We evaluated 12 patients with 14 ruptures of the pectoralis major muscle to compare surgical and conservative management of this injury. Because 9 of the injuries occurred while weight lifting, we performed an anatomic study on human hemithorax specimens during a simulated bench press to determine the mechanism of this rare occurrence. Excursion of individual pectoralis muscle fibers was measured at seven points along the broad muscle origin by the use of fine wires connected to the humeral insertion and to dial gauges on the study apparatus. Excursions in the concentric and eccentric phases of the lift were expressed as a percentage of resting fiber length. The short, inferior fibers of the muscle lengthened disproportionately during the final 30 degrees of humeral extension. We concluded that the inferior fibers have a mechanical disadvantage in the final portion of the eccentric phase of the lift, and application of high loads to these maximally stretched fibers produces rupture. We repaired five acute and two chronic ruptures, and measured peak torque and work production against the contralateral side using Cybex isokinetic testing. Surgically treated patients showed comparable torque and work measurements, while conservatively treated individuals demonstrated and marked deficit in both peak torque and work/repetition. We recommend repair of complete pectoralis muscle ruptures in active patients who require maximum strength in vocational or avocational activities.