Age-Related Reductions in Muscle Quality Influence the Relative Differences in Strength and Power

Abstract

Recent studies have demonstrated that age-related changes in the relative differences in isokinetic strength and power may reflect fast twitch fiber alterations. It is possible skeletal muscle ultrasound (US) imaging may reflect these changes. PURPOSE: The purpose of this study was to examine the influence of muscle quality on the relative differences in strength and power in younger and older adults. METHODS: Twenty young (20.1±1.5 yrs) and 20 older (69.5±3.1 yrs) healthy, recreationally active men performed two plantar flexion maximal voluntary isometric actions (MVCs) and three maximal isokinetic actions on a calibrated isokinetic dynamometer. Panoramic brightness-mode US images of the medial and lateral gastrocnemius were taken to determine subcutaneous fat corrected echo intensity (EI) to represent muscle quality. Isokinetic peak torque (PT), mean power (MP), and peak power (PP) at 0.52 rad·s-1 and 2.09 rad · s-1 were normalized as %MVC. Six separate 2-way mixed factorial analyses of variance were used to evaluate absolute and normalized PT, MP, and PP. Independent samples t-tests were used to compare isometric PT, EI, %decrease in PT and %increase in MP and PP between the younger and older men. Pearson’s correlation coefficients were used to examine the relationship between EI and the %decrease in PT, %increase in MP, and %increase in PP. An alpha of P≤0.05 was used to determine statistical significance. RESULTS: The younger men exhibited greater absolute isometric PT, isokinetic PT, MP, and PP at 0.52 rad·s-1 and 2.09 rad·s-1 (P=0.001-0.003). After normalizing to isometric PT, age-related differences were eliminated. The older men exhibited higher EI (P<0.001), greater %decrease in PT (43.6% vs. 38.9%; P=0.006), and lower %increase in MP (167.5% vs. 186.3%; P=0.049) and PP (125.5% vs. 144.5%; P = 0.006). Echo intensity was significantly related with the %decrease in PT (r=0.605; P<0.001), %increase in MP (r=-0.419; P=0.009), and %increase in PP (r=-0.605; P<0.001) from 0.52 rad·s-1 to 2.09 rad·s-1. CONCLUSIONS: The absolute age-related reductions in isokinetic strength and power are eliminated with normalization. However, the relative differences from slow to fast velocities may reflect the age-related decreases in fast-twitch fiber and subsequent alterations in muscle quality.