Motor-Unit Activity Differs With Load Type During a Fatiguing Contraction

Abstract

Mottram, Carol J., Jennifer M. Jakobi, John G. Semmler, and Roger M. Enoka. Motor-unit activity differs with load type during a fatiguing contraction. J Neurophysiol 93: [1381][1382][1383][1384][1385][1386][1387][1388][1389][1390][1391][1392] 2005. First published October 13, 2004; doi:10.1152/jn.00837.2004. Despite a similar rate of change in average electromyographic (EMG) activity, previous studies have observed different rates of change in mean arterial pressure, heart rate, perceived exertion, and fluctuations in motor output during the performance of fatiguing contractions that involved different types of loads. To obtain a more direct measure of the motor output from the spinal cord, the purpose of this study was to compare the discharge characteristics of the same motor unit in biceps brachii during the performance of two types of fatiguing contractions. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task) for a prescribed duration. The force and position tasks were performed in random order at a target force equal to 3.5 Ϯ 2.1% (mean Ϯ SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of the isolated motor unit. Each subject maintained the two tasks for an identical duration (161 Ϯ 96 s) at a mean target force of 22.2 Ϯ 13.4% MVC (range: 3-49% MVC). The dependent variables included the discharge characteristics of the same motor unit in biceps brachii, fluctuations in motor output (force or acceleration), mean arterial pressure, heart rate, and rating of perceived exertion. Despite similar increases in the amplitude of the averaged EMG (% MVC) for the elbow flexor muscles during both tasks (P ϭ 0.60), the rates of increase in mean arterial pressure (P Ͻ 0.001), rating of perceived exertion (P ϭ 0.023), and fluctuations in motor output (P ϭ 0.003) were greater during the position task compared with the force task. Consistent with these differences, mean discharge rate declined at a greater rate during the position task (P ϭ 0.03), and the coefficient of variation for discharge rate increased only during the position task (P ϭ 0.02). Furthermore, more motor units were recruited during the position task compared with the force task (P ϭ 0.01). These findings indicate that despite a comparable net muscle torque, the rate of increase in the motor output from the spinal cord was greater during the position task.

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

“…Given that similar levels of initial tonic excitation have been observed for the force and position tasks by this and previous studies Mottram et al, 2005), the available evidence does not support a change in fusimotor drive during the two tasks. Rather, the increase in reflex responsiveness to heteronymous nerve stimulation during the position task is likely mediated by reduced presynaptic inhibition of Ia afferent terminals.…”

“…Subsequent studies have demonstrated that the control strategy used during isometric contractions can alter the recruitment and discharge behavior of single motor units both with (Tax et al, 1989;Tax et al, 1990;Rudroff et al, 2006) and without (Mottram et al, 2005) changes in the distribution of activity among accessory muscles. Furthermore, the duration that sustained (Hunter et al, 2002;Rudroff et al, 2005) and intermittent (Sjogaard et al, 2000) contractions can be maintained before task failure is also influenced by the control strategy used to sustain the task.…”

Age-related changes in the rate of muscle activation and rapid force characteristics

“…These findings have been supported by Clark et al (2010Clark et al ( , 2011 who reported that healthy older adults exhibited no reduction in rapid muscle activation characteristics when compared to middle-aged adults. Although it is possible, however, that the limitations associated with surface EMG assessments may not be sensitive enough to elucidate some of these changes detected with intramuscular recordings (Mottram et al 2005;Klass et al 2008). These and previous findings may suggest that the agerelated declines in relative rapid force characteristics may be attributed to factors other than the impairment of rapid muscle activation in healthy older adults.…”

Influence of fatigue on the simulated relation between the amplitude of the surface electromyogram and muscle force

“…As motor units activated at the onset of a sustained contraction generally exhibit a decline in discharge rate, a target force is maintained by the progressive recruitment of additional motor units (Bigland-Ritchie et al 1983;Garland et al 1994;Carpentier et al 2001;Mottram et al 2005). The discharge rate of the newly recruited motor units initially increases, but then eventually also declines (Carpentier et al 2001).…”