PROLONGED-EXERCISE FATIGUE HAS BOTH CENTRAL AND PERIPHERAL COMPONENTS
Ross, E. A., Gregson, W., Williams, K., & George, K. (2009). Neuromuscular fatigue following repetitive endurance cycling. ACSM 56th Annual Meeting, Seattle, Washington. Presentation number 1987.
This study determined the effects of 20 days of repetitive endurance cycling within a 22-day period, on peripheral and central contributions to fatigue. Well-trained male cyclists (N = 8) completed 20 prolonged cycling stages interspersed by two rest days (days 9 and 16), which replicated the 2007 Tour de France route and schedule. Isometric knee extensor torque and electromyographic responses of the Vastus Lateralis in response to percutaneous electrical stimulation and transcranial magnetic stimulation were measured before, on day 9, day 16, and the second day following completion of the simulated "Tour". Post exercise measurements on days 9 and 16 were taken >18 hours after cessation of the previous exercise bout.
Maximal voluntary contraction of the knee extensors decreased by 20 ± 10% during the "Tour", but recovered after two days of rest. Peripherally evoked M-wave and potentiated twitch responses were also significantly decreased on days 9 and 16, up to 31 ± 21% and 22 ±18%, respectively, but returned to baseline values following two days of recovery. Voluntary activation was reduced to 75 ± 8% during the "Tour", and remained significantly depressed (79 ± 7%) following completion of the "Tour". Amplitude of motor evoked potentials, elicited by transcranial magnetic stimulation were also decreased by 44 ± 28% on day 9 and remained significantly depressed during the remainder of, and following, the "Tour".
Implication. A reduction in knee extensor strength occurs following repetitive prolonged cycling and is a result of both central and peripheral processes. A reduced sarcolemmal excitability and impairment of contractile mechanisms exists following repetitive prolonged exercise bouts, even after a substantial recovery period. Peripheral changes are accompanied by an enduring reduction in corticomotor output, which persists even following two days of rest.
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