There is little doubt that a soccer match requires a tremendous amount of energy. To supply those energy needs, the muscle relies heavily on muscle glycogen. For years, researchers have understood that muscle glycogen is very important to exercise performance. Athletes with more muscle glycogen can exercise longer and maintain skill and focus. And muscle glycogen can be increased with a diet high in carbohydrates. Most athletes also understand this as well. What is not known is exactly how glycogen affects exercise and fatigue. A newly published study from our laboratory at Virginia Tech sheds light on this question. We found that glycogen depletion during exercise may hinder performance by affecting the central nervous system and the brain more than the muscle.
For this study, we used rats as subjects and treadmill running as the exercise mode. We separated the rats into three groups. Two of the groups ran on the treadmill for 90 min in order to deplete their muscles of glycogen. After the exercise, one group was put on a high carbohydrate (CHO) diet and the other received no CHO. A third, control group was not exercised and received a traditional rat diet.
The next day, muscle glycogen levels of the high CHO and control groups were nearly 65% greater that the low CHO group. Thus, the low CHO animals began the day in a state of glycogen depletion.
We then put the animals back on the treadmill to see how long they could exercise. High CHO and control groups ran for about 3 hours before being exhausted. On the other hand, the low CHO group ran for only 35 min. This was not surprising. We’ve known for years that glycogen depletion reduces endurance performance. But, how does it do this?
We also found that when the low CHO animals reached the point of exhaustion, they were in a state of hypoglycemia – very low blood glucose or low blood sugar. This was not seen in the groups that started exercise with normal muscle glycogen. Hypoglycemia is known to have dramatic effects on the central nervous system. It makes one feel tired, lethargic and unmotivated. Could it be that the low CHO animals were experiencing central fatigue due to low blood sugar? That is, some sort of mental exhaustion rather than actual muscle fatigue was affecting their ability to continue running?
To test this idea, we put another set of animals through the same glycogen depleting exercise bout and diet routine. We then tested the calf muscles (gastrocnemius) with electrical stimulation. To do this, we anesthetized the animals and made the muscle contract with short pulses of electricity delivered to the nerve. This approach bypasses the brain so that we could focus on how the muscle performs without any involvement of the central nervous system.
When we did this, we found that the rate of muscle fatigue was nearly identical in all three groups. In short, neither glycogen depletion nor hypoglycemia affected how the muscle fatigued.
Our results show that when the muscle is “disconnected” from the brain, glycogen depletion and hypoglycemia had no effect on muscle performance. However, when the brain is involved as is during treadmill running, these two factors greatly influence exercise performance.
During exercise, as muscle glycogen levels drop, the muscle begins to rely on blood glucose for energy. When glycogen depletion nears, the muscle has to take up more and more glucose, resulting in hypoglycemia. Unfortunately, the brain uses blood glucose as a fuel to support many of the central nervous system’s functions, including psychological function. With a lack of glucose, lethargy as well as a lack of motivation and focus are hallmark symptoms. In this state, it is very difficult to engage in intense exercise, maintain motor skills and make proper decision. Thus, glycogen depletion during exercise may be “all in your head” rather than entirely within the muscle.
What this means for the player is that a poor diet, one that lacks carbohydrates results in muscle glycogen depletion during a match? The hypoglycemia or low blood sugar that results from glycogen depletion can affect the central nervous system. Clearly a lack of energy, loss focus and motivation and declining technical skills will negatively impact the player’s game. In the end, maintaining muscle glycogen through a solid diet can lead to better performance, especially near the end of the match.
Williams, JH, Batts TW, Lees SL (2013) Reduced muscle glycogen differentially affects exercise performance and muscle fatigue. ISRN Physiology, Article ID 371235.