Divergent response of low‐ versus high‐threshold motor units to experimental muscle pain: dfferential motor unit behavior during pain

Research output: Contribution to journalArticle


External organisations

  • University of Brescia
  • Imperial College London


During low force contractions, motor unit (MU) 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 MU behaviour in the presence of muscle pain at high forces. For this reason, we analyzed the tibialis anterior MU behaviour at low and high forces. Surface EMG signals were recorded from 15 healthy participants (26 (3) years, 6 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 MUs 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 < 0001). 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 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.


Original languageEnglish
Number of pages16
JournalThe Journal of Physiology
Early online date18 Mar 2020
Publication statusE-pub ahead of print - 18 Mar 2020


  • experimental pain, hypertonic saline, motor neuron, muscle, nociception, pain models