Muscle pain induced by hypertonic saline in the knee extensors decreases single-limb isometric time to task failure

Samuel A Smith,Dominic Micklewright,Alexis R Mauger,Samantha L Winter

doi:10.1007/s00421-020-04425-2

Samuel A Smith, Dominic Micklewright + Show 2 more

Open Access

https://doi.org/10.1007/s00421-020-04425-2

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Abstract

PurposeIncreased nociceptive activity and the experience of exercise-induced pain (EIP) may contribute to fatigue during endurance exercise. To investigate this, a pain model that produces pain similar to EIP and decouples its relationship to exercise intensity is required. This study (1) compared the quality of pain caused by a hypertonic saline injection into the vastus lateralis in resting and exercise conditions, and (2) investigated whether this pain contributes to changes in time to task failure.MethodsOn separate days, 18 participants completed a time to task failure at 20% maximal voluntary torque (MVT), a resting hypertonic saline intramuscular injection, and in a further three visits a time to task failure at 10% MVT following injection of isotonic saline, hypertonic saline or a control (no injection).ResultsIn a subset of eligible participants (n = 12), the hypertonic saline combined with 10% MVT produced a qualitative experience of pain (assessed by the McGill Pain Questionnaire) that felt similar to EIP. 10% MVT with hypertonic saline significantly elevated pain intensity in the first 20% of the time to task failure and caused a significantly (P < 0.05) shorter time to task failure (448 ± 240 s) compared with the isotonic saline (605 ± 285 s) and control (514 ± 197 s) conditions.ConclusionThese findings demonstrate that hypertonic saline increases the intensity of pain during exercise, which results in a faster occurrence of exercise-induced fatigue. These results provide important evidence supporting pain as a limiting factor in endurance performance.

Highlights

Intense and prolonged muscle contractions result in acute pain proportional to the intensity and duration of exercise (Cook et al 1997)
As the to task failure (TTF) task was limited to a maximum of 20 min, participants that met this cut-off in any condition did not reach task failure or ‘exhaustion’, which does not provide a true indication of endurance performance
The 20% maximal voluntary torque (MVT) task induced a significantly greater mean visual analogue scale (VAS), equivalent to between “somewhat strong” and “strong” pain intensity (t11 = 5.3, P < 0.001, CI.95 1.1, 2.6, d = 1.8), which peaked after a longer period of time (t11 = 5.6, P < 0.001, CI.95 64, 147, d = 1.7) and lasted for a shorter duration (t11 = − 3.9, P = 0.002, CI.95 − 175, − 49, d = 1.7) than the experimental muscle pain experienced in rest HYP

Summary

Introduction

Intense and prolonged muscle contractions result in acute pain proportional to the intensity and duration of exercise (Cook et al 1997). European Journal of Applied Physiology (2020) 120:2047–2058 is often accompanied by fatigue (Pollak et al 2014), which is defined as an exercise-induced reduction in the capacity to produce muscle force or power (Bigland-Ritchie and Wood 1984). This association has led to the suggestion that EIP may accelerate fatigue development during intense and prolonged exercise (Mauger 2014). It is suggested that the increased activation of group III and IV afferents inhibits central motor drive and the ability to recruit motor units (Amann et al 2011; Hureau et al 2019)

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