Injuries can occur at any point from the origin to the insertion and are graded based on the site and degree of the tear. A grade 1 injury is a contusion or strain without tearing. Grade 2 injuries are partial tears, usually of the muscle body or musculotendinous junction. Although grade 2 injuries produce less functional impairment than a complete rupture, the resulting weakness can considerably impair an active patient's ability to train and compete. Finally, grade 3 injuries are complete tears of the musculotendinous junction or an avulsion of the tendon.
Most complete tears involve an avulsion of the tendon from the humerus.
Separate processes may account for tendon ruptures and midsubstance tears. As the weight is lowered to the chest, fibers originating from the sternal head are maximally stretched. This may be particularly true with rapidly changing muscle mass when a person uses anabolic steroids or growth hormone. The patient usually has no warning of impending rupture. In most cases there are no preceding complaints relating to pectoralis problems.