Recruitment and rate coding organisation for soleus motor units across entire range of voluntary isometric plantar flexions

Tomomichi Oya; Stephan Riek; Andrew G Cresswell

doi:10.1113/jphysiol.2009.175695

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Recruitment and rate coding organisation for soleus motor units across entire range of voluntary isometric plantar flexions

Journal article

Peer reviewed

Recruitment and rate coding organisation for soleus motor units across entire range of voluntary isometric plantar flexions

Tomomichi Oya, Stephan Riek and Andrew G Cresswell

The Journal of Physiology, Vol.587(19), pp.4737-4748

2009

DOI: https://doi.org/10.1113/jphysiol.2009.175695

PMCID: PMC2768026

PMID: 19703968

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https://doi.org/10.1113/jphysiol.2009.175695View

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Abstract

Action Potentials

Young Adult

Motor Neurons - physiology

Humans

Adult

Male

Muscle, Skeletal - innervation

Electromyography

Electrophysiological Phenomena

Muscle, Skeletal - physiology

Recruitment, Neurophysiological - physiology

Isometric Contraction - physiology

Unlike upper limb muscles, it remains undocumented as to how motor units in the soleus muscle are organised in terms of recruitment range and discharge rates with respect to their recruitment and de-recruitment thresholds. The possible influence of neuromodulation, such as persistent inward currents (PICs) on lower limb motor unit recruitment and discharge rates has also yet to be reported. To address these issues, electromyographic (EMG) activities from the soleus muscle were recorded using selective branched-wire intramuscular electrodes during ramp-and-hold contractions with intensities up to maximal voluntary contraction (MVC). The multiple single motor unit activities were then derived using a decomposition technique. The onset-offset hysteresis of motor unit discharge, i.e. a difference between recruitment and de-recruitment thresholds, as well as PIC magnitude calculated by a paired motor unit analysis were used to examine the neuromodulatory effects on discharge behaviours, such as minimum firing rate, peak firing rate and degree of increase in firing rate. Forty-two clearly identified motor units from five subjects revealed that soleus motor units are recruited progressively from rest to contraction strengths close to 95% of MVC, with low-threshold motor units discharging action potentials slower at their recruitment and with a lower peak rate than later recruited high-threshold units. This observation is in contrast to the 'onion skin phenomenon' often reported for the upper limb muscles. Based on positive correlations of the peak discharge rates, initial rates and recruitment order of the units with the magnitude of the onset-offset hysteresis and not PIC contribution, we conclude that discharge behaviours among motor units appear to be related to a variation in an intrinsic property other than PICs.

Details

Title: Recruitment and rate coding organisation for soleus motor units across entire range of voluntary isometric plantar flexions
Authors: Tomomichi Oya (Author) - University of Queensland
Stephan Riek (Author) - University of Queensland
Andrew G Cresswell (Author) - University of Queensland
Publication details: The Journal of Physiology, Vol.587(19), pp.4737-4748
Publisher: Wiley-Blackwell Publishing Ltd.
DOI: 10.1113/jphysiol.2009.175695
ISSN: 1469-7793
PMID: 19703968; PMC2768026
Organisation Unit: Graduate Research School; University of the Sunshine Coast, Queensland
Language: English
Record Identifier: 99472602702621
Output Type: Journal article

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