The interrelationship among muscle mass, strength, and the ability to perform physical tasks of daily living in younger and older women

The purpose of this study was to objectively compare the difficulty and determine the contribution of strength and muscle mass to the performance of physical tasks of daily living in a group of younger and older women. A cross-sectional design was used. Volunteer participants were from the community of Birmingham, AL; there were 21 older (aged 60-75 years) and 20 younger (23-34 years) healthy women in the study. Subjects were matched for height and weight. Their testing included total and regional body composition evaluation by use of dual-energy x-ray absorptiometry, isometric strength tests of elbow flexors and knee extensors, and integrated electromyography (IEMG) evaluation while the subjects were standing from and sitting into a chair, and while they were carrying a small load (weight relative to strength). A two-way analysis of variance and a two-way analysis of covariance with repeated measures, Pearson product correlation, and first-order partial correlations were used to analyze the data. A significant inverse correlation was observed between age and isometric strength of both the knee extensors and elbow flexors. Adjusting for upper leg lean tissue did not change the significant inverse correlation between age and knee extensor strength. However, after an adjustment for arm lean tissue, there was no significant correlation between elbow flexor strength and age. Older women experienced significantly greater difficulty in standing than younger women as measured by quadriceps normalized IEMG (i.e., IEMG during task/IEMG during maximum isometric strength test). This difference persisted even after the covariate upper leg lean tissue was added to the model. No significant difference was observed between younger and older women for difficulty (biceps normalized IEMG) during the carry task after the covariate arm lean tissue was added to the model. The older women in this study had less strength in the knee extensors and experienced greater difficulty standing from a chair than the younger women, even after the covariate, upper leg lean tissue was added to the model. This suggests that other factors, in addition to loss of lean tissue, contribute to the age-related decline of muscular strength and the ability to perform tasks with the legs. In contrast, although elbow flexor strength declined, this appeared to be largely due to decreased arm lean tissue mass.