Net knee extension torque is a function of agonist extension torque and opposing antagonistic torque. It has been proposed that antagonistic activity may be functional in stabilizing and/or protecting the joint. The present study has examined the hypothesis that, after fatiguing the antagonist hamstring muscles, a higher net extension torque would be found, which would be related to the degree of fatigue. Moreover, it was investigated whether such an increase was different between knee angles.
Seven, recreationally active males performed isometric MVCs at the following knee angles: 90, 70, 50 and 30 deg knee flexion. Thereafter, subjects performed maximal effort dynamic flexion contractions (180 deg s-1) until torque had declined ~50% this was immediately followed by a knee extension MVC. This sequence was repeated four times and randomised for all knee angles. EMG measurements of m. biceps femoris were taken in extension contractions. Antagonist average rectified value (EMGar) was normalized to the maximal flexion EMGar at the same knee angle. Voluntary activation (VA) and maximal torque-generating capacity (MTGC) of the m. quadriceps were determined using superimposed electrical stimulation.
Dynamic torque decline after the fatiguing bout of flexion contractions was 48 ± 11 %. The Wilcoxon signed rank test showed no significant effect of fatigue at the different knee angles on EMGar, MVC torque and MTGC, nor was there a significant effect of knee angle on EMGar (P > 0.05). EMGar was (mean ± S.D.) 35 ± 14 and 36 ± 14 % pre- and post-fatigue, respectively. MVC torque was on average 6 ± 4 % lower, MTGC 5 ± 9 % higher and VA 8 ± 6 % lower after the fatiguing exercise. However, these changes were not significant (P > 0.05). Only at 50 deg knee angle was VA significantly lower after fatigue (P < 0.05).
Antagonistic activity was surprisingly high in the pre-fatigue state but, despite this, a 50 % reduction of antagonistic torque did not increase net extension torque. In fact, MVC torque tended to be slightly lower. This unexpected result could have been due to an inhibition of knee extensors to preserve balance around the joint. However, the change in VA was small compared with the 50 % torque decline. MTGC of the extensor muscle had not changed, thus the dynamic flexion contractions had not fatigued the extensor muscles.
The present results may be due to the recruitment of fatigue resistant motor units during the antagonist activation during MVC which are not, or very little, affected by the dynamic fatiguing protocol (Karatzaferi et al. 2001).
All procedures accord with current local guidelines.