Coaches and athletes need to know what fatigue is to understand all issues of the training process.

The whole training process is predicated on fatigue and on recovery from it–the changes of training load, means of recovery, frequency and sequence of exercises and workouts, periodization, and nutrition. Without understanding fatigue it is not possible to understand all these issues as well as the upsides and downsides of different supplements.

Fatigue is a decreased effort capacity of a body, or part of it, resulting from exertion or excessive stimulation. There are several types of fatigue: mental (boredom), sensory (a result of intense activity of one or more of the senses), emotional (a consequence of intense emotions, observed after performance at important sports competitions, or after executing movements that demand overcoming fear), and physical (caused by muscle work). While workouts cause all types of fatigue (in various degrees) the most obvious is physical fatigue. Physical fatigue directly affects the function and structure of the musculoskeletal system and muscle fatigue is its main component.

The above is just a brief introduction to the subject. The whole article on muscle fatigue is at http://www.stadion.com/muscle-fatigue/