Discharge properties of motor units during steady isometric contractions performed with the dorsiflexor muscles

Abstract

The purpose of this study was to record the discharge characteristics of tibialis anterior motor units over a range of target forces and to import these data, along with previously reported observations, into a computational model to compare experimental and simulated measures of torque variability during isometric contractions with the dorsiflexor muscles. The discharge characteristics of 44 motor units were quantified during brief isometric contractions at torques that ranged from recruitment threshold to an average of 22 ± 14.4% maximal voluntary contraction (MVC) torque above recruitment threshold. The minimal [range: 5.8-19.8 pulses per second (pps)] and peak (range: 8.6-37.5 pps) discharge rates of motor units were positively related to the recruitment threshold torque (R(2) ≥ 0.266; P < 0.001). The coefficient of variation for interspike interval at recruitment was positively associated with recruitment threshold torque (R(2) = 0.443; P < 0.001) and either decreased exponentially or remained constant as target torque increased above recruitment threshold torque. The variability in the simulated torque did not differ from the experimental values once the recruitment range was set to ∼85% MVC torque, and the association between motor twitch contraction times and peak twitch torque was defined as a weak linear association (R(2) = 0.096; P < 0.001). These results indicate that the steadiness of isometric contractions performed with the dorsiflexor muscle depended more on the distributions of mechanical properties than discharge properties across the population of motor units in the tibialis anterior.

Brain areas associated with force steadiness and intensity during isometric ankle dorsiflexion in men and women

“…The CV of torque was greatest at the lowest intensity of contraction (10 % MVC) (see Fig. 2c) for both men and women as found for several upper and lower limb muscles (Laidlaw et al 1999; Burnett et al 2000; Jones et al 2002; Taylor et al 2003; Tracy 2007b; Jesunathadas et al 2012). Several areas, including two motor areas, the putamen and contralateral superior frontal gyrus, as well as the insula and ipsilateral inferior lobes were associated with the CV of force when all the contraction intensities were pooled.…”

“…When the standard deviation is normalized to the mean force (coefficient of variation, CV), the force fluctuations do not increase with intensity and are usually larger at lower intensities of contraction (e.g., Taylor et al 2003; Moritz et al 2005; Tracy et al 2007; Jesunathadas et al 2012). CV of force is mediated primarily by low-frequency oscillations in neural drive (<2–3 Hz) seen as the oscillations of the trains of motor unit action potentials (Negro et al 2009; Dideriksen et al 2012), with a greater influence of synaptic noise and motor unit discharge rate variability at low forces (Jesunathadas et al 2012). The low-frequency oscillating neural drive reflects an integration of both descending and afferent inputs (Negro et al.…”

Changes in muscle fascicles of tibialis anterior during anisometric contractions are not associated with motor-output variability of the ankle dorsiflexors in young and old adults

“…The greater SD of acceleration for lengthening contractions performed with the first dorsal interosseous muscle has been attributed to differences in motor unit synchronization, coherence in the 2-12 Hz frequency band (Semmler 2002), and motor unit discharge variability Pasquet et al 2006). Although lower discharge rates are associated with greater discharge variability in the first dorsal interosseous (Barry et al 2007;Moritz et al 2005), and the discharge rates of motor units in tibialis anterior are reduced during lengthening contractions (Pasquet et al 2006), it is unknown if discharge variability is greater for motor units in tibialis anterior during lengthening contractions (Jesunathadas et al 2008). Furthermore, motor neurons that innervate tibialis anterior may not receive the types of synaptic input required to correlate discharge times and thereby contribute to differences during shortening and lengthening contractions.…”

Modelagem e simulação do sistema neuromuscular responsável pelo controle do torque gerado na articulação do tornozelo.

“…Do ponto de vista da modelagem do sistema neuromuscular também há uma grande diversidade de trabalhos que buscam entender como as redes neuronais ativam os músculos gerando força e EMG (Dideriksen et al, 2012;Farina et al, 2013;Jesunathadas et al, 2012;Jesunathadas et al, 2010;Jones et al, 2002;Moritz et al, 2005;Farina, 2011;Taylor et al, 2003;Baker, 2009b;Williams et al, 2010). Neste caso, há uma significativa simplificação dos músculos e, em geral, representam condições isométricas na qual o sistema esquelético se torna relativamente desnecessário para a análise.…”

“…Embora possa se ter uma ideia global do comportamento motor, algumas propriedades estatísticas da saída motora são negligenciadas neste último caso. Além disso, fenômenos emergentes que estão relacionados à estrutura e organização das redes neuronais medulares (Jesunathadas et al, 2012;Jones et al, 2002) passam a não exercer influência quando estas simplificações são realizadas.…”