Age-related changes in the relative differences in isokinetic strength and power may reflect fast twitch fiber function. We aimed to examine the influence of muscle quality on the relative differences in strength and power in younger and older men. Twenty younger (20.1 ± 1.5 yrs) and 20 older (69.5 ± 3.1 yrs) healthy, recreationally active men performed two plantarflexion maximal voluntary isometric contractions (MVCs) and three maximal concentric isokinetic contractions at a slow (0.52 rad·s− 1) and fast (2.09 rad·s− 1) velocity. Absolute and normalized (%MVC) isokinetic peak torque (PT), mean power (MP), peak power (PP), the relative differences in PT (%decrease), MP and PP (%increase) from 0.52 to 2.09 rad·s− 1, and electromyographic (EMG) amplitude were examined. Ultrasonography was used to determine subcutaneous fat corrected echo intensity (EI) to represent muscle quality. The younger men exhibited greater absolute isometric PT, isokinetic PT, MP, and PP at 0.52 and 2.09 rad·s− 1 (P = 0.001–0.003); but these differences were no longer present following normalization (P = 0.079–0.954). The older men exhibited similar EMG amplitude values but higher EI values (P < 0.001), a greater %decrease in PT (43.6% vs. 38.9%; P = 0.006), and a 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 related to 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 to 2.09 rad·s− 1. The absolute age-related reductions in isokinetic strength and power were no longer present following normalization to isometric strength. However, age-related differences in strength and power remained intact when examining the relative differences from slow to fast velocities, which appear to be influenced by the qualitative changes in skeletal muscle.