Implications of group III and IV muscle afferents for high-intensity endurance exercise performance in humans

We investigated the influence of group III/IV muscle afferents on peripheral fatigue, central motor drive (CMD) and endurance capacity during high-intensity leg-cycling. In a double-blind, placebo-controlled design, seven males performed constant-load cycling exercise (318 +/- 9W; 80% of peak power output (W-peak)) to exhaustion under placebo conditions and with lumbar intrathecal fentanyl impairing spinal mu-opioid receptor-sensitive group III/IV muscle afferents. Peripheral fatigue was assessed via changes in pre- vs. post-exercise quadriceps force in response to supramaximal magnetic femoral nerve stimulation (Delta Q(tw,pot)). CMD was estimated via quadriceps electromyogram. To rule out a direct central effect of fentanyl, we documented unchanged resting cardioventilatory responses. Compared to placebo, significant hypoventilation during the fentanyl trial was indicated by the 9% lower (V) over dot(E)/(V) over dot(CO2), causing a 5 mmHg increase in end-tidal P-CO2 and a 3% lower haemoglobin saturation. Arterial pressure and heart rate averaged 8 and 10% lower, respectively, during the fentanyl trial and these differences progressively diminished towards end-exercise. Although initially similar, the percent change in CMD was 9 +/- 3% higher at end-exercise with fentanyl vs. placebo (P < 0.05). Time to exhaustion was shorter (6.8 +/- 0.3 min vs. 8.7 +/- 0.3 min) and end-exercise Delta Q(tw,pot) was about one-third greater (-44 +/- 2% vs. -34 +/- 2%) following fentanyl vs. placebo. The rate of peripheral fatigue development was 67 +/- 10% greater during the fentanyl trial (P < 0.01). Our findings suggest that feedback from group III/IV muscle afferents limits CMD but also minimizes locomotor muscle fatigue development by stimulating adequate ventilatory and circulatory responses to exercise. In the face of blocked group III/IV muscle afferents, CMD is less inhibited but O-2 transport compromised and locomotor muscle fatigability is exacerbated with a combined net effect of a reduced endurance performance.