligaments, which consist of collagen and a somewhat elastic fiber known as elastin, connect bones to one another at joints. Although ligaments possess a certain degree of elasticity to facilitate joint movement, this flexibility is limited.

Connective tissue adaptations

The positive effect of exercise on connective tissue has been well documented. Physical training has been shown to cause an increase in tensile strength, size, resistance to injury, and the ability to repair damaged ligaments and tendons to regular tensile strength.

Proper training can alter the Golgi tendon organ and “push back” the “safety valve,” which shuts off muscle contractions. Not just any type of training alters the structure of connective tissue. Surprisingly, while endurance training has been shown to produce some adaptations, higher-intensity training is more likely to force these adaptations (Hatfield 2011).

Exercise has a positive effect on connective tissue

In other research, muscle, and tendon connective tissue has clearly demonstrated a capacity to adapt to mechanical loading and unloading in healthy individuals (Mackey et al. 2008).

For tendons, metabolic activity, circulatory responses, and collagen turnover are markedly increased after exercise. Chronic loading in the form of physical training leads to increased collagen turnover and some degree of net collagen synthesis. These changes modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress-susceptibility, and probably make it more load-resistant (Kjaer et al. 2006).

3.4.8 Muscular system

The muscular system is essential for all human movement, which involves ongoing muscle contraction and relaxation cycles.

In exercise physiology, muscles are the main operative tissue, expending energy, generating wastes, and requiring substantial nutrition. Muscles differ in appearance when observed under the microscope because of their underlying cellular structure. Two appearances are recognized: striated muscle tissue and smooth muscle tissue. Based on functional and structural differences, muscle tissue is divided into three types: skeletal, cardiac, and smooth.

In exercise physiology, muscles are the primary focus of study, as they are responsible for generating force and movement in the body during physical activity.

The muscular system plays a crucial role in facilitating movement and maintaining posture. It is responsible for a wide range of bodily actions, including walking, running, and blood circulation. Approximately 600 muscles collaborating alongside the skeletal system generate and coordinate motion throughout the body (Hatfield 2011).

keletal muscles are attached to bones by tendons and are considered voluntary, as the individual can generally control them. In contrast, smooth and cardiac muscles (cardiac muscle forms the heart’s walls) are involuntary, operating under the control of the autonomic nervous system (ANS)—the involuntary component of the nervous system. As a result, you don’t have to think to make your heartbeat. The anatomy of the cardiovascular system will be discussed in the chapter on the Carido-Respiratory System. All muscle types share the properties of extendibility, elasticity, excitability, and contractility.

3.4.9 Muscle Physiology

Muscle physiology is the study of the function and structure of muscles in the body.


Muscles are largely composed of protein, with a hierarchical system of organization from large groups to small fibers. A muscle is a group of motor units that are physically separated by a membrane from other groups of motor units.


The motor unit

Nearly all muscles, except for “involuntary muscles” like those found in the digestive system or heart (smooth and cardiac muscle tissue), require a signal from the brain to carry out work. This signal travels via the nervous system to the muscle fibers. The entire structure responsible for muscle contraction includes the nerve and the muscle fibers it stimulates.

This combination of nerve and stimulated muscle fibers is referred to as a motor unit. A motor unit comprises a single neuron and all the muscle fibers it innervates.

"Motor units are the basic functional units of muscle and the building blocks of movement."

R. M. Enoka

The quantity of muscle fibers stimulated by one nerve determines either:

a) the generated force or 

b) the delicate precision of movement when a small number of fibers are innervated.

The innervation ratio (muscle fibers per motor neuron) represents the number of muscle fibers stimulated by a single motor neuron. This ratio influences the fine motor control that a muscle has. For instance, the hand has a lower number of fibers per motor unit compared to the calf muscles.

When fibers receive a signal to contract, either all fibers stimulated by a specific motor unit contract or none will. This principle is known as the all-or-none law.

While a single motor unit won’t innervate all the muscle fibers within a particular muscle, the contracting fibers are dispersed throughout the muscle’s width and depth. This ensures an even contraction and helps prevent injury due to uneven contraction that might overemphasize one side of the muscle over the other. This concept is called non-contiguous innervation.

Each muscle cell is not directly connected to the central nervous system (CNS). Instead, impulses travel down nerve axons from the CNS, branching off to stimulate a group of muscle cells that contract in unison. To coordinate muscle movement, the CNS requires information about muscle length and tendon tension, which connects muscles to the skeleton. Specialized sensory organs called “muscle spindles, or as often referred to as muscle spindle cells,” provide this information by measuring muscle strain, allowing for the presetting of muscle tension to optimize movement and coordination.

Every skeletal muscle comprises interconnected tissues, including muscle fibers, blood vessels, nerve fibers, and connective tissue. The muscle also houses energy molecules like adenosine triphosphate (ATP) and creatine phosphate (CP), which are the body’s energy storage and utilization mechanisms. Energy is obtained for work by removing a phosphate molecule from the ATP.

