Divergent response of low‐ versus high‐threshold motor units to experimental muscle pain

Abstract

Key pointsr The neural strategies behind the control of force during muscle pain are not well understood as previous research has been limited in assessing pain responses only during low-force contractions.r Here we compared, for the first time, the behaviour of motor units recruited at low and high forces in response to pain.r The results showed that motor units activated at low forces were inhibited while those recruited at higher forces increased their activity in response to pain.r When analysing lower-and higher-threshold motor unit behaviour at high forces we observed differential changes in discharge rate and recruitment threshold across the motor unit pool.r These adjustments allow the exertion of high forces in acutely painful conditions but could eventually lead to greater fatigue and stress of the muscle tissue.Abstract During low-force contractions, motor unit discharge rates decrease when muscle pain is induced by injecting nociceptive substances into the muscle. Despite this consistent observation, it is currently unknown how the central nervous system regulates motor unit behaviour in the presence of muscle pain at high forces. For this reason, we analysed the tibialis anterior motor unit behaviour at low and high forces. Surface EMG signals were recorded from 15 healthy participants (mean age (SD) 26 (3) years, six females) using a 64-electrode grid while performing isometric ankle dorsiflexion contractions at 20% and 70% of the maximum voluntary force (MVC). Signals were decomposed and the same motor units were tracked across painful (intramuscular hypertonic saline injection) and non-painful (baseline, isotonic saline, post-pain) contractions. At 20% MVC, discharge rates decreased significantly in the painful condition (baseline vs. pain: 12.7 (1.1) Hz to 11.5 (0.9) Hz, P < 0.001). Conversely, at 70% MVC, discharge rates increased significantly during pain (baseline vs. pain: 19.7 (2.8) Hz to 21.3 (3.5) Hz, p = 0.029) and recruitment thresholds decreased (baseline vs. pain: 59.0 (3.9) %MVC to 55.9 (3.2) %MVC, p = 0.02). These results show that there is a differential adjustment between low-and high-threshold motor units during This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 2094E. Martinez-Valdes and others J Physiol 598.11 painful conditions. An increase in excitatory drive to high-threshold motor units is likely required to compensate for the inhibitory influence of nociceptive afferent inputs on low-threshold motor units. These differential mechanisms allow the force output to be maintained during acute pain but this strategy could lead to increased muscle fatigue and symptom aggravation in the long term.

Modulations in motor unit discharge are related to changes in fascicle length during isometric contractions

“…In this study we were able identify an average of 7 motor units across contractions, which is lower than the number of units identified with non-transparent HDEMG electrodes, where approximately 15 to 20 motor units can be identified on the tibialis anterior muscle on average per participant (Martinez-Valdes et al, 2020b). These differences can be attributed to a number of factors.…”

“…Each grid consists of 8 × 4 electrodes (1-mm diameter, 10-mm interelectrode distance in both directions) embedded into a layer of silicon rubber Figure 2A . The array was located centrally between the proximal and distal tendons of the muscle, with the columns oriented parallel to the tibia bone (Martinez-Valdes et al , 2020b). Skin-electrode contact was made by inserting conductive gel (Sonogel, Bad Camberg, Germany) into the electrode cavities with a mechanical pipette (Eppendorf, Hamburg, Germany) as seen previously (Botter et al , 2013).…”

Ankle Angle but Not Knee Angle Influences Force Fluctuations During Plantar Flexion

“…The purpose of the study was to evaluate the influence of changes in ankle-and knee-joint angles on force steadiness and the discharge characteristics of motor units (MU) in soleus when the plantar flexors performed steady isometric contractions. Submaximal contractions (5,10,20, and 40 % of maximum) were performed at two ankle angles (75 ° and 105 °) and two knee angles (120 ° and 180 °) by 14 young adults. The coefficient of variation of force decreased as the target force increased from 5 to 20 % of maximal force, then remained unaltered at 40 %.…”

Acute Quadriceps Muscle Pain Reduces Isometric Endurance Performance of the Contralateral Limb

“…An increase in either motor unit firing rate and/or the increase in the number of motor units may explain the increase in EMG amplitude. The reason for such change could be attributed to the centrally mediated inhibition caused by muscle pain (Farina et al, 2004;Liew et al, 2019;Martinez-Valdes et al, 2020) which could necessitate a greater need for central drive to ensure the maintenance of force. As a result, the earlier recruitment of more fatigable higher threshold motor units could lead to the earlier development of fatigue and a shortened TTF.…”