Excitatory drive to the alpha‐motoneuron pool during a fatiguing submaximal contraction in man.

Löscher, Wolfgang N.; Cresswell, Andrew G.; Thorstensson, Alf

doi:10.1113/jphysiol.1996.sp021214

Cited by 138 publications

(159 citation statements)

References 37 publications

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“…Here, it is likely that either more motor units are recruited to perform the same task or that the firing frequencies of the same recruited motor units are enhanced (Bigland-Ritchie et al 1986;Adam & De Luca 2005;Sogaard et al 2006;Missenard et al 2008;Contessa et al 2009). The resulting shift towards higher frequency components of the generated force post-fatigue, particularly at frequencies between 8 and 12 Hz, is consistent with the occurrence of physiological tremor (Freund 1983;Löscher et al 1996;McAuley & Marsden 2000;Kouzaki et al 2004). The results presented in this study indicate that fatigue perturbs the production of force, with the output fluctuating not only at a higher magnitude but also at a higher frequency, resulting in increased force signal complexity or noisiness (periodicity in the force signal), such as increased tremor in the force signal.…”

Section: Discussionsupporting

confidence: 77%

“…This variability seems to be associated with the physiological organization of the motor unit pool (Missenard et al 2009), including recruitment of the motor units (Adam & De Luca 2005;Moritz et al 2005;Tracy et al 2005) and synchronization of the motor unit pool (McAuley & Marsden 2000). From the results of this study, it seems that, during submaximal voluntary contraction, motor units are recruited in a pattern that ensures effective task performance (as demonstrated by measures of accuracy and steadiness in the time domain), but that the periodic structure of the generated force is affected by an inherent shakiness, or physiological tremor (Freund 1983;Löscher et al 1996;McAuley & Marsden 2000). Assessment in the frequency domain therefore appears to allow access to an understanding of shakiness or tremor in the generated force.…”

Section: Discussionmentioning

confidence: 85%

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Effect of fatigue on force fluctuations in knee extensors in young adults

Singh

Arampatzis

Duda

et al. 2010

Phil. Trans. R. Soc. A.

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This study investigated the hypothesis that fatiguing exercises led to increased force fluctuations during submaximal isometric knee extensions and to decreased accuracy and steadiness in the time and frequency domains. Sixteen young adults (eight males, eight females) were tested, in a seated posture with 90°knee flexion, to assess their ability to reproduce target extensor torques set at 15 per cent and 20 per cent of their maximum voluntary isometric contraction, both before and after fatiguing exercises. Normalized mean (NMAE) and peak (NPAE) of the absolute error were both used to quantify accuracy, whereas normalized standard deviation of the absolute error (NSAE) was used to quantify steadiness of the torque trials in the time domain. Mean and median power frequencies (MnPF, MdPF) and normalized peak power (NPkP) were used to assess the spectral structure of the torque signals. NMAE, NSAE and NPAE all showed excellent intra-as well as intersession reliabilities (intraclass correlation values greater than 0.75 and low standard error of measurement values), demonstrating repeatability of the test set-up. NMAE, NSAE and NPAE increased significantly postfatigue (greater than 42%), together with a shift towards higher frequency (MnPF and MdPF) components, indicating that the set-up was sensitive enough to detect the decreased force accuracy and steadiness of the musculature after fatigue. Increased force variability in both the time and frequency domains could therefore explain decreased steadiness after fatigue.

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Section: Discussionsupporting

confidence: 77%

Section: Discussionmentioning

confidence: 85%

Effect of fatigue on force fluctuations in knee extensors in young adults

Singh

Arampatzis

Duda

et al. 2010

Phil. Trans. R. Soc. A.

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“…Much of the indirect evidence favoring a reflex inhibitory pathway mediated by group III and IV muscle afferents has been obtained for other physiological extensors. For example H-reflexes evoked in ankle plantarflexors are depressed after fatigue (Garland and McComas, 1990;Garland, 1991;Loscher et al, 1996;Walton et al, 2002;Kuchinad et al, 2004). We suggested previously that the discrepancy between results based on the H-reflex and that for the CMEP in elbow flexors (Butler et al, 2003) could be attributable to presynaptic inhibition of group Ia afferent volleys by group III and IV muscle afferents (Pettorossi et al, 1999;Rossi et al, 1999;Rudomin and Schmidt, 1999).…”

Section: Discussionmentioning

confidence: 95%

Fatigue-Sensitive Afferents Inhibit Extensor but Not Flexor Motoneurons in Humans

Martin¹,

Smith²,

Butler³

et al. 2006

J. Neurosci.

161

159

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The role of group III and IV muscle afferents in controlling the output from human muscles is poorly understood. We investigated the effects of these afferents from homonymous or antagonist muscles on motoneuron pools innervating extensor and flexor muscles of the elbow. In study 1, subjects (n ϭ 8) performed brief maximal voluntary contractions (MVCs) of elbow extensors before and after a 2 min MVC of the extensors. During MVCs, electromyographic responses from triceps were evoked by stimulation of the corticospinal tracts [cervicomedullary motor evoked potentials (CMEPs)]. The same subjects repeated the protocol, but input from fatigue-sensitive afferents was prolonged after the fatiguing contraction by maintained muscle ischemia. In study 2, CMEPs were evoked in triceps during brief extensor MVCs before and after a 2 min sustained flexor MVC (n ϭ 7) or in biceps during brief flexor MVCs before and after a sustained extensor MVC (n ϭ 7). Again, ischemia was maintained after the sustained contractions. During sustained MVCs of the extensors, CMEPs in triceps decreased by ϳ35%. Without muscle ischemia, CMEPs recovered within 15 s, but with maintained ischemia, they remained depressed (by ϳ28%; p Ͻ 0.001). CMEPs in triceps were also depressed (by ϳ20%; p Ͻ 0.001) after fatiguing flexor contractions, whereas CMEPs in biceps were facilitated (by ϳ25%; p Ͻ 0.001) after fatiguing extensor contractions. During fatigue, inputs from group III and IV muscle afferents from homonymous or antagonist muscles depress extensor motoneurons but facilitate flexor motoneurons. The more pronounced inhibitory influence of these afferents on extensors suggests that these muscles may require greater cortical drive to generate force during fatigue.

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“…Although the greater recruitment of motor units during the position task can be explained by an increase in the descending drive to the motor neuron pool (Löscher et al 1996), the greater decrease in mean discharge rate and increase in discharge rate variability likely involves other mechanisms. Because the discharge of the same motor unit was monitored in the two tasks, the differential change in discharge rate likely involved synaptic mechanisms rather than intrinsic motor neuron properties.…”

Section: Discussionmentioning

confidence: 99%

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

Mottram¹,

Jakobi²,

Semmler³

et al. 2005

Journal of Neurophysiology

148

179

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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.

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Excitatory drive to the alpha‐motoneuron pool during a fatiguing submaximal contraction in man.

Cited by 138 publications

References 37 publications

Effect of fatigue on force fluctuations in knee extensors in young adults

Effect of fatigue on force fluctuations in knee extensors in young adults

Fatigue-Sensitive Afferents Inhibit Extensor but Not Flexor Motoneurons in Humans

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

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