Empirical and objective data suggest that muscle and connective tissue can undergo adaptations to physical training resulting in greater tissue mass and increased maximum tensile strength. These adaptations are especially apparent as a result of load bearing and resistive training. Furthermore, information is presented suggesting that pre-conditioning and in-season muscle conditioning, especially strength training, reduce injuries among athletes. Additionally, a theoretical model of training, "periodization", is offered as a method of increasing performance to maximum or optimal values while reducing overtraining and injury potential. Periodization of training can reduce overtraining potential and injury potential while optimizing performance by variation of volume, intensity, and exercise selection during a training program.