Long-term fuel sources for ATP generation, such as glucose or glycogen (multiple glucose molecules bound together) and triglycerides (fats), can also be stored in muscles. Furthermore, muscles can store oxygen for aerobic activities through a unique molecule called myoglobin.

Muscles are made up of progressively larger subunits, with the smallest being the muscle cell or muscle fiber. Each muscle consists of thousands of cylindrical muscle fibers that lie parallel to one another and span the entire muscle length.

Skeletal muscles are composed of varying numbers of muscle bundles, known as fasciculi, with individual bundles referred to as a fasciculus (singular). These fasciculi (plural) are separated and enveloped by the connective tissue layer called the perimysium. Within each fasciculus, individual muscle fibers are encased by the endomysium, which provides further protection and structure.

Muscle fibers are composed of myofibrils, small bundles of myofilaments responsible for muscle contraction. Myofilaments primarily consist of two types of proteins: myosin, which forms short, thick filaments, and actin, which forms long, thin filaments. Additionally, troponin and tropomyosin are two crucial proteins that make up myofibrils and play a vital role in the contractile response.

Skeletal muscles are enclosed by three layers of connective tissue (collectively, these layers are known as mysia) that provide structure, compartmentalize muscle fibers, and maintain the muscle’s integrity. The epimysium, a dense and irregular connective tissue sheath, wraps around each muscle, enabling powerful contraction and movement. The epimysium also isolates the muscle from surrounding tissues and organs for independent movement. The epimysium tapers and merges with intramuscular tissue sheaths to form a tendon. Tendons are fibrous extensions of skeletal muscles that attach muscles to bones at both ends. 

Another connective tissue layer, the perimysium, bundles up to 150 fibers into a fasciculus. Deeper, every individual muscle fiber is enveloped and isolated from its adjacent fibers by a slender layer of connective tissue known as the endomysium.

The muscle fiber represents the smallest individual component of a muscle. Each fiber can be further analyzed by examining its constituent parts to understand its specific properties. Enclosed by the endomysium—a connective tissue sheath—each muscle fiber possesses a cell membrane called the sarcolemma, which envelops the muscle cell’s contents.

Like other cells, the muscle cell’s interior consists of a dense protoplasm known as sarcoplasm. This substance, comparable to a gel, contains all the inorganic and organic materials within the cell. The sarcoplasm encompasses contractile proteins (actin and myosin), stored glycogen and fats, enzymes, cell nuclei, and specialized structures such as mitochondria and the sarcoplasmic reticulum.

Mitochondria are specialized cellular units responsible for generating adenosine triphosphate (ATP) through the aerobic energy system, while the sarcoplasmic reticulum is a vast network of tubular channels and vesicles that contribute to structural integrity and muscle contraction.

The primary function of a muscle is to produce tension, which is then transmitted to the tendons. This tension causes the tendons to exert a pulling force on the bones at their attachment points. While the origin, the attachment point nearest to the body’s center, remains relatively stable, the bone at the insertion point (the attachment point furthest from the body’s center) experiences movement.

The primary function of a muscle is to generate force and movement in the body

Mitochondria are unique cellular structures that generate ATP. At the same time, the sarcoplasmic reticulum consists of an intricate network of tubular channels and vesicles that contribute to structural integrity and muscle contraction.

Training-induced adaptations occur within individual muscle fibers and can range from alterations within the muscle fiber components, such as those found in the sarcoplasm, to changes in the size of each muscle fiber. These modifications can impact the muscle’s characteristics. For instance, enlarging the fibers within a muscle causes an increase in their diameter, leading to an expansion in fasciculi size and ultimately resulting in overall muscle growth. In addition, a boost in mitochondria enhances a muscle’s endurance capacity while storing more ATP accelerates muscle contraction speed. Furthermore, changes in enzymes and proteins within the muscle can also affect the contractile and growth properties of that specific muscle.

The mechanics of Muscular Contraction

Individual muscle fibers can be broken down into smaller units responsible for muscle fiber contraction. The sarcoplasm, which is the muscle cell’s protoplasm, primarily consists of hundreds to thousands of myofibrils. Think of myofibrils as thin wires forming a cable, or muscle fiber, which then combines to create an even larger cable, the muscle. These myofibrils, each about 1/1000 mm in diameter, contain myofilaments or contractile proteins that enable muscle fibers to contract. Myosin and actin are the primary myofilaments.

The muscle fiber contraction occurs as myosin (the thick filament) moves over actin (the thin filament). In addition to actin and myosin filaments, two other contractile proteins, troponin, and tropomyosin, surround the actin molecule. The arrangement of actin and myosin proteins gives muscles their characteristic striated appearance, marked by alternating light and dark bands.

