Accessory muscle activity contributes to the variation in time to task failure for different arm postures and loads

Abstract

Time to failure and electromyogram activity were measured during two types of sustained submaximal contractions with the elbow flexors that required each subject to exert the same net muscle torque with the forearm in two different postures. Twenty men performed the tasks, either by maintaining a constant force while pushing against a force transducer (force task), or by supporting an equivalent load while maintaining a constant elbow angle (position task). The time to failure for the position task with the elbow flexed at 1.57 rad and the forearm horizontal was less than that for the force task (5.2 +/- 2.6 and 8.8 +/- 3.6 min, P = 0.003), whereas it was similar when the forearm was vertical (7.9 +/- 4.1 and 7.8 +/- 4.5 min, P = 0.995). The activity of the rotator cuff muscles was greater during the position tasks (25.1 +/- 10.1% maximal voluntary contraction) compared with the force tasks (15.2 +/- 5.4% maximal voluntary contraction, P < 0.001) in both forearm postures. However, the rates of increase in electromyogram of the accessory muscles and mean arterial pressure were greater for the position task only when the forearm was horizontal (P < 0.05), whereas it was similar for the elbow flexors. These findings indicate that forearm posture influences the difference in the time to failure for the two fatiguing contractions. When there was a difference between the two tasks, the task with the briefer time to failure involved greater rates of increase in accessory muscle activity and mean arterial pressure.

Task- and time-dependent modulation of Ia presynaptic inhibition during fatiguing contractions performed by humans

“…Time to failure, which occurs when the participant is no longer able to match the task criterion, is briefer for the position task than for the force task (Baudry et al 2009b;Hunter et al 2002;Maluf et al 2005;Rudroff et al 2007). Moreover, previous work has indicated that the synaptic input received by motor neurons differs during force and position tasks (Baudry et al 2009b;Mottram et al 2005;Rudroff et al 2010) and suggests a more rapid increase in presynaptic inhibition of Ia afferents during the position task when sustained to failure (Klass et al 2008).…”

Spatial reorganisation of muscle activity correlates with change in tangential force variability during isometric contractions

“…Moreover, previous studies have highlighted the increased activity in muscles not contributing on the external torques in complex motor tasks [Flanders and Soechting, 1990;Zuylen et al, 1988]. Therefore, most likely the centroid difference is compensated by the increased activity of the auxiliary muscle, and it is mainly the auxiliary muscles which account for most of the tangential force variability [Poortvliet et al, 2013;Rudroff et al, 2007].…”

“…Moreover, it has been suggested that less fatigued accessory muscles could compensate for motor control deficiency [Poortvliet et al, 2013;Rudroff et al, 2007]. In this regard, increased muscle cooperation could enlarge the force variability, since a cooperative muscle would contribute to force on its direction of mechanical action [Kutch et al, 2008].…”

Reflex Inhibition in Human Biceps Brachii Decreases With Practice of a Fatiguing Contraction

“…Although the task was to maintain a constant force, the force exerted by a muscle or a group of muscles is never constant during a voluntary contraction but fluctuates about an average value due to the discharge characteristics of the activated motor units Moritz et al 2005). Furthermore the amplitude of the normalized (coefficient of variation) force fluctuations increases during a fatiguing contraction due to changes in motor-unit activity Mottram et al 2005;Rudroff et al 2007). In the current study, the coefficient of variation for force increased from 1.6 Ϯ 0.5% during the first 60 s of the fatiguing contraction performed in session 1 to 7.6 Ϯ 2.8% during the last 60 s. Although a similar value was reached during the last 60 s of session 3 (7.1 Ϯ 2.1%), the longer time to failure for the third session meant a slower rate of increase in the coefficient of variation for force.…”

“…Activation of this inhibitory pathway decreases the discharge rate of biceps brachii motor units during isometric contractions with the elbow flexor muscles. The strength of the inhibitory reflex onto biceps brachii motor neurons depends on the position of the forearm with greater inhibition when the forearm is neutral compared with supinated (Barry et al 2008), which might explain why the time to failure for a sustained elbow flexor contraction is longer when the forearm is in a supinated position than when it is in a neutral position (Rudroff et al 2005(Rudroff et al , 2007. A suppression of this inhibitory pathway with practice might attenuate the decline in the discharge rate of motor units, delay the recruitment of new motor units, and prolong the time to task failure.…”