Relationship Between Age and Grip Strength, Phase Angle, and Impedance Rate Measured by Bioelectrical Impedance Analysis in Japanese Individuals Aged 15-18 Years.

Although muscle mass increases during puberty, it plateaus in girls and slowly increases in boys after around 15 years of age. This study investigated the relationship between phase angle and impedance ratio as indicators of muscle quality and age in Japanese individuals aged 15-18 years and examined their relationship with grip strength. Participants included 838 high school, college, or university students (534 boys, 304 girls). Body composition, phase angle, and impedance ratio were measured using a multi-frequency bioelectrical impedance analyzer. Phase angle was calculated from the resistance and reactance to a 50-kHz alternating current, while impedance ratio was calculated from the impedance to 5- and 250-kHz currents. Grip strength was measured using a Smedley dynamometer. Significant effects of age were observed for body size, fat-free mass, grip strength, phase angle, and impedance ratio in the boys. Compared to 16-year-olds, 17-year-olds showed a significantly higher phase angle and significantly lower impedance ratio in whole body, upper limb, and lower limb. In contrast, no significant main effects of age were observed for any of these measures in the girls. Phase angle and impedance ratio were significantly correlated with grip strength at each age in both sexes. These results suggest that phase angle and impedance ratio can be used to assess muscle strength at any age during late adolescence. However, age-specific values may be preferable for boys in late adolescence when using the reference or percentile values for phase angle and impedance ratio. Key words Muscle quality " Muscle mass " Muscle strength " Puberty " Adolescence.