3.5 Skeletal muscles and muscle actions

3.5 Skeletal muscles and muscle actions

Welcome to this essential chapter, a critical component of your journey to becoming a successful personal trainer. The following pages will explore the prime movers and their synergists, providing a comprehensive understanding of muscle actions and their role in various exercises.

Combined with the knowledge acquired from previous chapters, this information will create a solid foundation for understanding kinesiology, biomechanics, and the basics of energy metabolism.

As we delve into the prime movers and their synergists, we aim to strike the perfect balance between providing you with a solid academic foundation and equipping you with the practical understanding necessary to excel as a personal trainer. Concentrating on these essential muscles will deepen your knowledge of muscle actions and biomechanics and enhance your ability to develop customized, effective training programs for your clients.

So, let’s embark on this fascinating exploration of the skeletal muscles and muscle actions that will be at the heart of your personal training career and, together, build a strong foundation for your understanding of human movement and performance.

Muscle terminology

Understanding the location of muscles and their relationship to the joints is crucial for comprehending the impact muscles have on joint movement. When a muscle contracts, it typically draws both ends towards the center, or the belly, of the muscle. If neither of the bones to which a muscle is attached were stabilized, both bones would move towards each other during contraction. However, in our bodies, one bone is generally more stabilized due to various factors, causing the less stabilized bone to move during contraction. The attachment points for a muscle are referred to as the insertion and the origin.

Origin

The origin refers to the proximal attachment, typically considered the least movable part or the point that connects closest to the midline or center of the body.

Insertion

The insertion denotes the distal attachment, which is usually regarded as the most movable part or the point that connects furthest from the midline or center of the body.

As a trainer, it is crucial to possess a foundational understanding of the anatomical structures that make up the human body and the fundamental terminology used to describe directions within the body. This knowledge is essential for the ability to understand the literature you will encounter throughout your learning journey.

Outlined below is the core terminology that you, as a PTBA Certified Trainer, must comprehend:

3.5.1 Essential skeletal muscles for personal trainers

Having a look at the provided images of the human body, it seems that learning all the muscles is overwhelming. However, from our experience as educators in the fitness industry, we know that learning about human physiology and anatomy gets easier if you invest slightly more time into it and repeatedly go over the chapters.

We are trying to provide you with the easiest way possible to learn and understand these crucial topics. Therefore we encourage you to go beyond online learning and participate in our live courses.

In the subsequent pages of this textbook, we will cover the essential muscles that you, as a personal trainer, need to understand in detail.

Muscle shapes and fiber arrangement

Skeletal muscles come in various shapes and configurations, each serving a unique function and contributing to the overall versatility of our musculoskeletal system. The different shapes of skeletal muscles include convergent, fusiform, parallel, unipennate, bipennate, multipennate, and circular:

  1. Convergent muscles: These muscles have a broad origin that tapers towards a narrow insertion point. Their fibers converge towards a single tendon, allowing for a versatile range of motion. An example of a convergent muscle is the pectoralis major.
  2. Fusiform muscles: These muscles have a spindle-like shape, with a wide central belly that tapers towards the ends. Their fibers run parallel to the muscle’s long axis, resulting in a strong, focused contraction. The biceps brachii is an example of a fusiform muscle.Parallel muscles: Parallel muscles have fibers that run parallel to the length of the muscle. These muscles typically have a uniform width and can contract over a considerable distance, providing a good range of motion. The rectus abdominis is an example of a parallel muscle.
  3. Unipennate muscles: The fibers are arranged diagonally along one side of a central tendon. This arrangement increases the muscle’s overall strength, as more fibers can be packed into a given area. An example of a unipennate muscle is the extensor digitorum longus.
  4. Bipennate muscles: Bipennate muscles have fibers that run diagonally on both sides of a central tendon, resembling a feather. This arrangement allows for even more fibers, resulting in a stronger muscle. The rectus femoris is an example of a bipennate muscle.
  5. Multipennate muscles: Multipennate muscles have multiple tendons with fibers running diagonally between them. This configuration further maximizes the muscle’s strength and force production. The deltoid muscle is an example of a multipennate muscle.
  6. Circular muscles: Also known as sphincter muscles, circular muscles have fibers arranged in concentric rings. These muscles control the opening and closing of various orifices in the body, such as the mouth and the eyes. The orbicularis oris and orbicularis oculi (the muscles around our eyes) are examples of circular muscles.

