Power is diminished more dramatically at higher contraction velocities in older adults. It has been suggested that this may reflect age-related changes in single myofiber morphology or composition. PURPOSE: To examine power, muscle activation, and single myofiber morphology and composition between young (YM) and older (OM) males. METHODS: Power, or torque × velocity, was recorded during isokinetic knee extensions at 60°·s-1 and 180°·s-1 in healthy, untrained YM (n=15; 20.7±2.2 yrs) and OM (n=15; 71.6±3.9 yrs). The relative increase in power from 60°·s-1 to 180°·s-1 was recorded for each participant. Electromyography amplitude of the vastus lateralis (VL) was normalized to its peak from a maximal isometric contraction to calculate muscle activation. VL tissue samples were obtained from a sub-sample (YM=13; OM=11) via microbiopsy and immunofluorescence was used to identify type I and IIa myofibers for subsequent analysis of cross-sectional area (CSA). Independent samples t-tests were used to compare groups and select correlations were assessed. RESULTS: Relative increase in power was greater in YM (159% vs. 115%; p=0.005). Muscle activation was similar between groups (p\u3e0.05). Individual fiber type compositions and CSA were similar between groups (p\u3e0.05), but type IIa:type I myofiber size ratio was lower in OM (-31.15%; p=0.002). Myofiber size nor composition data correlated with the relative power increase (p\u3e0.05). CONCLUSION: OM had smaller type IIa myofibers relative to type I myofiber size, which may reflect age-related motor unit remodeling. Nevertheless, and albeit a smaller sample size, myofiber size nor composition were associated with the age-related diminishment in relative power increase
