Adjustments Differ Among Low-Threshold Motor Units During Intermittent, Isometric Contractions

Abstract

Farina D, Holobar A, Gazzoni M, Zazula D, Merletti R, Enoka RM. Adjustments differ among low-threshold motor units during intermittent, isometric contractions. J Neurophysiol 101: 350 -359, 2009. First published November 12, 2008 doi:10.1152/jn.90968.2008. We investigated the changes in muscle fiber conduction velocity, recruitment and derecruitment thresholds, and discharge rate of low-threshold motor units during a series of ramp contractions. The aim was to compare the adjustments in motor unit activity relative to the duration that each motor unit was active during the task. Multichannel surface electromyographic (EMG) signals were recorded from the abductor pollicis brevis muscle of eight healthy men during 12-s contractions (n ϭ 25) in which the force increased and decreased linearly from 0 to 10% of the maximum. The maximal force exhibited a modest decline (8.5 Ϯ 9.3%; P Ͻ 0.05) at the end of the task. The discharge times of 73 motor units that were active for 16 -98% of the time during the first five contractions were identified throughout the task by decomposition of the EMG signals. Action potential conduction velocity decreased during the task by a greater amount for motor units that were initially active for Ͼ70% of the time compared with that of less active motor units. Moreover, recruitment and derecruitment thresholds increased for these most active motor units, whereas the thresholds decreased for the less active motor units. Another 18 motor units were recruited at an average of 171 Ϯ 32 s after the beginning of the task. The recruitment and derecruitment thresholds of these units decreased during the task, but muscle fiber conduction velocity did not change. These results indicate that low-threshold motor units exhibit individual adjustments in muscle fiber conduction velocity and motor neuron activation that depended on the relative duration of activity during intermittent contractions.

Changes in motor unit behavior following isometric fatigue of the first dorsal interosseous muscle

“…Despite the absence of recruitment compression postrecovery, the percent increase in the mean amplitude of the MUs recruited at each threshold, averaged over all subjects, remained elevated, particularly at the latter stages of recruitment. This suggests that fatigue-induced lowering of the recruitment threshold of larger MUs was also employed postrecovery to compensate for the continued force impairment of those MUs already activated (Adam and De Luca 2003;Calder et al 2008;Carpentier et al 2001;Farina et al 2009). The slope of the regression fit to the MUAP amplitude and threshold relationship exhibited an overall tendency to increase directly postfatigue, which corresponds with the reported recruitment compression.…”

“…Later recruited MUs have been reported to fire at a constant rate or to increase steadily (Garland et al 1994), to exhibit no significant change in discharge rate (Adam and De Luca 2005), or to show varying rates with time (Carpentier et al 2001). Although a decrease in the recruitment threshold of higher threshold MUs following fatigue is generally reported (Adam and De Luca 2003;Calder et al 2008;Carpentier et al 2001;Jensen et al 2000), some studies have observed increased recruitment thresholds for early recruited MUs (Carpentier et al 2001;Farina et al 2009). Others have reported a homogenous and monotonic decrease in the recruitment threshold of their sample MU population as fatigue progressed (Adam and De Luca 2003;Christova and Kossev 1998).…”

Muscle fatigue – from motor units to clinical symptoms

“…The second issue involves the fatigability of fatigue-resistant motor units. When human volunteers performed a series of 25 ramp-up and ramp-down contractions that were minimally fatigable (9% decline in maximal voluntary contraction [MVC] force), Farina et al (2009) found that the adjustments were greatest for the first recruited motor units. As the target force for the ramp contractions was only 10% MVC force, the involved motor units were presumably fatigue resistant based on the motor unit-typing scheme.…”

“…Despite the emphasis in the fatigue literature on the impairment of physiological processes distal to the muscle fiber action potential (Ferreira and Reid, 2008;Fitts, 2008;Place et al, 2010), even actions that involve only a modest demand are accompanied by adjustments in motor unit activity (Farina et al, 2009). The magnitude of these adjustments is underscored by the influence of load compliance on the duration that a submaximal, isometric contraction can be sustained.…”

Relationships Between Surface Electromyography and Strength During Isometric Ramp Contractions

“…In the isometric there is a consensus response to an increase in sEMG as to alter some characteristic muscle joint as the increase in signal amplitude [13,24,25,26] changes the length and range of motion [27] and temperature [28,29,30].…”