Understanding the diverse shapes of skeletal muscles is essential for personal trainers, as it provides valuable insights into the function and capabilities of each muscle.

Muscles of the upper body - anterior view

Pectoralis major

Origin: Portions of the clavicle and sternum, costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle

Insertion: Lateral lip of the bicipital groove of the humerus

Actions:

  1. Horizontal adduction at the glenohumeral joint (shoulder joint)
  2. Flexion at the glenohumeral joint
  3. Internal rotation of the humerus

Pectoralis minor

Origin: Anterior surfaces of the 3rd, 4th, and 5th ribs, near their costal cartilages

Insertion: Coracoid process of the scapula

Actions:

  1. Stabilizes the scapula by drawing it inferiorly and anteriorly against the thoracic wall
  2. Assists in depressing the scapula (moving it downward)
  3. Assists in protraction of the scapula (moving it forward)

Biceps brachii

Origin:

  1. Long head: Supraglenoid tubercle of the scapula
  2. Short head: Coracoid process of the scapula

Insertion: Radial tuberosity and bicipital aponeurosis (which blends with the forearm fascia)

Actions:

  1. Flexion at the elbow joint
  2. Supination of the forearm (radioulnar joint)
  3. Weak flexion at the glenohumeral joint (shoulder joint)

Brachialis

Origin: Anterior surface of the humerus, particularly the lower half

Insertion: Coronoid process and tuberosity of the ulna

Actions:

  1. Flexion at the elbow joint (primary flexor)

Brachioradialis

Origin: Lateral supracondylar ridge of the humerus

Insertion: Styloid process of the radius

Actions:

  1. Flexion at the elbow joint (particularly when the forearm is in a semi-pronated or semi-supinated position)
 
 

Rectus abdominis

Origin: Pubic symphysis and pubic crest

Insertion: Xiphoid process of the sternum and costal cartilages of the 5th, 6th, and 7th ribs

Actions:

  1. Flexion of the lumbar spine (trunk flexion)
  2. Compresses the abdominal cavity, assisting in forced expiration, defecation, urination, and childbirth

External oblique

Origin: Outer surfaces of the 5th to 12th ribs

Insertion: Linea alba, pubic tubercle, and the anterior half of the iliac crest

Actions:

  1. Unilateral contraction: Lateral flexion of the trunk and rotation to the opposite side
  2. Bilateral contraction: Flexion of the trunk and compression of the abdominal cavity, assisting in forced expiration, defecation, urination, and childbirth

Internal oblique

Origin: Inguinal ligament, iliac crest, and the thoracolumbar fascia

Insertion: Linea alba, pubic crest, pecten pubis, and the lower borders of the 10th, 11th, and 12th ribs

Actions:

  1. Unilateral contraction: Lateral flexion of the trunk and rotation to the same side
  2. Bilateral contraction: Flexion of the trunk and compression of the abdominal cavity, assisting in forced expiration, defecation, urination, and childbirth

Transversus abdominis

Origin: Inguinal ligament, iliac crest, thoracolumbar fascia, and the inner surfaces of the 7th to 12th costal cartilages

Insertion: Linea alba, pubic crest, and the pecten pubis via the conjoint tendon

Actions:

  1. Compression of the abdominal cavity, assisting in forced expiration, defecation, urination, and childbirth (does not contribute to trunk flexion, extension, or rotation)

Serratus anterior

Origin: Outer surfaces of the 1st to 8th (or 9th) ribs, at the junction of the rib and costal cartilage

Insertion: Anterior surface of the medial border of the scapula

Actions:

  1. Protraction of the scapula (moving it forward and laterally along the thoracic wall)
  2. Upward rotation of the scapula (during arm elevation)
  3. Stabilizes the scapula against the thoracic wall

Deltoid

Origin:

