The variable roles of the upper and lower subscapularis during shoulder motion

The varied roles of the subscapularis muscle as an internal rotator of the humerus, a shoulder abductor, a humeral head depressor and an anterior stabiliser may be a result of differing innervation and lines of torque between its superior and inferior components. The aims of the study were to investigate the differences in the level of muscle activation between the upper and lower subscapularis during abduction, flexion, internal and external rotation movements, and temporal characteristics during abduction and flexion.

Intramuscular electrodes recorded electromyographic muscle activity from the upper and lower subscapularis muscles of the dominant throwing arm of twenty-four normal subjects. Participants completed ten repetitions of four shoulder movements - abduction, flexion, internal rotation and external rotation. Muscle activity was expressed as a percentage of maximum voluntary isometric contraction.

The lower subscapularis was found to activate at a higher level than the subscapularis during abduction, flexion and external rotation movements and this was significant during concentric and eccentric phases of abduction and flexion (<0.001). During internal rotation, upper subscapularis muscle activity mirrored that of lower subscapularis, with a mean difference of 1.14%. Neither upper nor lower subscapularis had onset data commencing prior to the abduction movement; however upper subscapularis activated significantly later than lower subscapularis (P=0.018).

The lower subscapularis has significantly higher muscle activity during shoulder elevation and this might reflect its greater role as a humeral head depressor and anterior stabiliser.