Actin and myosin are arranged longitudinally to create the smallest contractile unit of the muscle fiber, known as the sarcomere. Each sarcomere measures 2.6 micrometers in length, and these compact units repeat along the entire muscle length. Actin filaments are positioned and anchored at both ends of the sarcomere. The anchoring point is referred to as the “Z-line.” Six actin filaments encircle each myosin filament, while three myosin filaments surround each actin filament.

The sarcomere’s dark region, corresponding to the alignment of myosin filaments, is called the “A-band.” The light band, which fits areas in two adjacent sarcomeres containing only actin filaments, is known as the “I-band.” The sarcomere’s center, where only myosin is present, is referred to as the “H-zone.” During muscle contraction, both the H-zone and I-band will decrease in size. See Figure below.

Each muscle fiber features a network of tubules that transport ions and molecules necessary for muscle fiber contraction. This network, known as the sarcoplasmic reticulum, runs parallel to and envelopes each myofibril. It consists of tubules that end as vesicles near the Z-lines, where the actin filaments align. These tubules store calcium ions (Ca2+), which are released as a signal to initiate muscle contraction.

T-tubules (short for transverse tubules) are positioned perpendicular to the sarcoplasmic reticulum and terminate near the Z-line. When an electrical signal to contract a muscle is received, it travels down the T-tubules through a process called depolarization. This signal is then transferred to the sarcoplasmic reticulum, which releases Ca2+, triggering muscle contraction. This intricate system of tubules allows the muscle to contract simultaneously, preventing one part of the muscle from contracting before another.

Muscular contraction

There are multiple theories on muscular contraction, but the sliding filament theory is the most widely accepted. The filaments described above can interlock and slide over each other to accommodate muscle stretching. During contraction, they slide into one another, creating cross-links between the actin and myosin filaments. It’s crucial to note that while the sarcomere shortens, the individual proteins and filaments do not change in length; they slide alongside each other. These cross-links are quickly broken, forming new links further along the filaments. The process of breaking these cross-links allows the two filaments to move toward one another, causing the muscle to shorten (contract). This process is known as the sliding filament theory.

There are two main theories of muscular contraction: the sliding filament theory and the cross-bridge theory.


According to this theory, a myofibril contracts as the actin and myosin filaments slide over each other. Chemical bonds and receptor sites on the myofilaments attract one another, sustaining the contraction until fatigue disrupts the process. The strength of contraction in a whole muscle largely depends on the number of muscle fibers involved: the more muscle fibers participating, the stronger the contraction.

The filament sliding process of contraction can only occur when the myosin-binding sites on the actin filaments are exposed through a series of steps initiated by the entry of Ca++ into the sarcoplasm. Tropomyosin wraps around the actin filament chains, covering the myosin-binding sites and preventing actin from binding to myosin. The troponin-tropomyosin complex relies on calcium ion binding to TnC to regulate cross-bridges formation between myosin heads and actin filaments.

When calcium is present, cross-bridge formation and filament sliding will occur.

Here is a simplified version of the most common model for skeletal muscle contraction and relaxation:

  1. The brain sends an electrical signal along the motor unit through a nerve to the muscle fibers it innervates.
  2. The electrical stimulus triggers the release of calcium ions, which then cause troponin to move tropomyosin out of the way.
  3. With tropomyosin pushed aside, the myosin filament can bind with the actin filament, forming a “cross-bridge.”
  4. ATP is broken down, releasing energy that enables the myosin head to pull the actin over the myosin, a process referred to as “sliding filaments.”
  5. The sliding filaments cause the muscle fibers to bundle together, known as “fiber-bundling.”

The term “contraction” does not always imply the shortening of a muscle. In a technical sense, it refers to the development of tension within a muscle. There are two main types of contractions. An isometric contraction occurs when the muscle develops tension without shortening. In contrast, an isotonic contraction occurs when the muscle shortens while maintaining constant tension.

Energy for contraction comes from the chemical reactions between the food components we consume and the oxygen we breathe. Consequently, blood is necessary to deliver essential nutrients and oxygen to the muscles and to remove waste products. The biochemical process of energy production shall be addressed in detail in Chapter 6.

Types of Muscle Fibers

Skeletal muscle fibers can be classified based on two criteria: 1) how fast fibers contract relative to others and 2) how fibers regenerate ATP.

Within this criteria, we divide muscle fiber types into two main groups:

  1. Fast-twitch muscle fibers
  2. Slow-twitch muscle fibers

The Fast twitch muscle fiber group is then further divided into two subcategories, resulting in three main muscle fiber types used in the literature:

  1. Type IIx muscle fiber type (also referred to as FT as Fast Twitch, FF as Fast Fatigable, white muscle fibers or often named Type IIb)
  2. Type IIa muscle fiber type (also referred to as intermediate fibers or FOG as Fast Oxidative Glycolytic)
  3. Type I muscle fiber type (also referred to as ST as Slow Twitch, slow Oxidative, or red muscle fibers)

Type IIx muscle fibers are those that produce maximal force and fatigue rapidly. In contrast, the Type IIa fibers adopt specific characteristics of the slow twitch fibers and don’t fatigue as rapidly.

