The Strategy of the Brain to Maintain the Force Production in Painful Contractions-A Motor Units Pool Reorganization

• Published in: Cells (Basel, Switzerland) Vol. 11; no. 20; p. 3299
• Main Authors: Becker, Klaus, Goethel, Márcio, Fonseca, Pedro, Vilas-Boas, João Paulo, Ervilha, Ulysses
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 20.10.2022; MDPI
• Subjects: Action potential; Brain; Care and treatment; Data collection; Decomposition; Electromyography; Electromyography - methods; experimental pain; Firing rate; Health aspects; Humans; hypertonic saline; Motor Neurons - physiology; motor unit; Motor units; muscle; Muscle contraction; Muscle pain; Muscle, Skeletal - physiology; Myalgia; Nervous system; Neuromuscular diseases; Pain; pain models; Range of motion; Saline Solution; Software; Portugal; United States > US; motor unit; pain models; experimental pain; hypertonic saline; muscle
• ISSN: 2073-4409; 2073-4409
• DOI: 10.3390/cells11203299

A common symptom in neuromuscular diseases is pain, which changes human movement in many ways. Using the decomposed electromyographic signal, we investigate the strategy of the brain in recruiting different pools of motor units (MUs) to produce torque during induced muscle pain in terms of firing rate (FR), recruitment threshold (RT) and action potential amplitude (MUAP ). These properties were used to define two groups (G1/G2) based on a K-means clusterization method. A 2.0 mL intramuscular hypertonic (6%) or isotonic (0.9%) saline solution was injected to induce pain or act as a placebo during isometric and isokinetic knee extension contractions. While isometric torque decreases after pain induction with hypertonic solution, this does not occur in isokinetic torque. This occurs because the MUs re-organized after the injection of both solutions. This is supported by an increase in RT, in both G1 and G2 MUs. However, when inducing pain with the hypertonic solution, RT increase is exacerbated. In this condition, FR also decreases, while MUAP increases only for G1 MUs. Therefore, this study proposes that the strategy for maintaining force production during pain is to recruit MUs with higher RT and MUAP .