Muscle mass and peripheral fatigue: a potential role for afferent feedback?

The voluntary termination of exercise has been hypothesized to occur at a sensory tolerance limit, which is affected by feedback from group III and IV muscle afferents, and is associated with a specific level of peripheral quadriceps fatigue during whole body cycling. Therefore, the purpose of this study was to reduce the amount of muscle mass engaged during dynamic leg exercise to constrain the source of muscle afferent feedback to the central nervous system (CNS) and examine the effect on peripheral quadriceps fatigue.

Eight young males performed exhaustive large (cycling - BIKE) and small (knee extensor - KE) muscle mass dynamic exercise at 85% of the modality-specific maximal workload. Pre- vs. post-exercise maximal voluntary contractions (MVC) and supramaximal magnetic femoral nerve stimulation (Q(tw,pot)) were used to quantify peripheral quadriceps fatigue.

Significant quadriceps fatigue was evident following both exercise trials; however, the exercise-induced changes in MVC (-28 ± 1% vs. -16 ± 2%) and Q(tw,pot) (-53 ± 2% vs. -34 ± 2%) were far greater following KE compared to BIKE exercise, respectively. The greater degree of quadriceps fatigue following KE exercise was in proportion to the greater exercise time (9.1 ± 0.4 vs. 6.3 ± 0.5 min, P < 0.05), suggestive of a similar rate of peripheral fatigue development.

These data suggest that when the source of skeletal muscle afferent feedback is confined to a small muscle mass, the CNS tolerates a greater magnitude of peripheral fatigue and likely a greater intramuscular metabolic disturbance. An important implication of this finding is that the adoption of small muscle mass exercise may facilitate greater exercise-induced muscular adaptation.