For the professional or amateur athlete, there's no thrill like reaching the finish line. And there's nothing more frustrating than not reaching the finish because of the debilitating effects of muscle fatigue. It's long been understood that dehydration and carbohydrate depletion are the main causes of exercise-induced fatigue. Athletes know they can prolong their activity by loading up on carbohydrates and drinking plenty of fluids. However, recent studies also reveal there are additional key factors that contribute to fatigue during prolonged exercise.

Dehydration

During exercise, the body loses water through sweating and evaporation. Sweat is the way your body keeps from overheating as sweat glands release perspiration that evaporates, cooling the skin and the blood underneath. The cooled blood then flows back to cool the body's core. Even mild dehydration can impair athletic performance. To restore the body's fluids that are lost during exercise, athletes should consume beverages that contain agents such as glucose and sodium, two ingredients found in most sports or energy drinks. These agents help maintain blood volume and aid in the absorption of water into the body.

Overheating

During exercise, an athlete's body temperature, typically about 98.6 degrees, can increase to temps up to 104 degrees or more, especially during intense exercise. While a certain percentage of blood is used to regulate body temperature, large quantities of blood are still required to meet the energy and metabolic needs of working muscles. These demands can overtax the circulatory system, resulting in inadequate removal of body heat and a rise in an athlete's body temperature. Research has proven that athletes involved in endurance sports can experience risks of overheating. Studies indicated that athletes who drank fluids during a two-hour run lowered their body temps by two degrees compared to those who did not rehydrate.

Depletion of Muscle Fuels

During intense short-term exercise, fatigue can result from depletion of glycogen. Glucose is the predominant fuel source for muscles in the first 10 seconds to three minutes of intense exercise. During long-term exercise, the aerobic pathway kicks in for energy production. In addition to glucose, fatty acids and amino acids are burned as fuel for aerobic metabolism, providing a wider range of energy resources.

However, glycogen depletion contributes to muscle fatigue even during long-term exercise. During studies, when athletes exercised to near exhaustion at 80 percent of their maximum capacity, the glycogen content of their muscles dropped to near zero in about 90 minutes. Through carbohydrate loading, endurance was increased and glycogen storage capacity was enhanced. These results suggest glycogen is a crucial fuel for energy production.

To preserve glycogen, some athletes adopt the method of training the muscles to become more efficient in using fat as a fuel source by completing several extended training sessions, each lasting more than two hours. This method stimulates the enzymes responsible for the conversion of fat into energy, which enables athletes to burn a higher percentage of fat and conserve glycogen for more strenuous efforts.

Low Blood Glucose

In addition to providing energy for muscles, glucose is also a source of energy for the brain and nervous system. In fact, 50 to 60 percent of the glucose supplied by the liver is used strictly for brain and nervous system function.

During longer exercise sessions, glycogen stores run low. This reliance on muscle glycogen is balanced by an increased reliance on blood glucose for fuel. After two to three hours of exercise, the majority of carbohydrate energy appears to be derived from glucose, which is transported from circulating blood into exercising muscles. This causes blood glucose to decline to relatively low levels. Fatigue occurs because there is not enough blood glucose available to compensate for the depleted muscle glycogen.

The use of sports drinks, carbohydrate gels and sports bars help athletes keep good blood glucose levels elevated to maintain central nervous system function and provides carbohydrates to working muscles. Studies show that athletes are capable of absorbing up to 80 grams of carbohydrates per hour during exercise, delaying fatigue by as much as 30 to 60 minutes.

Central Fatigue

Recent research has also examined mental fatigue during exercise. Although it does not affect the muscles directly, central fatigue can reduce an athlete's capacity to perform. Doctors have uncovered a correlation between levels of the amino acid tryptophan in the brain and the degree of mental fatigue. Once tryptophan enters the brain, it can depress the central nervous system, causing fatigue. Supplementation of branched chain amino acids (BCAAs) helps to regulate the entry of tryptophan into the brain and has proven to increase performance for athletes. Additional research is being conducted in this area.

By making sure you're properly hydrated before and during exercise, and by consuming enough of the nutrients your body needs to fuel activity, you can increase your chances of beating muscle fatigue and reaching the finish line a winner.