(c) Figure Q2.2. Arm abduction.
Studies have shown that the force generated by individual rotator cuff muscles is proportional to the deltoid force at a given abduction angle. The ratios between the muscles are shown in Table Q2.2. Calculate the muscle forces of supraspinatus, infraspinatus and subscapularis and bone-on-bone joint contact force (Jx, Jy, and Jz) of the glenohumeral joint for an abduction angle of 90°. You can further assume that deltoid and supraspinatus act only in the x-y plane, infraspinatus and subscapularis act only in the x-z plane, and teres minor can be neglected, since it is much smaller in size than the other rotator cuff muscles.
Table Q2.2. Moment arms and loading ratios of muscles.
MuscleMoment arm (cm)Loading ratio 90° AbductionAngle with respect to the humerus (°)
Deltoid2.21.022
Supraspinatus1.50.268
Infraspinatus1.20.7335
Subscapularis3.650.2417
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