McArdle, W. D., Katch, F. I., & Katch, V. L. (2004). Exercise physiology (5th ed.). Philadelphia, PA: Lippincott Williams & Wilkins.

In their discussion of strength training, McArdle, Katch, and Katch (2004, p. 513) concluded that training 4-5 times per week produced less strength development than a 2-3 times per week regimen. Strength training is particularly hard on muscles, causing significant damage particularly at the near-cellular level. Training too frequently may prevent adequate recovery and tissue restoration, which contributes to the retardation of progress in neuromuscular and structural adaptations and strength development.

Heavy exercise produces muscle damage in the form of "minute tears or damage to contractile components with the accompanying release of creatine kinase (CK), myoglobin (Mb), and troponin I, the muscle-specific marker of muscle fiber damage" (McArdle, Katch, & Katch, pp. 540). When coupled with extreme stretching and ranges of movement, tearing of portions of the muscle's connective tissue harness also occurs (p. 540).

Muscle damage through strength training is not caused by the absolute forces created in an exercise but rather by the overall strain produced in the training session (p. 541). Large amounts of heavy resistance training that produces significant fatigue in many muscle groups will be more damaging than will a few attempts at maximum lifts. Damage is often felt by athletes as post-exercise stiffness or soreness (Delayed-onset Muscle Soreness DOMS) particularly when beginning training or attempting a new or altered exercise. The length of time needed to repair damage through weight training depends upon the extent of strain incurred in the training session. When strain is excessive, up to three days might be needed in normal individuals although individual differences exist and longer times have been recorded.

In athletes who are experienced in strength-training, cellular damage can occur through hard resistance training despite modest to minimal post-exercise soreness. This leads to a substantial threat to a pitcher's health when weight training occurs amongst pitching practices and games. If a player is suffering micro-tears in tissues due to high-strain strength training or stretching, the explosive actions of a pitch can overload the damaged tissues causing greater or significant injury. This is one reason why modern baseball conditioning and practices appear not to have reduced injuries. Despite claims that strength training and stretching prevent injury, that claim is unsubstantiated by any research. On the other hand, excessive exercising of tissues that are predisposed to injury because of their damaged state is an hypothesis that also has not been evaluated.

Implication. Heavy strength training damages muscle tissues that have to be repaired during recovery. If another activity is performed during that recovery period, the likelihood of injury is increased.

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