6.2 Effects of intensity and duration on the energy system and fuel interaction during physical activity
The body can switch between different energy systems depending on the activity being performed. For example, as discussed earlier, the phosphagen (ATP-PCr) and anaerobic glycolytic systems can satisfy the high amount of energy required during explosive or high-intensity exercises (sprinting, jumping, throwing, or 100-200 meters of running, weightlifting, etc.). On the other hand, the energy demand is lower at lower-intensity exercises such as walking and jogging, and the energy will be primarily supplied through the oxidative system.
However, the ATP demand rises when the intensity progressively increases from jogging to higher running speeds. Therefore the energy supplied solely through the aerobic energy pathway can’t keep up with the rate at which ATP is demanded. The oxidative system can yield a higher amount of ATP but is slower in production. In that case, the body will tap into the glycolytic pathway assistance to cover the increased energetic requirements (Chandler and Brown 2013).
During high-intensity and relatively prolonged exercise, such as a 200-400 meters race or when performing repetition-to-failure sets with 80% of 1 RM for some resistant exercise, the energetic demands will be derived mainly from the ATP-phosphagen and anaerobic glycolytic system with minimal participation of the oxidative pathway (Brooks et al. 2004).
All three energy systems are active at a given time (this is the so-called energy continuum). However, one system will predominate over the others based on the intensity and characteristics of the activity. Even though exercise intensity, duration, and type play an important role in determining which energetic system predominates at any time, intensity is the most important factor for determining which energy is mainly activated (Chandler and Brown 2013).
The table below presents basic estimates of the duration during which each energy system dominates ATP delivery. As previously mentioned, factors such as intensity, duration, and type of activity play a significant role in determining the predominant energy system for ATP production. Additionally, an individual’s fitness level and training history also influence the duration of each energy system. The ATP-PCr phosphagen system typically provides sufficient energy for 8-10 seconds of maximal contractions, while the anaerobic glycolytic pathway can sustain constant contractions for approximately 80 seconds. Beyond 80-90 seconds, the aerobic energy systems increasingly dominate ATP provision to our muscles, enabling sustained muscular contractions.
