Force-velocity relationships and fatiguability of strength and endurance-trained subjects.

The isokinetic forces, produced by a single as well as repetitive maximal knee extensions, and cross-sectional area (CSA) of the quadriceps femoris muscles were measured in 36 young adult males aged from 18 to 24 years: strength-trained (weight resistance-trained subjects, WT, N = 8), endurance-trained (middle distance runners, RUN, N = 13), and age-matched untrained subjects (UNT, N = 15). The CSA was measured using a B-mode ultrasound technique at the midpoint of the thigh length. The isokinetic forces at three velocities (1.05, 3.14 and 5.24 rad.s-1) were significantly correlated to CSA in all subjects (r = 0.01-0.849, p < 0.05). The absolute values in force development at three test velocities were significantly higher in WT than in RUN and UNT. While average values of force output per unit CSA were almost the same among the subject groups at test velocities of 3.14 and 5.24 rad.s-1, it was significantly higher in WT than in UNT at 1.05 rad.s-1. The repetitive maximal task consisted of 50 consecutive maximal knee extensions with constant velocity of 3.14 rad.s-1. The mean value of force (MF) of every 5 consecutive and all trials, respectively, was significantly correlated to CSA in all subjects (r = 0.448-0.885, p < 0.05). The average percentage of decline of MF with 50 maximal contractions was significantly lower in RUN (36 +/- 5%, mean +/- SE) than in WT (59 +/- 2%) and UNT (47 +/- 3%), and in UNT than in WT. The average values of MF per unit CSA for RUN became significantly higher than those for WT and UNT on and after the 36th-40th trial. Thus, force output during a single as well as repetitive maximal contractions was significantly correlated to muscle CSA. In terms of force per unit muscle CSA, however, WT and RUN showed specific force-velocity relation and fatiguability during repetitive maximal contractions, respectively, which could be considered as a result of muscle adaptation to their own training regimens.