When an athlete becomes fatigued, performance suffers. Everyone knows this is true, but why does it happen? Muscular fatigue is caused by a decrease in muscular excitability, or the muscle’s ability to respond rapidly to a stimulating agent telling the muscle to contract. That stimulating agent is calcium. Unfortunately, exercise depletes calcium from muscle cells, causing a decrease in muscular excitability and an increase in fatigue. In fact, a 50% decrease in calcium can result in an 80% reduction in force. The key to fighting muscular fatigue is to increase muscular excitability. Increases in muscular excitability lead to an increase in the intensity of muscle contractions.