Motor Unit Recruitment Strategies Are Altered during Deep-Tissue Pain

Abstract

Muscle pain is associated with decreased motor unit discharge rate during constant force contractions. As discharge rate is a determinant of force, other adaptations in strategy must explain force maintenance during pain. Our aim was to determine whether motor unit recruitment strategies are altered during pain to maintain force despite reduced discharge rate. Motor unit discharge behavior was recorded in two muscles, one with (quadriceps) and one without [flexor pollicis longus (FPL)] synergists. Motor units were recruited during matched low-force contractions with and without experimentally induced pain, and at higher force without pain. A total of 52 and 34 units were recorded in quadriceps and FPL, respectively, during low-force contractions with and without pain. Of these, 20 quadriceps and 9 FPL units were identified during both trials. The discharge rate of these units reduced during pain in both muscles [quadriceps: 8.7 (1.5) to 7.5 (1.3) Hz, p Ͻ 0.001; FPL: 11.9 (1.5) to 10.0 (1.7) Hz, p Ͻ 0.001]. All remaining units discharged only with or without pain, but not in both conditions. Only one-third of the additional units recruited during pain (quadriceps n ϭ 7/19, FPL n ϭ 3/15) were those expected given orderly recruitment of the motor unit pool as determined during higher-force contractions. We conclude that reduced motor unit discharge rate with pain is accompanied by changes in the population of units used to maintain force. The recruitment of new units is partly inconsistent with generalized inhibition of the motoneuron pool predicted by the "pain adaptation" theory, and provides the basis for a new mechanism of motor adaptation with pain.

Interactions between Pain and the Motor Cortex: Insights from Research on Phantom Limb Pain and Complex Regional Pain Syndrome

“…Moreover, recent studies using intra-muscular electromyography (EMG) recordings have shown that pain (induced in either muscular or non-muscular tissue) results in changes in the motorunit recruitment strategy, revealing that the effect of pain is not limited to a uniform inhibition of the motoneuron pool but, rather, includes more subtle changes in the distribution of output to the motoneuron pool. 20,21 However, it is still unclear whether these alterations in motor function observed at the muscular level reflect changes at the peripheral, spinal, or cortical level.…”

Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle

“…) or as de-recruitment of motor units (Hug et al 2013;Minami et al 2013;Tucker et al 2009) for the painful muscle. In such cases, maintenance of the net motor output (e.g., a similar movement/position) is achieved by redistribution of the activity to other motoneurons or to other muscles (Hug et al 2013(Hug et al , 2014Minami et al 2013;Tucker et al 2009).…”

Experimental knee joint pain during strength training and muscle strength gain in healthy subjects: A randomized controlled trial

“…It has recently been demonstrated that patients with knee OA who have quadriceps activation failure acquire greater volitional quadriceps activation and muscle strength gain from an intervention aimed at blocking pain sensation centrally in combination with exercise (49). Second, the average pain intensity was 16.2 mm, which is lower than that reported in previous studies (25)(26)(27)36). The time course of experimental IPFP pain shows that maximum pain intensity is reached within 5 minutes, and then gradually declines over the following 10 -12 minutes (23).…”

“…Second, the "pain adaptation" theory hypothesizes that pain inhibits agonistic muscles and facilitates antagonistic muscle activity (35). Excitation of group III and IV afferents can reduce muscle force, electromyographic activity, and discharge rate of some motor units (14,17,25,27,36), and reduce quadriceps activation. Our data do not support this hypothesis, as uniform inhibition of the quadriceps in the pain group would be expected to reduce the training volume and a parallel lower strength gain over time compared to the control group, but no differences in training volume and a larger strength gain were found in the pain group.…”

“…Rather than uniform effects (inhibition or excitation) of pain on the motoneuron pool, it is proposed that pain causes a redistribution of activity within and between muscles (37). As force is determined by the number of motor units recruited, their discharge rate, and the contractile properties of muscle (38), the ability to maintain force despite a decreased motor unit discharge rate can only occur if there is a simultaneous change either in muscle fiber contractile properties and/or changes in motor unit recruitment strategies (36). As changes in muscle contractile properties have been largely excluded (39), changed recruitment is the most likely explanation.…”