All heads of subscapularis have their largest internal rotation
moment arm (approximately 2.5 cm) and torque generation at 0°
abduction. As abduction angle increases, moment arms of inferior
and middle heads stay relatively constant, while moment arm of
superior head progressively decreases until it is about 1.3 cm
at 60° shoulder abduction (Otis JC, et al. 1994). However
no significant difference in upper subscapularis activity was
observed during shoulder internal rotation exercises at 0°,
45°, or 90° adduction (Kadaba MP, et al. 1992; Decker
MJ, et al. 2003).
The literature is mixed and inconclusive as to what postion
to perform internal rotation exercises (0° versus 90°
adduction). However it is interesting to note greater assistance
of other muscles (Pectarlis Major, Latissimus Dorsi, and Teres
Major) when peforming shoulder internal rotation exercises at
0° as opposed to 90° shoulder abduction (Decker MJ, et
al. 2003).
Subscapularis also generates abduction torque during arm elevation,
particularly superior head when shoulder is externally rotated
(Otis JC, et al. 1994). Subscapularis has peak abductor moment
arm of 1 cm at 0° abduction, with a gradual decrease to 0
cm at 60° abduction in neutral rotation (Liu J et al. 1997).
