Abstract
Spinal cord excitability, as assessed by modulation of Hoffmann (H-) reflexes, is reduced with fatiguing isometric contractions. Furthermore, spinal cord excitability is reduced during non-fatiguing arm and leg cycling. Presynaptic inhibition of Ia terminals is believed to contribute to this suppression of spinal cord excitability. Electrical stimulation to cutaneous nerves reduces Ia presynaptic inhibition, which facilitates spinal cord excitability, and this facilitation is present during arm cycling. Although it has been suggested that reducing presynaptic inhibition may prolong fatiguing contractions, it is unknown whether sensory stimulation can alter the effects of fatiguing exercise on performance or spinal cord excitability. Thus, the aim of this experiment was to determine if sensory stimulation can interfere with fatigue-related suppression of spinal cord excitability, and alter fatigue rates during cycling sprints. Thirteen participants randomly performed three experimental sessions that included: unloaded cycling with sensory stimulation (CONTROL + STIM), sprints with sensory stimulation (SPRINT + STIM) and sprints without stimulation (SPRINT). Seven participants also performed a fourth session (CONTROL), which consisted of unloaded cycling. During SPRINT and SPRINT + STIM, participants performed seven, 10 s cycling sprints interleaved with 3 min rest. For CONTROL and CONTROL + STIM, participants performed unloaded cycling for ~30 min. During SPRINT + STIM and CONTROL + STIM, participants received patterned sensory stimulation to nerves of the right foot. H-reflexes and M-waves of the right soleus were evoked by stimulation of the tibial nerve at multiple time points throughout exercise. Sensory stimulation facilitated soleus H-reflexes during unloaded cycling, whereas sprints suppressed soleus H-reflexes. While receiving sensory stimulation, there was less suppression of soleus H-reflexes and slowed reduction in average power output, compared to sprints without stimulation. These results demonstrate that sensory stimulation can substantially mitigate the fatiguing effects of sprints.
Highlights
It is well established that intermittent, maximal effort bouts of exercise result in decrements in performance
The main results from this experiment show that sensory stimulation can alter the effects of fatigue; spinal cord excitability is reduced by fatigue and enhanced by sensory stimulation
With increasing bouts of intermittent sprints, power output decreased significantly. This is a consistent finding with previous work that has examined the fatigue of intermittent sprints during arm (Pearcey et al, 2016) and leg cycling (Billaut et al, 2006; Billaut and Basset, 2007; Racinais et al, 2007a; MendezVillanueva et al, 2008; Girard et al, 2013a,b; Pearcey et al, 2015b)
Summary
It is well established that intermittent, maximal effort bouts of exercise result in decrements in performance. In many cases, these performance decrements are due to neuromuscular fatigue that is a combination of peripheral and central fatigue mechanisms (Billaut et al, 2006; Billaut and Basset, 2007; Racinais et al, 2007a; Mendez-Villanueva et al, 2008; Girard et al, 2013a,b; Pearcey et al, 2015b, 2016; Monks et al, 2017). It seems likely that H-reflex amplitudes would be reduced, it is currently unknown whether cycling sprints affect spinal reflex excitability in humans
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