Age-related differences in isometric and dynamic strength and endurance

To measure age-related skeletal muscle changes, 30 healthy, moderately active women performed isometric and dynamic strength and endurance tests of their left quadriceps femoris muscle groups. Fifteen of the subjects were ages 20 to 29 years and 15 were ages 50 to 80 years. A cybex II isokinetic dynamometer was used to measure the torque output isometrically with the knee at 90 degrees of flexion on three trials. In addition, each subject performed three trials from 90 degrees of knee flexion to full knee extension and back to 90 degrees of knee flexion with the velocity of the isokinetic dynamometer set at 10 rpms. Endurance time was calculated to be the length of time a torque output of at least 50 percent of the maximal strength could be maintained. The t tests done revealed a significant difference between the two groups on isometric and dynamic strength, while no significant difference was found on isometric or dynamic endurance. Statistically correcting for the extraneous variables, height and weight, through a partial correlation revealed a significant negative correlation between age and strength. The younger subjects had higher torque outputs, both isometrically and dynamically, than the older subjects. The same partial correlation between isometric endurance and age and dynamic endurance and age demonstrated no significant correlation. The fact that endurance did not change in this older population while strength did may be the result of fast-twitch and slow-twitch muscle fibers changing with age.