1. The pectoralis major muscle is not necessary for normal shoulder function but is required in the performance of athletics or other strenuous activity. 2. Normal tendons do not rupture except at the musculotendinous junction or the tendon insertion. 3. A muscle can hold about four times as much as it can lift. Ruptures of muscles in healthy athletes are due to the application of excessive weight when the muscle is holding at its maximum power. 4. A roentgenogram of the chest will reveal the absence of the shadow of the pectoralis major muscle if rupture has occurred, and this can be an aid in diagnosis.