The strength of active external rotation and of abduction of the
shoulder when the humerus was in the plane of the scapula (30 degrees of
horizontal flexion anterior to the coronal plane) was measured
isokinetically and isometrically in thirty-nine normal volunteers, who were
stratified by age and sex. The angles at which peak torque was produced
were similar when tested isokinetically and isometrically; these angles
were similar for external rotation (at 60 and 30 degrees of internal
rotation) and for abduction (at 30 and 60 degrees of abduction). Isometric
peak torque was greater than slow-speed (90 degrees per second) isokinetic
peak torque, which in turn was greater than fast-speed (210 degrees per
second) isokinetic peak torque. There were highly significant differences
in strength, measured isokinetically and isometrically, between younger and
older men and between older men and older women. The variability of normal
values for torque was similar in each group. Repeat testing demonstrated a
high reliability of isokinetic measurements and of isometric measurements
at angles within the range of the production of peak torque. Complete
testing was performed in four normal volunteers before and after a block of
the suprascapular nerve. The supraspinatus and infraspinatus components of
the rotator cuff contributed a variable proportion to the total strength of
abduction (25 to 50 per cent) and external rotation (50 to 75 per cent)
throughout the range of motion. This study demonstrated that both
isokinetic and isometric testing in the scapular plane are valid methods
for measurement of the strength of external rotation and abduction of the
shoulder. The data support standardization of the positions for testing the
strength of motions of the shoulder: isometric strength of external
rotation should be measured in the scapular plane with the shoulder in 45
degrees of abduction and 45 degrees of internal rotation; isometric
strength of abduction, in the scapular plane with the shoulder in 45
degrees of abduction; and isokinetic strength of external rotation and
abduction, in the scapular plane at 90 degrees per second.