Heightened Sensory Signaling Does Not Alter Critical Torque

Abstract

Peripheral metabolic perturbations have been shown to activate group III and IV sensory afferents and lead to a decrease in central motor drive and an earlier onset of fatigue. Critical power or its isometric analog, critical torque (CT), is an important parameter of aerobic metabolic function and a strong predictor of endurance performance. PURPOSE: This study sought to examine the effects of heightened afferent signaling on voluntary CT and electrically stimulated CT. METHODS: Eleven participants performed a voluntary (VOL) 5-min all-out and a 5-min stimulated (STIM) isometric knee extension test with contralateral blood flow occlusion (CBFO) following fatiguing exercise and without occlusion (CONTROL). Stimulated CT was also assessed with occlusion applied to the ipsilateral leg (IBFO). Both the VOL and STIM protocols were performed at a 3-sec to 2-sec work to rest cycle. Stimulated contractions were performed at 50 Hz at a current eliciting 25% of MVC. RESULTS: There was a significant effect of time (p < 0.001) for VOL in both the CBFO and CONTROL conditions with torque declining to a critical level. VOL CT did not differ between the two conditions (360 ± 119 vs. 343 ± 108 Nm; p = 0.28). A significant condition x time interaction (p = 0.009) was found for STIM. Initial torque did not differ among the conditions (p = 0.876), but CT values were lower (p < 0.001) in IBFO (19.2 ± 15.8 Nm) compared to CONTROL (57.7 ± 15.6 Nm) and CBFO (62.1 ± 17.7 Nm). The CONTROL and CBFO conditions did not differ (p = 0.347). CONCLUSIONS: Heightened group III and IV afferent signaling has been shown to increase central fatigue and reduce endurance capacity. Our findings indicate the reduced endurance capacity is not due to reductions in CT—suggesting participants were able to maintain central drive to skeletal muscle. Our findings that stimulated CT fell with IBFO provides further evidence validating our stimulated CT test and it demonstrates its sensitivity to oxygen delivery.