Quantifying the effect of age and contraction mode on the force-velocity-power relationship in the knee extensors

PURPOSE: To assess and compare the force-velocity-power relationship (F-V-P) using isotonic and isokinetic knee extensions in 11 younger (19-30y) and 11 older (69-81y) men. METHODS: The construction of the F-V-P curves were attained with each participant performing maximal, explosive knee extensions at 8 different loads (isotonic) and 8 different velocities (isokinetic) on a Cybex HUMAC NORM dynamometer. Maximal voluntary contraction (MVC or Po), maximal shortening velocity (Vmax), maximum power, optimal torque (Popt) and velocity (Vopt) were assessed and compared. RESULTS: The older men were 43% weaker, 18% slower and ~54% less powerful than the younger men for both isotonic and isokinetic contractions. For the older men, Popt was 35% and 38% lower in the isotonic and isokinetic F-V-P relationships, respectively. The Vopt of the older men were 36% and 25% slower, respectively in the isotonic and isokinetic power curves. Interestingly, the Vopt achieved during isokinetic testing was 16% and 36% higher for both young (400 vs 346°/s) and old (300 vs 221°/s), respectively, compared to the isotonic Vopt. CONCLUSION: For the isotonic modality, the weaker and slower knee extensors of the older men contributed equally to lower power whereas for the isokinetic modality, age-related muscle weakness contributed more than velocity to the reduction in power. The muscle behaves differently between modalities, which may explain parts of the results. The isotonic modality better modelled the F-V relationship, with fewer inherent limitations compared to the isokinetic modality