Central Nervous System Fatigue During Upper-body Maximal Intensity, Intermittent Sprints.

Abstract

PURPOSE: To investigate central nervous system fatigue by assessing 1) neuromuscular fatigue of the elbow flexors and 2) corticospinal excitability, during upper-body maximal intensity intermittent-sprints (IS). METHODS: 19 participants completed (Part A) 10 trials of 10s sprints on an arm cycle ergometer interspersed with 180s of recovery. Mean power was recorded for each IS. Elbow flexors maximum voluntary contraction (MVC) force and percent voluntary activation were recorded prior to the first IS and immediately after IS 5 and 10. 7 participants completed (Part B) the same sprint protocol, and we used transcranial magnetic stimulation, transmastoid electrical stimulation and brachial plexus electrical stimulation to produce motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs) and maximal M-waves (Mmax) in the biceps brachii during a 5% MVC elbow flexion contraction. All MEPs and CMEPs were normalized to Mmax. RESULTS: Part A: There was clear evidence of fatigue induced by the sprint protocol. Mean power decreased by 11.5% (p = 0.002), and 8.4% (p < 0.001) from IS 1 to 10 and IS 5 to 10, respectively. MVC force decreased by 7.2% (p = 0.003), 12.9% (p = 0.001), and 6.0% (p = 0.01) from pre-IS to post-IS 5, pre-IS to post-IS 10, and post-IS 5 to post-IS 10, respectively. Percent voluntary activation decreased by 1.85% (p = 0.027) and 3.92% (p = 0.042) at post-IS 5 and 10, respectively, compared to pre-IS. Part B: External manifestations of fatigue were matched by decreased supraspinal excitability compared to pre-IS. MEP amplitudes decreased by 28.7% (p = 0.002) and 42.2% (p = 0.002) at post-IS 1 and 5. Despite that, CMEP amplitudes did not differ (p > 0.142) at any time point from pre-IS. MEP/CMEP ratios decreased by 29.8% (p < 0.001) and 31.6% (p = 0.015) at post-IS 5 and 10, respectively compared to pre-IS. Mmax amplitudes did not differ (p > 0.264) at any time point. CONCLUSION: The IS protocol produced a decrease in power, force and percent voluntary activation, illustrating neuromuscular fatigue of the elbow flexors. The differential effect on MEPs and CMEPs suggests that the fatigue may largely be attributed to decreases in supraspinal drive that limits motor output during IS.