Abstract
PurposeMuscle pain can impair exercise performance but the mechanisms for this are unknown. This study examined the effects of muscle pain on neuromuscular fatigue during an endurance task.MethodsOn separate visits, twelve participants completed an isometric time-to-task failure (TTF) exercise of the right knee extensors at ~ 20% of maximum force following an intramuscular injection of isotonic saline (CTRL) or hypertonic saline (HYP) into the vastus lateralis. Measures of neuromuscular fatigue were taken before, during and after the TTF using transcranial magnetic stimulation (TMS) and peripheral nerve stimulation.ResultsThe mean pain intensity was 57 ± 10 in HYP compared to 38 ± 18 in CTRL (P < 0.001). TTF was reduced in HYP (4.36 ± 0.88 min) compared to CTRL (5.20 ± 0.39 min) (P = 0.003). Maximum voluntary force was 12% lower at minute 1 (P = 0.003) and 11% lower at minute 2 in HYP (P = 0.013) compared to CTRL. Voluntary activation was 4% lower at minute 1 in HYP compared to CTRL (P = 0.006) but not at any other time point (all P > 0.05). The TMS silent period was 9% longer at 100 s during the TTF in HYP compared to CTRL (P = 0.026).ConclusionMuscle pain reduces exercise performance through the excacerbation of neuromuscular fatigue that is central in origin. This appears to be from inhibitory feedback from group III/IV nociceptors which acts to reduce central motor output.
Highlights
Exercise requires repeated or sustained muscular contractions and can cause a progressive decline in the force-generating capacity of a muscle, known as exercise-induced fatigue (Gandevia 2001)
For pain quality assessed by the McGill long form questionnaire, Cramping (50%), Aching (58%), Tiring (58%) and Intense (50%) were the most common words selected in CTRL whereas in hypertonic saline (HYP), Cramping (50%), Aching (42%), Grueling (42%), Intense (67%) were most selected
For EMG amplitude during the submaximal to-task failure (TTF), there was a condition × time interaction (F1, 11 = 5.018, P = 0.047, η2p = 0.313). EMGRMS was not different at minute 1. EMGRMS increased in amplitude at task failure for both conditions, EMGRMS was lower in HYP compared to CTRL (Fig. 4)
Summary
Exercise requires repeated or sustained muscular contractions and can cause a progressive decline in the force-generating capacity of a muscle, known as exercise-induced fatigue (Gandevia 2001). Strenuous exercise is usually accompanied by exerciseinduced pain. The naturally occurring and non-damaging exertional pain accompanying strenuous exercise (exerciseinduced pain) can be described as “aching” or “cramping” and increases as a function of time/exercise intensity (Cook et al 1997; Smith et al 2020). The feeling of exerciseinduced pain arises from the accumulation of noxious biochemicals, reduced muscle pH and increases in intramuscular pressure which stimulates group III/IV nociceptive afferents (O’Connor and Cook 1999; Mense 2008). Since exercise-induced pain and exercise intensity (and the development of fatigue) are associated, it may be possible that exercise-induced pain contributes to the fatigue process, this is not known
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