The purpose of this study was to determine the interindividual variability of the upper and lower limb muscle size in young men. Subjects were 655 Japanese men aged 18-39 years. They were sedentary and mildly to highly active individuals, including college athletes of various sports. Muscle thicknesses at each of the anterior and posterior portions of the upper arm, thigh, and lower leg were measured using B-mode ultrasonography. Interindividual variability of muscle thickness was evaluated by coefficients of variation (CVs). The CVs of muscle thicknesses were found to be in the order of upper arm posterior (17.7%), thigh anterior (14.8%), thigh posterior (12.6%), upper arm anterior (12.2%), lower leg anterior (9.8%), and lower leg posterior (9.4%). The CVs were significantly different between each pair of measurement sites except for those of upper arm anterior-thigh posterior and lower leg anterior-posterior. These differences remain significant even when the muscle thicknesses were normalized to the segment length. The observed differences in the size variability can be interpreted as muscle-related differences in hypertrophic responsiveness to resistance training. The muscle-dependent size variability may be related to the differences in the fiber-type composition and/or muscle usage in daily life among examined muscle groups.

This study investigated the effect of gender on age-related changes in muscle thickness (Mt) in the elderly, with relation to the differences due to the location of muscle groups. Mt values at the forearm, upper arm anterior and posterior, abdomen, subscapular, thigh anterior and posterior, and lower leg anterior and posterior were determined using ultrasonography in 188 men and 204 women aged 65 to 79 yrs. The men had thicker muscles than the women, with a greater relative difference in the upper limb and trunk compared to the lower limb muscles. For the men, Mt values at the upper limb, thigh and abdomen were negatively correlated to age (r=-0.330 to -0.214, p<0.05). For the women, only the Mt at the thigh anterior showed a signifi cant negative correlation with age (r=-0.346, p<0.05). At the thigh anterior, there was no signifi cant difference between both genders in the slope of the regression line for the relationship between age and percentages of Mt to the mean value for the subjects aged 65 to 69 yrs of men and women, respectively, suggesting that the rate of the age-related loss of Mt at this site was independent of gender. These results were the same even when Mt was expressed as the value relative to (body mass) 1/3 , calculated to normalize the infl uence of the difference in body mass. The fi ndings obtained here indicated that, in an age span from 65 to 79 yrs, 1) gender difference in muscle thickness was more apparent in the upper limb and trunk than in the lower limb muscles, and 2) men showed signifi cant age-related losses in muscle thickness at more body sites as compared to women, especially at the upper limb and abdomen.

This study aimed to investigate the effects of age and gender on the power generation capabilities of lower limb muscles in the elderly aged from 65 to 79 yrs. In addition to isometric knee extension torque (KT) and leg extension power (LP), mechanical power during movements specifi c to maximal walking (WP) and running (RP) without arm movements were determined using a non-motorized treadmill in 140 men (71.3±4.0 yrs, mean ±SD) and 172 women (70.2±3.7 yrs). The measured variables were expressed as relative to body mass and then their percentages to mean values for the subjects aged 65 to 69 yrs of men and women, respectively, were calculated. The percentages of all measured variables except for KT in the women were negatively correlated to age in both men (r =-0.207 to -0.375, p<0.05) and women (r =-0.228 to -0.263, p<0.05). In the slope of regression line for the relationship between age and the percentage for each of LP, WP, and RP, no signifi cant differences were found between the men and women and between the variables. In the sub-sample of the subjects, adjusted by their body height and mass, the men (n=31) showed signifi cantly higher values than the women (n=34) in all variables except for WP, with a greater relative difference in LP and RP than in KT. The fi ndings obtained here indicate that, at least in the elderly aged from 65 to 79 yrs, 1) the rate of age-related loss in the power generation capability of lower limb muscles is independent of gender and movement patterns, and 2) power development in the movement form of maximal walking is less infl uenced by gender as compared to that of maximal running as well as knee extension torque and leg extension power.