  1. Anterior fibers: Anterior border of the lateral third of the clavicle
  2. Middle fibers: Lateral aspect of the acromion process
  3. Posterior fibers: Spine of the scapula

Insertion: Deltoid tuberosity of the humerus

Actions:

  1. Anterior fibers: Flexion and internal rotation of the humerus at the glenohumeral joint (shoulder joint)
  2. Middle fibers: Abduction of the humerus at the glenohumeral joint (after the initial 15-30 degrees performed by the supraspinatus)
  3. Posterior fibers: Extension and external rotation of the humerus at the glenohumeral joint

Muscles of the upper body - posterior view

Trapezius

Origin:

  1. Upper fibers: External occipital protuberance, medial portion of the superior nuchal line, and the ligamentum nuchae
  2. Middle fibers: Spinous processes of the 7th cervical vertebra (C7) and the first three thoracic vertebrae (T1-T3)
  3. Lower fibers: Spinous processes of the 4th to 12th thoracic vertebrae (T4-T12)

Insertion:

  1. Upper fibers: Lateral third of the clavicle and the acromion process of the scapula
  2. Middle fibers: Spine of the scapula
  3. Lower fibers: Tubercle at the apex of the scapular spine

Actions:

  1. Upper fibers: Elevation and upward rotation of the scapula
  2. Middle fibers: Retraction (adduction) of the scapula
  3. Lower fibers: Depression and upward rotation of the scapula
  4. Upper and lower fibers together: Rotate the scapula upward during arm elevation

Rhomboid

Rhomboid muscles consist of two separate muscles: rhomboid major and rhomboid minor. The actions of both muscles are the same.

Rhomboid major:

Origin: Spinous processes of the 2nd to 5th thoracic vertebrae (T2-T5)

Insertion: Medial border of the scapula, from the level of the scapular spine to the inferior angle of the scapula

Rhomboid minor:

Origin: Spinous processes of the 7th cervical vertebra (C7) and the 1st thoracic vertebra (T1)

Insertion: Medial border of the scapula, at the base of the scapular spine

Actions:

  1. Retraction (adduction) of the scapula
  2. Elevation of the scapula
  3. Downward rotation of the scapula

Supraspinatus

Origin: Supraspinous fossa of the scapula

Insertion: Greater tubercle of the humerus (on the superior facet)

Actions:

  1. Abduction of the humerus at the glenohumeral joint (shoulder joint) – initiates the first 15-30 degrees of abduction
  2. Stabilizes the glenohumeral joint by helping to hold the head of the humerus in the glenoid cavity

 

Infraspinatus

Origin: Infraspinous fossa of the scapula

Insertion: Greater tubercle of the humerus (on the middle facet)

Actions:

  1. External (lateral) rotation of the humerus at the glenohumeral joint (shoulder joint)
  2. Stabilizes the glenohumeral joint by helping to hold the head of the humerus in the glenoid cavity

Teres minor

Origin: Lateral border of the scapula (middle half)

Insertion: Greater tubercle of the humerus (on the inferior facet)

Actions:

  1. External (lateral) rotation of the humerus at the glenohumeral joint (shoulder joint)
  2. Stabilizes the glenohumeral joint by helping to hold the head of the humerus in the glenoid cavity

 

Subscapularis

Origin: Subscapular fossa of the scapula 

Insertion: Lesser tubercle of the humerus (on the anterior facet) 

Actions:

  1. Internal (medial) rotation of the humerus at the glenohumeral joint (shoulder joint)
  2. Adduction of the humerus at the glenohumeral joint
  3. Stabilizes the glenohumeral joint by helping to hold the head of the humerus in the glenoid cavity
  4. Assists in extension of the humerus at the glenohumeral joint

Teres major

Origin: Inferior angle of the scapula 

Insertion: Intertubercular groove of the humerus (on the medial lip) 

Actions:

  1. Adduction of the humerus at the glenohumeral joint (shoulder joint)
  2. Internal (medial) rotation of the humerus at the glenohumeral joint
  3. Extension of the humerus at the glenohumeral joint
  4. Assists in horizontal adduction of the humerus at the glenohumeral joint
  5. Stabilizes the humeral head in the glenoid cavity of the scapula

