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.