Inhibition of motor unit firing during experimental muscle pain in humans

Sohn, Min Kyun; Graven‐Nielsen, Thomas; Arendt‐Nielsen, Lars; Svensson, Peter

doi:10.1002/1097-4598(200008)23:8<1219::aid-mus10>3.0.co;2-a

Cited by 130 publications

(92 citation statements)

References 35 publications

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“…Conversely, the discharge rates of motor units located in the caudal region did not change in response to pain. In comparison, the decreases in motor unit discharge rate reported in other muscles for similar levels of acute pain (visual analogue scale: 3-6) are in the range 7-13% (Farina et al 2004;Hodges et al 2008;Sohn et al 2000). Careful examination of the results shown in these studies did not reveal indications of nonuniform motor unit inhibition, except to some degree for the medial gastrocnemius [see Fig.…”

Section: Discussionmentioning

confidence: 73%

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

Dideriksen

Holobar

Falla

2016

Journal of Neurophysiology

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Dideriksen JL, Holobar A, Falla D. Preferential distribution of nociceptive input to motoneurons with muscle units in the cranial portion of the upper trapezius muscle. J Neurophysiol 116: 611-618, 2016. First published May 25, 2016 doi:10.1152/jn.01117.2015Pain is associated with changes in the neural drive to muscles. For the upper trapezius muscle, surface electromyography (EMG) recordings have indicated that acute noxious stimulation in either the cranial or the caudal region of the muscle leads to a relative decrease in muscle activity in the cranial region. It is, however, not known if this adaption reflects different recruitment thresholds of the upper trapezius motor units in the cranial and caudal region or a nonuniform nociceptive input to the motor units of both regions. This study investigated these potential mechanisms by direct motor unit identification. Motor unit activity was investigated with high-density surface EMG signals recorded from the upper trapezius muscle of 12 healthy volunteers during baseline, control (intramuscular injection of isotonic saline), and painful (hypertonic saline) conditions. The EMG was decomposed into individual motor unit spike trains. Motor unit discharge rates decreased significantly from control to pain conditions by 4.0 Ϯ 3.6 pulses/s (pps) in the cranial region but not in the caudal region (1.4 Ϯ 2.8 pps; not significant). These changes were compatible with variations in the synaptic input to the motoneurons of the two regions. These adjustments were observed, irrespective of the location of noxious stimulation. These results strongly indicate that the nociceptive synaptic input is distributed in a nonuniform way across regions of the upper trapezius muscle.

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Section: Discussionmentioning

confidence: 73%

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

Dideriksen

Holobar

Falla

2016

Journal of Neurophysiology

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“…This led to the assumption that experimental muscle pain altered MU recruitment, inhibiting muscle activity (Wang et al 2000a). However, direct measurement of MU activity in the masseter muscle indicated no effect of experimental muscle pain on the recruitment curve while decline of MU firing rates was observed with pain (Sohn et al 2000). Although the effect of pain observed by Sohn et al (2000) was similar for different MUs at different contraction levels, it may be not evident when different MU samples are studied in painful and nonpainful contractions (Birch et al 2000).…”

Section: Introductionmentioning

confidence: 66%

Effect of Experimental Muscle Pain on Motor Unit Firing Rate and Conduction Velocity

Farina¹,

Arendt‐Nielsen²,

Merletti³

et al. 2004

Journal of Neurophysiology

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147

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. Effect of experimental muscle pain on motor unit firing rate and conduction velocity. J Neurophysiol 91: 1250 -1259, 2004. First published November 12, 2003 10.1152/jn.00620.2003. The aim of this human study was to investigate the relationship between experimentally induced muscle pain intensity (i.e., amount of nociceptive activity) and motor unit (MU) firing decrease and MU conduction velocity (CV). In 12 healthy subjects, nociceptive afferents were stimulated in the right tibialis anterior muscle by three intramuscular injections of hypertonic saline (0.2, 0.5, and 0.9 ml) separated by 140 s. The subjects performed six isometric contractions (20 s long) at 10% of the maximal voluntary contraction during the experimental muscle pain. The same set of six contractions was performed without any infusion before the painful condition on the right leg. The procedure was repeated for the left leg with infusion of isotonic (nonpainful) saline. Intramuscular and surface electromyographic (EMG) signals were collected to assess MU firing rate and CV. The firing rate of the active MUs [range: 7.4 -14.8 pulses/s (pps)] did not change significantly in the three control conditions (without infusion for the right and left leg and with infusion of isotonic saline in the left leg). There was, on the contrary, a significant decrease (on average, mean Ϯ SE, 1.03 Ϯ 0.21 pps) of the firing rates during the painful condition. Moreover, MU firing rates were inversely significantly correlated with the subjective scores of pain intensity. Single MU CV was 3.88 Ϯ 0.03 m/s (mean Ϯ SE, over all the MUs) with no statistical difference among any condition, i.e., the injection of hypertonic saline did not alter the muscle fiber membrane properties of the observed MUs. Progressively increased muscle pain intensity causes a gradual decrease of MU firing rates. This decrease is not associated with a change in MU membrane properties, indirectly assessed by CV. This study demonstrates a central inhibitory motor control mechanism with an efficacy correlated to the nociceptive activity.

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“…Force produced by contracting muscle depends on the number, contractile properties, and discharge rate of motor units (Burke, 1981;Stuart and Enoka, 1983). Consistent with our findings, most previous studies (Sohn et al, 2000;Farina et al, 2005;Hodges et al, 2008) have shown decreased motor unit discharge rate during pain when force was maintained. Birch et al (2000) showed no change in discharge rate when averaged across the whole pool, but this could be explained by recruitment of new units and the increase in discharge rates of a few units as identified here.…”

Section: Discussionmentioning

confidence: 99%

“…Discharge rate of motor units is directly related to force production (Stuart and Enoka, 1983). However, discharge rate is decreased during contractions with matched force when pain is induced experimentally in the test muscle (Sohn et al, 2000;Farina et al, 2005). While decreased discharge rate has been considered to support the pain adaptation theory, it is not known how force is maintained under these conditions.…”

Section: Introductionmentioning

confidence: 99%

Motor Unit Recruitment Strategies Are Altered during Deep-Tissue Pain

Butler²,

et al. 2009

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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.

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Inhibition of motor unit firing during experimental muscle pain in humans

Cited by 130 publications

References 35 publications

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

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

Effect of Experimental Muscle Pain on Motor Unit Firing Rate and Conduction Velocity

Motor Unit Recruitment Strategies Are Altered during Deep-Tissue Pain

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