First of all, I think it is important that the athlete understand how their body produces energy. It can be broken down into two categories: aerobic and anaerobic respiration. Aerobic requires oxygen and is usually needed for long distance/endurance activities. Anaerobic does not require oxygen and is usually required for sports that consist of short bursts of intense activity. One chemical essential to energy production is adenosine triphosphate (ATP). As ATP breaks down energy is liberated. To create more energy the products of the breakdown of ATP, adenosine diphosphate and phosphate, recombine and split again. Another way of restoring ATP is the breakdown of creatine phosphate (cp) molecules. As they spilt, the energy they release re synthesizes an ATP molecule. And yet another method for ATP replenishment is a process known as glycolysis. A sugar molecule known as glycogen breaks down to produce energy for the resynthesis of ATP. Glycolysis produces a chemical called lactic acid. Lactic acid causes the pain that an athlete experiences while exercising. A good athlete will, through training, develop a high tolerance for lactic acid. Training will also help to increase an athlete's VO2 max, or the maximum amount of oxygen a person an breathe in to use during exercise. I would also consult an athlete on their diet. It should consist of 30% fat (energy stores, can be converted to glucose), 55% carbohydrates (another source of sugars) and 15% protein (replenishes, helps form tissues in the body).