Overtraining is a complicated subject that can involve both psychological and physiological mechanisms. Although much about it still isn't very well understood, there is little doubt that overtraining can have a host of wide ranging effects. Simply stated, overtraining is the product of performing too much strenuous physical activity. However, the exact threshold for overtraining varies from person to person. Everyone responds differently to exercise.
Some people can tolerate large volumes of training while others much less.
What's more, factors such as nutritional status, sleeping patterns, hormonal and enzymatic concentrations, muscle fiber composition, and previous training experience all can have an impact on recuperative capacity and, therefore, the point at which overtraining rears its ugly head. As the name implies, it is localized to a specific muscle or muscle group without affecting other bodily systems. It generally strikes those who are involved in serious strength training programs, especially bodybuilders, powerlifters and fitness competitors. Localized overtraining is bound to occur when the same muscle group is trained on successive days. The idea behind a split routine is to provide an optimal balance of volume and recovery.