Triceps brachii

Origin: Long head – infraglenoid tubercle of the scapula; 

Lateral head – posterior shaft of the humerus, above the radial groove; 

Medial head – posterior shaft of the humerus, below the radial groove 

Insertion: Olecranon process of the ulna 

Actions:

  1. Extension of the elbow joint
  2. Long head assists in adduction and extension of the humerus at the glenohumeral joint (shoulder joint)

Latissimus dorsi 

Origin: Spinous processes of lower thoracic vertebrae, iliac crest, and lower three or four ribs 

Insertion: Floor of the Intertubercular groove of the humerus

 Actions:

  1. Adduction of the humerus at the glenohumeral joint (shoulder joint)
  2. Extension of the humerus at the glenohumeral joint
  3. Internal (medial) rotation of the humerus at the glenohumeral joint
  4. Assists in forced inspiration by pulling the inferior aspect of the thorax inferiorly

Erector spinae 

Origin: A broad tendon that attaches to the posterior iliac crest, posterior sacrum, and lumbar spinous processes. It then divides into three columns:

  1. Iliocostalis – angles of the ribs and cervical transverse processes
  2. Longissimus – thoracic transverse processes and ribs, cervical transverse processes and mastoid process of the temporal bone
  3. Spinalis – spinous processes of the thoracic and cervical vertebrae 

Insertion: 

Iliocostalis – angles of the ribs and cervical transverse processes; 

Longissimus – thoracic transverse processes and ribs, cervical transverse processes and mastoid process of the temporal bone; 

Spinalis – spinous processes of the thoracic and cervical vertebrae

Actions:

  1. Bilateral contraction – extension of the vertebral column (trunk)
  2. Unilateral contraction – lateral flexion of the vertebral column (trunk)
  3. Unilateral contraction of the cervical portion – rotation of the head to the same side

Muscles of the lower body - anterior view

Rectus femoris 

Origin: Anterior inferior iliac spine (AIIS) and part of the superior acetabulum of the pelvis 

Insertion: Quadriceps tendon to the patella and, ultimately, the tibial tuberosity via the patellar ligament 

Actions:

  1. Extension of the knee joint
  2. Flexion of the hip joint (when the knee joint is flexed)
  3. Anterior pelvic tilt

Vastus lateralis 

Origin: Greater trochanter, intertrochanteric line, and linea aspera of the femur 

Insertion: Quadriceps tendon to the patella and, ultimately, the tibial tuberosity via the patellar ligament 

Actions:

  1. Extension of the knee joint
  2. Stabilizes the knee joint by preventing lateral patellar dislocation during knee flexion
  3. Assists in hip flexion

 

Vastus intermedius 

Origin: Anterior and lateral surfaces of the body of the femur, deep to the rectus femoris muscle 

Insertion: Quadriceps tendon to the patella and, ultimately, the tibial tuberosity via the patellar ligament 

Actions:

  1. Extension of the knee joint
  2. Stabilizes the patella by maintaining its position within the femoral groove during knee flexion and extension
  3. Assists in hip flexion

Vastus medialis 

Origin: Medial lip of the linea aspera, intertrochanteric line, and medial supracondylar line of the femur 

Insertion: Quadriceps tendon to the patella and, ultimately, the tibial tuberosity via the patellar ligament 

Actions:

  1. Extension of the knee joint
  2. Stabilizes the patella by medially directing its movement during knee flexion and extension
  3. Assists in hip flexion

Adductor magnus 

Origin: Inferior pubic ramus and ischial tuberosity of the pelvis 

Insertion: Gluteal tuberosity and linea aspera of the femur 

Actions:

  1. Adduction of the hip joint
  2. Extension of the hip joint (when the hip is flexed)
  3. Anterior pelvic tilt
  4. Medial rotation of the hip joint (with hip extension)
  5. Lateral rotation of the hip joint (with hip flexion)

Adductor longus 

Origin: Pubic bone, inferior pubic ramus, and pubic symphysis 

Insertion: Linea aspera of the femur 

Actions:

  1. Adduction of the hip joint
  2. Assists in medial rotation of the hip joint
  3. Assists in hip flexion

 

