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), enduraince-trained (middle distance runners, RUN, N = 13), and
age-matched untrained subjects (UNT, N = 15). The CSA was measuired 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 = 0.801 - 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 %, X ± 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.
Key words
Isokinetic contraction - muscle CSA - B-mode ultrasound - specificity