Twitch speed refers to the pace at which a particular force is produced and its sustained duration. This speed is determined by the entire motor unit, which consists of the neuron and the muscle fibers the motor neuron innervates. On behalf of these characteristics, as mentioned earlier, we derive two main classifications of muscle fiber types.

Twitch is the response of the fiber to a single stimulating nervous impulse (Whitney 1958).

We have summed the main characteristics of each main muscle fiber group in Table below:

In the human body, various muscles contain differing proportions of slow-twitch, intermediate, and fast-twitch fibers. Your genetics will partly dictate the dominance or percentage of each fiber type in specific muscles. Muscles demanding greater endurance are expected to have a higher percentage of slow-twitch fibers. For instance, the rectus abdominis (abdominal muscle) possesses many slow-twitch fibers, as it is responsible for long-term stabilization and is also a part of the respiration muscles. Similarly, lower body muscles, engaged throughout the day for standing and walking, usually contain a higher percentage of slow-twitch fibers. Conversely, upper body muscles, like the pectoralis major, are primarily used for short-duration, high-intensity activities and thus have a higher percentage of fast-twitch fibers.

While the predominance of specific fiber types in certain muscles is generally consistent across individuals, each person is born with slightly different percentages of muscle fiber types. Those with more fast-twitch fibers excel in short-term, high-intensity activities. In contrast, individuals with a higher proportion of slow-twitch fibers perform better in longer-duration, lower-intensity activities. Although training can influence fiber development, it does not convert fast-twitch fibers into slow-twitch ones.

Fast-twitch fibers

Type IIx fast-twitch fibers (FT, fast fatigue, or white fibers) can produce rapid, powerful contractions. This is because they quickly transmit electrochemical signals that prompt them to contract. Fast-twitch motor units innervate between 300 and 800 fibers, compared to the 10 to 180 fibers recruited by slow-twitch motor units.

Fast-twitch fibers contract quickly and powerfully but fatigue faster than slow-twitch fibers

This extensive recruitment and the composition of fast-twitch fibers allow them to generate significantly more force and experience greater growth than slow-twitch fibers.

Fast-twitch fibers exhibit high myosin ATPase activity, an enzyme that enables rapid ATP splitting for muscle contraction. The faster the myosin ATPase activity, the quicker the fiber can split ATP and access the energy needed for muscle fiber contraction and relaxation. Additionally, fast-twitch fibers display an accelerated rate of calcium release and uptake by the sarcoplasmic reticulum, facilitating rapid contraction. As a result, fast-twitch fibers can reach peak contraction force within 34 to 50 milliseconds. In contrast, slow-twitch fibers generally take around 110 milliseconds to achieve peak contraction.Furthermore, the peak contraction force generated by fast-twitch fibers is significantly greater than that of slow-twitch fibers.

Fast-twitch fibers primarily generate ATP through anaerobic glycolysis, relying on stored ATP and creatine phosphate. This dependency on anaerobic metabolism makes fast-twitch fibers well-adapted for short-term sprint activities and forceful muscle contractions, which largely depend on anaerobic metabolism. However, due to their limited capacity to regenerate energy (ATP) from aerobic sources, glycolytic fibers fatigue more rapidly, typically within 30 seconds of activity.

Intermediate Fibers (fast Oxidative Glycolytic)

Type IIa fast-twitch fibers, also known as Fast Oxidative Glycolytic (FOG) fibers, exhibit characteristics that fall between those of slow oxidative fibers (Type I) and fast glycolytic fibers (Type IIx). These fibers generate ATP relatively rapidly, allowing them to produce comparatively high levels of tension. Due to their oxidative nature, they do not fatigue as quickly as the Fast Glycolytic Type IIx fibers.

Type IIa fast-twitch fibers contract quickly, generate high force, and have greater fatigue resistance than type IIx fibers. They use both anaerobic and aerobic metabolism to produce energy.

Fast oxidative fibers are primarily employed for movements that demand more energy than postural control but less than muscle actions for explosive movements. They provide a balance between power and endurance, making them suitable for activities requiring moderate force and sustained effort. We refer to this type of activity as muscular endurance or anaerobic endurance.

Training can influence the properties of Type IIx fibers, enabling them to develop more oxidative characteristics and transition into Type IIa fibers. Endurance training, in particular, stimulates the development of the oxidative capacity in these fibers by increasing the number of mitochondria and enhancing the blood supply to the muscle. This adaptation allows the muscle to utilize oxygen better and resist fatigue, ultimately improving its overall performance in activities requiring both strength and endurance.

Slow-twitch Fibers (slow Oxidative)

Type I slow-twitch fibers (ST, slow oxidative, or red) exhibit slower contraction speeds, taking approximately 110 milliseconds to reach peak contraction force and producing lower peak contraction force than fast-twitch fibers. However, they have longer twitch durations.

