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

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 specialreference to aging. The cross-sectional area of the combined infraspinatus and teres minor muscles weremeasured, in the sagittal oblique magnetic resonance images, in eighty-one patients with a mean age of44.06 years (range 19 - 74). These areas were correlated with the measured isokinetic strength in externalrotation at angular velocities of 60 deg/s and 180 deg/s using Cybex 770 NORM. The results show that therewas a gradual decrease in size of the muscles as the age of the individual increases. A strong correlation wasfound between aging and combined cross-sectional area and peak torque as well. The correlation betweencombined cross-sectional area and peak torques at both angular velocities were less strong. Further, thecorrelation between the peak torque/cross-sectional area ratio with aging was also less significant, whichmay imply that the decrease in the muscle strength was greater than the change in muscle area. Our resultssuggest that there may be other qualitative and biochemical factors that may determine the true strengthof the muscles in the aged population.Key Words: Aging, muscle, cross sectional area, strength, shoulder.