Effect of eccentric muscle force on shoulder internal impingement in a cadaveric model of throwing

Muscle imbalance due to muscle weakness of the eccentric muscles is considered to contribute to rotator cuff or labral injuries. This study assessed the effect of eccentric internal rotator muscle force on shoulder internal impingement in the late cocking position of the throwing motion.

Seven fresh-frozen cadaveric shoulders were tested in the late cocking position, with loading of deltoid, pectoralis major, latissimus dorsi, teres major, and all rotator cuff muscles. The area of internal impingement was calculated using 3-dimensional location data. Contact pressure in the glenohumeral joint was measured with a pressure sensor. To assess the effects of eccentric internal rotator muscles, all data were compared between initial muscle forces, calculated using data from asymptomatic throwers, and the increased or decreased eccentric force of the subscapularis, pectoralis major, latissimus dorsi, and teres major muscles.

Even at initial muscle forces, an internal impingement area (53.7 ± 29.3 mm²) was found at the late cocking position in all specimens. Increased eccentric forces of the subscapularis, latissimus dorsi, and pectoralis major muscles significantly decreased the internal impingement area (P = .01-.02) and glenohumeral contact pressure (P = .02-.04) in the late cocking position. Decreased eccentric subscapularis muscle force in the late cocking position resulted in a significant increase in glenohumeral contact pressure compared with the initial condition (P = .03).

The internal impingement area and glenohumeral contact pressure decreased significantly with increased forces of the eccentric internal rotator muscles of the shoulder joint.