Isokinetic External Rotation Strength of Shoulder: Correlation with Age and Muscle Size

Abstract

The purpose of this study was to determine the correlation between quantitative (cross sectional areas)
evaluation of the posterior cuff muscles and mechanical strength in asymptomatic shoulders with special
reference to aging. The cross-sectional area of the combined infraspinatus and teres minor muscles were
measured, in the sagittal oblique magnetic resonance images, in eighty-one patients with a mean age of
44.06 years (range 19 - 74). These areas were correlated with the measured isokinetic strength in external
rotation at angular velocities of 60 deg/s and 180 deg/s using Cybex 770 NORM. The results show that there
was a gradual decrease in size of the muscles as the age of the individual increases. A strong correlation was
found between aging and combined cross-sectional area and peak torque as well. The correlation between
combined cross-sectional area and peak torques at both angular velocities were less strong. Further, the
correlation between the peak torque/cross-sectional area ratio with aging was also less significant, which
may imply that the decrease in the muscle strength was greater than the change in muscle area. Our results
suggest that there may be other qualitative and biochemical factors that may determine the true strength
of the muscles in the aged population.

Key Words: Aging, muscle, cross sectional area, strength, shoulder.