Infraspinatus is somewhat more effective external rotator
at lower shoulder abduction angles. All heads of the infraspinatus
have their largest ER moment arm (approximately 2.2 cm) and generate
their greatest torque at 0° shoulder abduction. As shoulder
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°
abduction. (Otis JC, et al. 1994)
Exercises in 90° abducted position are typically incorporated
to simulate position and strain on shoulder during overhead activities
such as throwing.
Infraspinatus exerts an average of 5.71 and 6.5 times more
shoulder external rotation force than does Teres Minor in both
0° and 90° abducted shoulder positions, respectively
(Hughes RE & An KN 1996).
Infraspinatus appears to be involved in abduction on scapular
plane (Hughes RE & An KN, 1996; Otis JC, et al. 1994) with
very small moment arm at 0° and approximately 1 cm abductor
moment arm beyond 15° shoulder abduction (Liu J et al. 1997).