Locomotor muscle fatigue modifies central motor drive in healthy humans and imposes a limitation to exercise performance

Abstract

We asked whether the central effects of fatiguing locomotor muscle fatigue exert an inhibitory influence on central motor drive to regulate the total degree of peripheral fatigue development. Eight cyclists performed constant-workload prefatigue trials (a) to exhaustion (83% of peak power output (W(peak)), 10 +/- 1 min; PFT(83%)), and (b) for an identical duration but at 67% W(peak) (PFT(67%)). Exercise-induced peripheral quadriceps fatigue was assessed via changes in potentiated quadriceps twitch force (DeltaQ(tw,pot)) from pre- to post-exercise in response to supra-maximal femoral nerve stimulation (DeltaQ(tw,pot)). On different days, each subject randomly performed three 5 km time trials (TTs). First, subjects repeated PFT(83%) and the TT was started 4 min later with a known level of pre-existing locomotor muscle fatigue (DeltaQ(tw,pot) -36%) (PFT(83%)-TT). Second, subjects repeated PFT(67%) and the TT was started 4 min later with a known level of pre-existing locomotor muscle fatigue (DeltaQ(tw,pot) -20%) (PFT(67%)-TT). Finally, a control TT was performed without any pre-existing level of fatigue. Central neural drive during the three TTs was estimated via quadriceps EMG. Increases in pre-existing locomotor muscle fatigue from control TT to PFT(83%)-TT resulted in significant dose-dependent changes in central motor drive (-23%), power output (-14%), and performance time (+6%) during the TTs. However, the magnitude of locomotor muscle fatigue following various TTs was not different (DeltaQ(tw,pot) of -35 to -37%, P = 0.35). We suggest that feedback from fatiguing muscle plays an important role in the determination of central motor drive and force output, so that the development of peripheral muscle fatigue is confined to a certain level.

Figure 1. Schematic illustration of the experimental design

As the first step, the prefatigue trials were carried out in the indicated order and separated by at least 48 h. For the second step various performance trials were conducted in random order and separated by at least 48 h.

Figure 2. Capillary blood lactate during the two constant-workload prefatiguing trials (PFT_83%, 347 ± 14 W; PFT_67%, 276 ± 10 W; 10.1 ± 0.5 min each) followed by the two experimental time trials (PFT_83%-TT and PFT_67%-TT)

The control time trial (Ctrl) was performed without pre-existing locomotor muscle fatigue. The two experimental time trials were started after a 4 min resting phase following the prefatiguing exercises which resulted in a reduction in potentiated twitch force (Q_tw,pot) of about 36 and 20% for PFT_83% and PFT_67%, respectively. *P < 0.05, ‡P < 0.05 versus Ctrl.

Figure 3. Peripheral quadriceps fatigue expressed as a percent change in Q_tw,pot from pre- to 4 min post-exercise

The two constant-workload trials (PFT_83%, 347 ± 14 W; and PFT_67%, 276 ± 10 W; 10.1 ± 0.5 min each) were performed to induce a pre-existing level of peripheral fatigue. The control performance trial (Ctrl) was conducted without pre-existing locomotor muscle fatigue. The two experimental performance trials (PFT_83%-TT and PFT_67%-TT) were conducted with a pre-existing level of peripheral quadriceps fatigue. The time trials started after a 4 min resting phase following either PFT_83% or PFT_67%. Note that despite significantly different levels of pre-existing locomotor muscle fatigue, resulting in substantially different exercise performances, end-exercise locomotor muscle fatigue was almost identical between the three performance trials and the PFT_83% prefatiguing trial (dashed line) supporting the hypothesis of an existing critical threshold of fatigue. n = 8; *P < 0.01.

Figure 4. Myoelectrical activity

A, integrated EMG (iEMG), and B, mean power frequency (MPF), of vastus lateralis used to illustrate the development of peripheral locomotor muscle fatigue during the two constant-workload prefatigue trials (PFT83%, 347 ± 14 W; and PFT67%, 276 ± 10 W). Values are normalized to the first minute of exercise. Mean values for iEMG and MPF during each muscle contraction (cycle revolution) were calculated and averaged over each 60 s period. *P < 0.01.

Figure 5. Effect of pre-existing locomotor muscle fatigue on neural drive and power output during a 5 km time trial

The control time trial was performed without pre-existing locomotor muscle fatigue. The two experimental time trials (PFT_83%-TT and PFT_67%-TT) were performed with different levels of pre-existing quadriceps fatigue (ΔQ_tw of about −36 and −20% for PFT_83%-TT and PFT_67%-TT, respectively). A, effects of pre-existing locomotor muscle fatigue on group mean iEMG of vastus lateralis normalized to the iEMG obtained during pre-exercise (unfatigued) maximal voluntary contractions (MVC) of the quadriceps. Each point represents the mean iEMG of the preceding 0.5 km section. Mean iEMG during the time trial was significantly reduced from Ctrl to PFT_83%-TT. B, group mean variations in power output during the 5 km time trial with three different levels of pre-existing fatigue. Group mean power output was 347 ± 14 W, 298 ± 14 W and 332 ± 18 W (P < 0.05) for Ctrl, PFT_83%-TT and PFT_67%-TT, respectively; n = 8.

Figure 6. Summary of effects of pre-existing level of locomotor muscle fatigue on group mean neural drive and mean power output during a 5 km time trial, and end-exercise locomotor muscle fatigue (percentage reduction in Q_tw,pot)

The control 5 km time trial was started without pre-existing locomotor muscle fatigue. The remaining time trials were started with significant levels of locomotor muscle fatigue (ΔQ_tw,pot−20 and −36% for PFT_67%-TT and PFT_83%-TT, respectively, P < 0.05).

Figure 7. Group mean data (n = 8) showing the relationship between Q_tw,pot as measured prior to the start of a 5 km time trial, and mean iEMG of vastus lateralis during subsequent time trials

Pre-existing levels of fatigue (i.e. reductions in pre-time trial Q_tw,pot from Ctrl) was induced via constant-workload exercise.