Stegeman, J. (translated by J. S. Skinner). (1981) Exercise physiology. Chicago, IL: Year Book Medical Publishers. (p. 267)

As muscle adapts to exercise stress it cycles through a diminution in performance capacity caused by a training stimulus and recovery and overcompensation caused by the body's attempt to adapt to that specific stress. This leads to a continuous cycle involving natural breakdown and gain in performance capacity that results from functional stimuli of a specific nature. If strength stimuli are applied then only strength is improved; if endurance stimuli are applied, then only endurance is improved. The body adapts to adequately cope with the specific forms of exercise stress which are applied. The adaptive process does not include any capacity that extends beyond the specific training stress. Thus, there is no basis to expect training effects from one form of exercise to transfer to any other form of exercise. Training is absolutely specific (Noakes, 1986).

However, training is not very beneficial when stimuli are mixed. When training tasks vary and do not provide important repetitions or volume of an experience, the body continually attempts to adapt to changing conditions. While adaptations of a specific nature do not occur, general fatigue (e.g., reduced glycogen levels, accrual of lactic acid) usually increases. If the volume of mixed work is sufficiently high, a general change in the physical status of the body is achieved requiring restitution during recovery. Unfortunately, there is no accompanying skill or resource utilization improvement. Mixed programs are satisfactory for general fitness improvements. They are not beneficial for specific performance improvements.

The best description of the effect of mixing training stimuli is "mixed training produces mixed results." When specific training objectives are desired, no matter what the phase of training, training stimuli should feature repetitions or extensive volumes without interruptions from other stimuli which disrupt the adaptation signals being generated. Consistent stimulus demands need to produce a level of fatigue that debilitates performance to the level that a particular quality can no longer be sustained because of technique degradation and despite increased effort. That level is often termed the "training threshold." It is harmful to fatigue athletes beyond that threshold or "cut-off" stage.

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