Slow-twitch fibers can generate additional ATP through aerobic metabolism by breaking down sugars and fats. This capability is akin to having their own fuel station rather than being restricted to a small fuel tank. Producing their own ATP allows slow-twitch fibers to excel in extended endurance activities (aerobic endurance). Although slow-twitch fibers differ in strength, speed, and power potential compared to fast-twitch fibers, individuals can still achieve respectable speeds using slow-twitch fibers, albeit not as rapidly as with well-trained fast-twitch or intermediate fibers.

Slow-twitch fibers are muscle fibers that contract slowly but have a high capacity for endurance.

For instance, an elite male runner (primarily relying on intermediate and fast-twitch fibers during competition) completes a mile in around three minutes and forty-five seconds. In contrast, an elite male marathon runner maintains four-and-a-half to five-minute miles for 26.2 miles. While much slower than the miler, the marathon runner’s times are still impressively fast per mile.

Slow-twitch fibers predominantly generate energy for ATP resynthesis through aerobic energy pathways that break down glucose and fatty acids, providing higher sustained levels of ATP and enabling prolonged endurance. Unlike fast-twitch fibers, slow-twitch fibers exhibit low myosin ATPase activity levels. Consequently, slow-twitch fibers produce less force of contraction for high-power, speed, and strength activities than fast-twitch fibers. However, slow-twitch fibers have a relatively high concentration of mitochondria and myoglobin and high capillary density. These features allow slow-twitch fibers to efficiently use oxygen and fats to supply the energy required for long-term, sustained endurance activities. Additionally, slow-twitch fibers have a smaller diameter than fast-twitch and intermediate fibers.

Did you know? Costill et al. (1976) found that untrained individuals had a 50/50 ratio of fast (type IIA and IIX) to slow twitch (type I) fibers. However, in the athletic population long and middle distance runners had 60-70% slow twitch fibers, while sprinters demonstrated an 80% fast twitch fiber makeup. Moreover, elite weight and power lifters have been found to have a significantly greater fast twitch fiber makeup (60%) than endurance athletes (40%) (Widrick, 2020)

The size principle of fiber recruitment

The force output of a muscle is related to the stimulus it receives. As a result, different muscle fibers exhibit varying susceptibility to recruitment, with Type I and Type IIx fibers demonstrating lower levels of susceptibility.

The size principle of fiber recruitment is also known as the Henneman Principle. According to this principle, motor units are activated in a specific sequence based on their recruitment thresholds, force output, and firing rates, starting with smaller units. This results in task-appropriate recruitment, providing two significant physiological benefits.

First, it minimizes fatigue experienced by an organism by initially using fatigue-resistant muscle fibers and only activating fatigable fibers when high forces are necessary. This strategic recruitment ensures that muscles maintain efficiency and endurance during various activities.

Secondly, the relative change in force produced by additional recruitment remains relatively constant. As an illustration, consider a scenario where all motor units produce a similar force. Recruiting an additional unit might increase the force by 10% when only ten motor units are active but generate only a 1% increase when 100 are active. By adhering to the size principle, the incremental increase in force remains relatively consistent as more motor units are engaged, allowing for better control and modulation of muscle force during different tasks.

As most muscles comprise a mix of Type I and Type II fibers, force production can range from very low to very high. Therefore, to reach a high-threshold motor unit, all the motor units below it must be sequentially recruited.

Mantilla, 2014

This principle can be illustrated by comparing picking up a phone to curling a dumbbell. Lower threshold motor units are recruited for the relatively light task of picking up a phone. In contrast, higher threshold motor units are activated for the more strenuous activity of curling a dumbbell. This systematic recruitment of motor units ensures efficient muscle force production while minimizing fatigue during various tasks, from light daily activities to high-intensity exercises.

The stretch reflex

The stretch reflex is an inherent protective mechanism within the neuromuscular system, facilitated by muscle spindles, which are proprioceptors located in the muscle belly. Unlike the Golgi tendon organ (GTO), which aligns with the muscle’s force plane, the muscle spindle runs parallel to it.

This reflex functions similarly to the Golgi tendon organ, safeguarding against overload and injury through the stretch reflex action, such as the knee-jerk response physicians use to assess muscle response adequacy. 

In addition, the stretch reflex serves as a regulatory mechanism that allows muscles to automatically adapt to variations in load and length without needing signals from higher-order centers in the nervous system, like the brain.

Additional proprioceptors can be found in and around all body joints, continually providing the nervous system with information about the joint’s spatial relationship to the rest of the body, including movement, position, and speed.

Adaptations to training

Exercise triggers a series of metabolic responses that influence the body’s anatomy, physiology, and biochemistry. The extent of these changes largely depends on whether the exercise is anaerobic or aerobic. The type and duration of exercise physically stimulate muscles to develop either more fast or slow twitch muscle fibers, which dictate the primary energy source used.

