Load Type Influences Motor Unit Recruitment in Biceps Brachii During a Sustained Contraction

Abstract

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task (P=0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% (P=0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (-16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.

Handedness but not dominance influences variability in endurance time for sustained, submaximal contractions

“…Consistent with previous findings (Baudry et al 2009;Hunter et al 2005Hunter et al , 2008Klass et al 2008;Rudroff et al 2010), the current study found that endurance time for the position task was briefer than that for the force task when the two tasks involved submaximal, isometric contractions with the elbow flexor muscles of both the dominant and nondominant arms of left-and right-handed participants. As observed in prior studies when the two fatiguing contractions were performed at 15-20% MVC force with the elbow flexor muscles (Hunter et al 2002;Rudroff et al 2011), the mechanical fluctuations (coefficient of variation for force), RPE, and MAP all increased more rapidly during the position task than the force task.…”

“…Previous work has shown that the strategies used to sustain force and position control differ during submaximal isometric contractions, even when the net muscle torque is similar across conditions. The rates of change in motor unit activity and modulation of reflex pathways, for example, are more rapid during position control (Baudry et al 2011;Mottram et al 2005;Rudroff et al 2010), which results in a briefer endurance time for a position task at low to moderate target forces (Baudry et al 2009;Hunter et al 2005Hunter et al , 2008Klass et al 2008;Rudroff et al 2010). Given that the motor performance capabilities of the nondominant arm of left-handed subjects are superior to those of righthanded subjects (Brouwer et al 2001;Caroselli et al 2006;Hoffmann 1997;Hoffmann et al 1997;Judge and Stirling 2003;Przybyla et al 2012), we expected to find that lefthanded subjects would exhibit less of a difference in the endurance time for sustained isometric contractions that have been shown to involve unique control strategies.…”

Eletromiografia do bíceps braquial em contrações dinâmicas

“…Nos exercícios máximos, ou próximos dessa intensidade, é necessário recrutar quase todas as UMs para gerar, ou manter, elevados níveis de força muscular 12,27 , causando aumento do valor RMS. O mecanismo fisioló-gico responsável pelo aumento da amplitude durante tarefas fatigantes não está claro.…”

“…O mecanismo fisioló-gico responsável pelo aumento da amplitude durante tarefas fatigantes não está claro. Contudo, é sugerido que o aumento na atividade elétrica ocorre devido ao progressivo recrutamento adicional de UM 12,27 , ou aumento da frequência de disparo das UMs já recrutadas 7 . Ocorre um aumento na amplitude da EMG em baixas bandas de frequência e uma redução relativa das bandas de altas frequências 11 .…”

Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

“…It is known, however, that load compliance influences reflex responsiveness (Akazawa et al 1983;Baudry et al 2009a;Maluf et al 2007;Perreault et al 2008) and that motor unit recruitment threshold force, rate of recruitment, mean discharge rate, and discharge variability are all influenced by load compliance (Baudry et al 2009b;Mottram et al 2005;Tax et al 1989Tax et al , 1990Theeuwen et al 1994). Moreover, both reflex responses (Corden and Lippold 1996;Earles et al 2001;Mynark and Koceja 2001;Wolfarth et al 1997) and motor unit properties Erim et al 1999;Galganski et al 1993;Kamen et al 1995;Semmler et al 2000;Vaillancourt et al 2003) change with advancing age.…”