High-intensity exercise is fatiguing. And there are reasons your body may succumb to the exertion of high-intensity physical activity. Fatigue can be caused by your body's inability to meet metabolic needs, by an unmet demand for oxygen or from having too little of certain minerals. With regular exercise training, your body adapts to the stress and can perform a longer bout of exercise without fatiguing.
Glycogen Depletion
The first and foremost source of energy that your body calls on during exercise, specifically high-intensity exercise, is stored muscle glycogen. It is readily available, making it the most efficient source of energy. Yet glycogen is the limiting factor in intensity and duration of exercise. When muscle glycogen stores are depleted, you cannot produce as much power, and you will suffer from fatigue.
Lactic Acid
During high-intensity exercise, your body cannot always keep up with the demand for oxygen. When this occurs, your cells turn to anaerobic energy production. That means producing energy for muscular contractions without the use of oxygen. A by-product of anaerobic metabolism is lactic acid. When your body is unable to remove more lactic acid from your bloodstream than your cells are producing, it begins to accumulate. Lactic acid in your bloodstream increases the pH of your blood, making exercise painful and fatiguing.
Calcium
Calcium plays an important role during muscular contractions. When calcium inside of your muscle cell is low, the force of contractions is limited. This is due to calcium's role in the sliding filament theory of muscular contractions. When your muscles contract, a small filament known as myosin grabs onto actin, another small filament, and they pull each other, causing your muscle to shorten. Myosin can only attach to actin under the existence of calcium. Without enough calcium, your muscular contractions become compromised, resulting in fatigue.
Adaptations to Exercise
Regular high-intensity exercise manipulates your body into adapting to the stress of physical activity. To perform high-intensity exercise for a longer period, your body adapts to increase its muscle glycogen stores and increase its ability to clear lactic acid from your bloodstream.
References
- "Exercise Testing and Prescription"; David C. Nieman; 2007
- "Exercise Physiology"; George A. Brooks, Thomas D. Fahey, Kenneth M. Baldwin; 2005