High-intensity exercise promotes the development of fast-twitch muscle fibers (Type II). In contrast, low-intensity exercise fosters the growth of slow-twitch muscle fibers (Type I). Alongside these physical changes, various hormonal shifts occur during exercise and rest periods. These changes benefit from and are facilitated by a nutrient profile that aligns with the type of metabolic demands imposed by the exercise.

- Aerobic adaptations

Aerobic exercises, such as endurance training or cardiovascular workouts on a treadmill, offer numerous benefits, including fat-burning, enhanced cardiovascular health, and improved recovery capabilities.

Aerobic activity pumps oxygen through the body, expanding the number and size of blood vessels. Blood vessels deliver oxygen and nutrients to muscles and remove waste products, promoting muscle growth, repair, and recovery. Without incorporating aerobic exercise into your training program, your body cannot establish new supply routes for your developing muscles. Type I fibers exhibit a higher oxidative capacity than Type II fibers before and after training. While strength and hypertrophy training induce somewhat similar muscular adaptations, the adaptations of aerobic exercise are distinct. Gradual conversion of Type IIx fibers to Type IIa fibers may occur. This is significant because Type IIa fast oxidative fibers have a greater oxidative capacity than Type IIx fast glycolytic fibers.

Aerobic training also triggers crucial metabolic changes within the body. At the cellular level, aerobic exercise adaptations include an increase in the size and number of mitochondria and higher myoglobin content. When mitochondria – the cellular furnaces where fat and other nutrients are burned – receive more oxygen, the muscle tissue’s aerobic capacity is enhanced. Additionally, aerobic exercise elevates myoglobin levels, a protein that transports oxygen from the bloodstream to muscle fibers. Ultimately, this adaptation increases the levels and activity of enzymes involved in the aerobic metabolism of glucose.

- Anerobic adaptations

Anaerobic training significantly enhances the body’s ability to develop explosive strength and maximize short-term energy systems. Some of the primary changes observed due to anaerobic exercise include increased size and a number of fast-twitch fibers. Moreover, anaerobic activity leads to a higher tolerance for elevated blood lactate levels, an increase in enzymes involved in the anaerobic phase of glucose breakdown (glycolysis), and a rise in the resting levels of ATP, CP, creatine, and glycogen content within muscles.

Lastly, anaerobic adaptations encompass a surge in growth hormone and testosterone levels following brief sessions (45-75 minutes) of high-intensity weight training. Growth hormone, testosterone, insulin, and insulin-like growth factor-1 are the four hormones directly responsible for muscle hypertrophy.

Muscular Hypertrophy

Muscular hypertrophy is simply the enlargement of muscle fibers. When muscles grow, new cells are rarely formed; instead, structural proteins are added to a muscle fiber, increasing its diameter.

The primary increase in fiber size is due to an expansion in the cross-sectional area of the muscle fibers, or hypertrophy. This increase in diameter is thought to occur through the remodeling of proteins within the cell or an increase in the size and number of myofibrils or both.

Although the exact mechanism is still unknown, it is believed that hormonal signals triggered by heavy resistance stimulate the uptake of amino acids. As a result, contractile proteins may accumulate through either an increased synthesis rate, a decreased protein breakdown rate, or both.

According to newer research, differences in hypertrophic adaptations are more known. This field is getting more exposure thanks to an extraordinary scientist, Dr. Brad Schoenfeld.

In search of information about muscular hypertrophy, one can easily find two different hypertrophic adaptations to training. The theory itself has been around for decades.

However, since scientific research in muscular hypertrophy is not of high priority and, therefore, hardly financed, we have had to wait for the research to emerge.

Due to the latest research, we can now say that sarcoplasmic and contractile, often called myofibrillar hypertrophy, is a “thing.”

Dr. Brad Schoenfeld’s research on muscular hypertrophy, as detailed in his book “Science and Development of Muscle Hypertrophy,” identifies several mechanisms that contribute to contractile and sarcoplasmic hypertrophy.

In summary, sarcoplasmic hypertrophy is a type of muscle growth characterized by an increase in the non-contractile components of muscle cells. Dr. Brad Schoenfeld’s research highlights several mechanisms contributing to this form of hypertrophy, including increased muscle glycogen storage, enhanced intracellular hydration, and accumulation of other sarcoplasmic components.

n contrast, contractile hypertrophy is a type of muscle growth characterized by increased size and the number of myofibrils within muscle cells, leading to increased muscle strength. Research highlights several mechanisms contributing to this form of hypertrophy, including mechanical tension, muscle damage, and metabolic stress.

According to Dr. Schoenfeld, the subsequent research discovered that a high-load, low-volume protocol led to hypertrophy in type-2 muscle fibers without significant increases in sarcoplasmic proteins and fluid.

Studies indicate that bodybuilding-style training routines, characterized by higher volume and moderate loads – the typical pump-training routines, result in greater sarcoplasmic growth. In contrast, powerlifting-style programs involving lower volume and heavier loads may yield more significant myofibrillar gains.

