Exercise should be invigorating, but if your workout causes fatigue, a number of factors may be at play. Post-workout fatigue is a symptom, not the problem. Nutrition, hydration, workout intensity, workout technique, weather and workout timing are potential energy drainers, but, in some cases, undiagnosed injuries and health conditions contribute to lack of energy after exercise.
An understanding of how your body's systems function during exercise will help you understand the cause of post-workout fatigue. Your muscles use a chemical called adenosine triphosphate, or ATP, as their energy source. Manufacturing ATP requires oxygen, waste elimination and heat dissipation. Anything that interferes with the ATP production process will have an adverse effect on your post-workout energy level.
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Carbohydrates, protein and fats replenish ATP, explains sports coach Phil Davies, creator of the Sport-Fitness Advisor website. fats are stored in the adipose tissue. They provide fuel for long duration events, but their energy release is slow, making them inappropriate for short-duration, high-intensity activities. A quarter-mile sprint fueled predominately by fats will probably cause post-workout fatigue. Protein, like fat, also supplies energy for long-duration exercise. Carbohydrates are eventually converted into glucose, which provides an immediate energy source. A heavy training session that depletes carbohydrate stores may cause a lack of post-exercise energy.
There's a reason why the term "burning calories" is used to describe the function of aerobic exercise. Calories are a form of heat. When you workout, you temporarily raise your body temperature, but the inability to dissipate heat interferes with ATP production. Dehy dration while exercising in high temperatures will therefore impede heat dissipation, limiting ATP production and cause a lack of energy after your workout.
Adjusting your dietary and hydration practices may prevent post-workout fatigue, but you may also need to adjust your workout. An intense aerobic workout, performed at 95 to 90 percent of your maximum heart rate, may produce lactic acid, explains performance coach Brian MacKenzie, creator of the Brian Mac Sports Coach website. Lactic acid changes the pH balance in your muscles, which affect their ability to contract. Cooling down after an aerobic workout will dissipate lactate acid and normalize the pH balance in your muscles.
Conditions such as chronic fatigue syndrome may delay post-workout recovery, says Lorna Paul, lead author of a study published in the "European Journal of Neurology". Paul reported a diminished number of post-exercise maximum voluntary contractions in patients diagnosed with chronic fatigue syndrome. Iron deficiency anemia, common in women of menstruating age, may also cause fatigue, says specialists at the University of Maryland Medical Center