Effect Of Age On High-velocity Power And Myofiber Morphology And Composition

Abstract

Muscle power, or torque multiplied by velocity, is an important determinant of physical functioning in older adults. The ability to produce power is diminished in older adults compared to younger adults, especially at higher contraction velocities. It has been suggested that this may reflect age-related changes in single myofiber morphology and/or composition. PURPOSE: To examine power, muscle activation, and single myofiber morphology and composition in young (YM) and older (OM) males. METHODS: Power was recorded during isokinetic knee extensions at 60 deg/sec and 180 deg/sec 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 deg/sec to 180 deg/sec was recorded for each participant. Electromyography amplitude of the vastus lateralis was normalized to its peak from a maximal isometric contraction to calculate muscle activation. Vastus lateralis 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 > 0.05). The average number of myofibers analyzed per participant was similar between groups (YM = 562.3 ± 280.3, OM = 576.6 ± 201.4; p > 0.05). Individual fiber type composition and CSA were similar between groups (p > 0.05), but type IIa:type I myofiber size ratio was lower in OM (-31.15%; p = 0.002). Neither myofiber size nor composition data correlated with the relative power increase (p > 0.05). CONCLUSIONS: OM had smaller type IIa myofibers relative to type I myofiber size, which may reflect age-related motor unit remodeling. Nevertheless, neither myofiber size nor composition were associated with the age-related diminishment in relative power increase. These data indicate that factors other than myofiber size or composition may be responsible for the impaired ability to increase power at a higher velocity in OM, but our smaller sample size is an important consideration.