Schoenfeld’s key takeaway is that sarcoplasmic hypertrophy indeed exists. It is likely influenced by the type of training undertaken, such as high-volume, pump-focused workouts. However, achieving this growth requires consistently high volume over weeks or months. It is essential to remember that no significant muscle growth, regardless of the type, occurs short term!

In addition to enlarging muscle fibers, increasing the number of muscle cells also contributes to muscle growth. This process is called hyperplasia. During this process, individual muscle cells split to form two cells. This phenomenon has been observed in animals, but the proof of the same mechanism in humans is inconsistent. However, even for those individuals who exhibit this phenomenon, hyperplasia accounts for less than 5% of all muscle growth.

 

Muscle atrophy is when structural proteins are lost, and muscle mass decreases (reduction in muscle diameter)—atrophy results from disuse. A notable example is when a limb is cast for several weeks. During that period, the muscles under the cast were not used at all, leading to a significant reduction in diameter compared to the limb that was not immobilized. The loss of muscle size and strength in older adults is mainly due to a lack of use. Aging does cause some decrease in size and strength. However, individuals without the strength to perform daily tasks are primarily in that situation due to muscular disuse. Regular resistance training can increase their strength and functionality. In such cases, resistance training may start simply as practicing standing up.


Sarcopenia is defined by the loss of muscle mass and function. This is a muscle disease (muscle failure) rooted in adverse muscle changes that accrue across a lifetime; sarcopenia is common among adults of older age but can also occur earlier in life (Cruz et at. 2019).

The Importance of Fibre Type to Fitness Training

A fitness training program must consider muscle fiber types to be successful. Some research shows that training can induce muscle fiber shifting, but the results are inconclusive (Widrick et al., 2002). However, training can definitely enhance specific characteristics of a muscle’s fibers based on the type of training.

Individuals training for endurance activities should focus on slow-twitch fibers. Endurance training generally induces a fiber-type shift toward a more oxidative phenotype, which is logical from the specificity of the training standpoint, considering the increased oxygen consumption during endurance exercise (Plotkin et al., 2021).

In contrast, those training for high-speed and power activities should concentrate on fast-twitch fibers. Training programs should be predominantly focused on high-intensity, low-volume, and/or high-velocity exercises. Strength and power athletes should also limit low-intensity training protocols or high-volume combined with high-intensity intermittent training programs, which may facilitate a shift of fast twitch to slow twitch fiber phenotypes (Widrick et al., 2002)

Everyone needs to train all fiber types from the functionality and health perspective. Still, most sports, activities, and goals will require primarily training one fiber type over another. If an athlete’s performance reveals a weakness, it may indicate insufficient training of a particular fiber type.

For instance, an endurance athlete consistently out-sprinted by competitors might need to increase speed and strength by training fast-twitch fibers, incorporating more sprints with extended rest periods in practice sessions.

Applying muscle fiber knowledge is also crucial when training clients with general fitness goals like weight loss and toning up. Strength activities increase lean tissue and muscle tone. Exercises that fatigue muscles stimulate most fibers, promoting muscle hypertrophy. Enhanced muscle stimulation and increased muscle mass boost metabolism, raising total calorie burn throughout the day and thus increasing calorie deficits at the same levels of energy intake. This makes the nutritional aspects of weight loss easier.

It’s essential to remember that slow-twitch fibers utilize aerobic systems and train the body to use fat as an energy source effectively. Yet, the calories burned will be fewer compared to fast-twitch muscle fibers. In addition, regular aerobic training is crucial for increased health benefits.

At the PTBA, we encourage personal trainers to use the techniques and methods most resonating with their clients and adopt methods that lead to high consistency.

3.4.10 Muscular Tissues

Cardiac Muscle Tissue

Cardiac muscle tissue, also known as striated-involuntary muscle tissue, forms the walls of the heart. Its primary function is to contract the heart, enabling blood circulation throughout the body. The cells of cardiac muscle tissue are typically branched and possess centrally-located nuclei. They often interconnect and merge with one another. Remarkably, cardiac muscle tissue exhibits high resistance to fatigue, requiring only brief periods of rest between contractions. In fact, during intense physical activity, the skeletal muscles tire before the cardiac muscle tissue.

Smoth Muscle Tissue

Smooth muscle tissue, also known as smooth-involuntary muscle tissue, is located in the walls of various tubular structures throughout the body, such as the digestive, respiratory, and genitourinary tracts. Additionally, it can be found in the walls of blood vessels, large lymphatic vessels, glandular ducts, and the intrinsic eye muscles (iris and ciliary body), as well as in the erector muscle of hair follicles. Smooth muscle tissue is vital in moving substances through these tracts, regulating blood vessel diameter, transporting substances along glandular ducts, adjusting pupil diameter and lens shape in the eye, and erecting hairs. Smooth muscle cells are elongated with pointed ends and contain a single nucleus per cell. These cells contract at a slower pace than striated muscle cells, allowing them to resist fatigue more effectively.

