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

Abstract

. 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.

Experimental muscle pain increases normalized variability of multidirectional forces during isometric contractions

“…Multiple neuromuscular mechanisms could be responsible for the pain-induced increase in force fluctuations. First, experimental muscle pain reduces motor unit firing rates and elicits a spatial reorganization of muscle activity during sustained contractions, with no changes in muscle fibre membrane properties (Farina et al 2004;Madeleine et al 2006). …”

“…Thus, it is likely that multiple mechanisms, such as recruitment of additional motor units (Graven-Nielsen et al 1997) , reduced motor unit firing rates (Farina et al 2004), and increased motor unit synchronization (Hodges et al 2008), acted simultaneously and balanced the magnitude of the detected surface EMG, overcoming potential inhibitory effects associated with pain. This is in line with recent findings by Tucker and Hodges (2009), who reported non-uniform changes in the motoneuron pool detected using fine-wire intramuscular electrodes during painful knee extensions, while the amplitude of bipolar surface EMG was not affected.…”

“…It has been suggested that, during painful contractions, the central nervous system is able to maintain a constant submaximal force level by reducing motor unit firing rates (Farina et al 2004), in addition to a spatial redistribution of activity within and between muscles (Madeleine et al 2006;Falla et al 2007;Tucker and Hodges 2010). As different muscle compartments have distinct directional alignments of force production (Herrmann and Flanders 1998), the redistribution of muscle activity is likely to elicit directional changes in the resultant force vector (Kutch et al 2008;Tucker and Hodges 2010).…”

Muscle Dysfunction in Cervical Spine Pain: Implications for Assessment and Management

“…These include reflexmediated adaptation of motor neuron discharges to pain, 22,65 alterations in cortical excitability and changes in the descending drive to muscles, 44 changes in muscle spindle sensitivity through sympathetic activation, 59 as well as other adverse effects of stress, fear, and anxiety.…”

Intra- and interexaminer reliability of four manual shoulder maneuvers used to identify subacromial pain

“…When these maneuvers are performed by for example the PT in patients with suspected subacromial impingement, no pain or pain combined with varying degree of muscular weakness is the main response. The muscular weakness could be a result of muscular-tendonal changes and/or probably due to neuro-muscular inhibition in the presence of pain (Farina D et al, 2004). Accordingly, pain or no pain as test response seems more relevant, since muscle force is hard to evaluate in the presence of pain.…”