Adductor brevis 

Origin: Pubic bone and inferior pubic ramus 

Insertion: Pectineal line and linea aspera of the femur 

Actions:

  1. Adduction of the hip joint
  2. Assists in hip flexion and medial rotation of the hip joint

Sartorius 

Origin: Anterior superior iliac spine (ASIS)

Insertion: Medial surface of the tibia (via the pes anserinus) and the anterior surface of the medial condyle of the femur 

Actions:

  1. Flexion of the hip joint
  2. Abduction of the hip joint
  3. External (lateral) rotation of the hip joint
  4. Flexion of the knee joint (when the hip joint is flexed and the knee joint is extended)
  5. Medial rotation of the knee joint (when the hip joint is flexed and the knee joint is extended)

Tibialis anterior 

Origin: Lateral condyle and upper two-thirds of the lateral surface of the tibia, interosseous membrane 

Insertion: Medial cuneiform and base of the first metatarsal bone 

Actions:

  1. Dorsiflexion of the ankle joint
  2. Inversion of the foot at the subtalar and midtarsal joints
  3. Assists in supporting the medial longitudinal arch of the foot
  4. Assists in ankle joint stabilization during walking and running

Muscles of the lower body - posterior view

Gluteus maximus 

Origin: Posterior ilium, sacrum, and coccyx 

Insertion: Gluteal tuberosity and iliotibial band of the femur 

Actions:

  1. Extension of the hip joint
  2. Assists in external (lateral) rotation of the hip joint
  3. Assists in lateral stabilization of the hip joint
  4. Assists in maintaining an upright posture

Gluteus medius 

Origin: External surface of the ilium between the iliac crest and the posterior gluteal line 

Insertion: Greater trochanter of the femur 

Actions:

  1. Abduction of the hip joint
  2. Medial rotation of the hip joint (anterior fibers)
  3. Lateral rotation of the hip joint (posterior fibers)
  4. Assists in hip joint stabilization during walking, running, and standing on one leg

 

Gluteus minimus 

Origin: External surface of the ilium between the anterior and inferior gluteal lines 

Insertion: Greater trochanter of the femur 

Actions:

  1. Abduction of the hip joint
  2. Medial rotation of the hip joint
  3. Assists in hip joint stabilization during walking, running, and standing on one leg

Biceps femoris 

Origin: 

Long head – ischial tuberosity; 

Short head – linea aspera, lateral supracondylar ridge of the femur

Insertion: Head of the fibula and lateral condyle of the tibia 

Actions:

  1. Extension of the hip joint
  2. Flexion of the knee joint
  3. External (lateral) rotation of the knee joint (when the knee is flexed)
  4. Assists in hip joint stabilization

Semitendinosus 

Origin: Ischial tuberosity 

Insertion: Medial surface of the upper tibia (via the pes anserinus) 

Actions:

  1. Extension of the hip joint
  2. Flexion of the knee joint
  3. Internal (medial) rotation of the knee joint (when the knee is flexed)
  4. Assists in hip joint stabilization

 

Semimembranosus 

Origin: Ischial tuberosity 

Insertion: Posterior medial condyle of the tibia 

Actions:

  1. Extension of the hip joint
  2. Flexion of the knee joint
  3. Internal (medial) rotation of the knee joint (when the knee is flexed)
  4. Assists in hip joint stabilization

Gastrocnemius 

Origin: Medial and lateral condyles of the femur 

Insertion: Calcaneus via the Achilles tendon 

Actions:

  1. Plantarflexion of the ankle joint
  2. Assists in knee joint flexion (when the foot is dorsiflexed)
  3. Assists in maintaining an upright posture
  4. Assists in venous and lymphatic return from the lower extremities

Soleus 

Origin: Head and proximal shaft of the fibula, proximal two-thirds of the tibia, and the interosseous membrane 

Insertion: Calcaneus via the Achilles tendon 

Actions:

  1. Plantarflexion of the ankle joint
  2. Stabilizes the ankle joint during standing, walking, and running
  3. Assists in venous and lymphatic return from the lower extremities
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