Skeletal Muscle Tissue

Skeletal muscle tissue (striated-voluntary muscle tissue) is found attached to bones, in extrinsic eyeball muscles, and in the upper third portion of the esophagus. Skeletal muscle tissue functions to move the bones and eyes. It also moves food during the first part of swallowing.

Skeletal muscle tissue is made up of long muscle cells (muscle fibers) that bear the unique characteristic of containing many nuclei, called multinucleate. As a result, skeletal muscle tissue cannot sustain prolonged all-out effort contractions, as they easily fatigue.

3.4.11 Interactions of Skeletal Muscles in the Body

The moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. Muscle pull rather than push. Upon activation, the muscle pulls the insertion toward the origin.

Although several muscles may be involved in an action, the principal muscle involved is the prime mover or agonist. During forearm flexion, for example, lifting a cup, a muscle named the biceps brachii is the prime mover. Because the brachialis can assist it, the brachialis is called a synergist in this action.

Most muscles work in pairs of agonists and antagonists. The muscles involved in causing a movement are the agonists. An antagonist is a muscle with the opposite action of the prime mover.

(1) they maintain the body or limb position, such as holding the arm out or standing upright; and

(2) they control rapid movement, as in shadow boxing, without landing a punch or having the ability to check the motion of a limb.

Types of muscles according to their function in a movement:

Some muscles do not pull against the skeleton for movements, such as the muscles of facial expressions. The insertions and origins of facial muscles are in the skin so that certain individual muscles contract to form a smile or frown, form sounds or words, and raise eyebrows.

Four helpful rules can be applied to all major joints except the ankle and knee because the lower extremities are rotated during development:

  1. A muscle that crosses the anterior side of a joint results in flexion, which decreases joint angle with movement. For example, the muscles crossing the anterior proportion of the elbow joint cause elbow flexion.
  2. A muscle that crosses the posterior side of a joint results in extension, increasing joint angle with movement. For example, the muscles crossing the posterior proportion of the elbow joint cause elbow extension.
  3. A muscle that crosses the lateral side of a joint results in abduction, which results in the body part moving away from the midline of the body. For example, the deltoid muscle on the shoulder joint’s lateral side causes the shoulder’s abduction.
  4. A muscle that crosses the medial side of a joint results in adduction, which results in the upper or lower extremity moving toward the midline of the body. For example, the teres major muscle on the medial side of the humerus causes shoulder adduction.

3.4.12 Naming of the muscles

Anatomists name the skeletal muscles according to several criteria, each of which describes the muscle in some way. These include naming the muscle after its shape, size, fiber direction, location, number of origins, or muscle action:

  • Muscle Shape: The names of some muscles reflect their shape. For example, the deltoid is a large, triangular-shaped muscle that covers the shoulder. It is named so because the Greek letter delta is a triangle.
  • Muscle Location: The skeletal muscle’s anatomical location or its relationship to a particular bone often determines its name. For example, the frontalis muscle is located on top of the skull’s frontal bone. Other examples are muscles of the arm that include the term brachii (of the arm):
    • Some muscles indicate their position relative to the body’s midline, which is related to muscle location: lateralis (to the outside or away from the midline) and medialis (medial or toward the midline).
    • The location of a muscle’s attachment can also appear in its name. When the name of a muscle is based on the attachments, the origin is always named first. For instance, the sternocleidomastoid muscle of the neck has a dual origin on the sternum (therefore referred to as “sterno”) and clavicle (therefore referred to as “cleido”), and it inserts on the mastoid proportion of the temporal bone.
  • Muscle Size: For the buttocks, the size of the muscles influences the names: gluteus maximus (largest), gluteus medius (medium), and gluteus minimus (smallest). Another example are the pectoral muscles, including the pectoralis major and minor.
  • Muscle length: Names are often used to indicate length related to muscle length. For example, brevis (short) and longus (long).
  • Muscle Fiber Direction: The direction of the muscle fibers and fascicles are used to describe muscles. For example, the abdominal muscles all indicate the direction of the fibers, such as the rectus (straight), the obliques (at an angle) and the transverse (horizontal) muscles of the abdomen.
  • Number of Muscle Origins (or muscles in a group): Some muscle names indicate the number of muscle origins or the number of muscles in a group, depending upon one’s perspective. One example is the quadriceps, a group of four muscles on the thigh’s anterior proportion (front). Other examples include the biceps brachii and the triceps brachii. The prefix bi indicates that the muscle has two origins, and tri indicates three origins.
  • Muscle action: When muscles are named for the movement they produce, one can find action words in their name. Some examples are flexors (decrease the angle at the joint), extensors (increase the angle at the joint), abductors (move the bone away from the midline), or adductors (move the bone toward the midline).
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