Motor units in the human medial gastrocnemius muscle are not spatially localized or functionally grouped

Abstract

Key pointsr Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories or regions, with low-threshold units preferentially located distally.r We used intramuscular recordings to measure the territory of muscle fibres from MG MUs and determine whether these MUs are grouped by recruitment threshold or joint action (ankle plantar flexion and knee flexion).r The territory of MUs from the MG muscle varied from somewhat localized to highly distributed, with approximately half the MUs spanning at least half the length and width of the muscle. There was also no evidence of regional muscle activity based on MU recruitment thresholds or joint action.r The CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.Abstract Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories, with low-threshold units preferentially located distally. In this study, subjects (n = 8) performed ramped and sustained isometric contractions (ankle plantar flexion and knee flexion; range: ß1-40% maximal voluntary contraction) and we measured MU territory size with spike-triggered averages from fine-wire electrodes inserted along the length (seven electrodes) or across the width (five electrodes) of the MG muscle. Of 69 MUs identified along the length of the muscle, 32 spanned at least half the muscle length (ࣙ 6.9 cm), 11 of which spanned all recording sites (13.6-17.9 cm). Distal fibres had smaller pennation angles (P < 0.05), which were accompanied by larger territories in MUs with fibres located distally (P < 0.05). There was no distal-to-proximal pattern of muscle activation in ramp contraction (P = 0.93). Of 36 MUs identified across the width of the muscle, 24 spanned at least half the muscle width (ࣙ 4.0 cm), 13 of which spanned all recording sites (8.0-10.8 cm). MUs were not localized (length or width) based on recruitment threshold or contraction type, nor was there a relationship between MU territory size and recruitment threshold (Spearman's rho = −0.20 and 0.13, P > 0.18). MUs in the human MG have larger territories than previously reported and are not localized based on recruitment threshold or joint action. This indicates that the CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

Is there sufficient evidence to claim muscle units are not localised and functionally grouped within the human gastrocnemius?

“…Even though the results presented by Héroux et al . () suggest the territory of single motor units may span proximo‐distal regions as large as the gastrocnemius length, they did not access the distribution of fibres within the territory. Interestingly, in relation to the noise level, the amplitude of action potentials of individual motor units was different across detection sites; potentials with greater amplitude seem to have been detected locally, i.e.…”

“…The detailed work presented by Héroux et al . () clearly provides good insight into the behaviour of small populations of motor units along medial gastrocnemius during relatively low level isometric contractions. Employing intramuscular EMG to study individual motor units does not, however, enable the study of a spectrum of functionally significant, mechanically demanding tasks, nor does it allow the full spectrum of motor units to be included in the evaluation (i.e.…”

“…Similar results, conflicting interpretations and equivocal implications: this is the issue triggered by the recent, interesting study published by Héroux et al . ().…”

“…This evidence is consistent with Héroux et al . (), where 32 out of 69 motor units spanned at least 6.9 cm, reported as being half the muscle length. Critically, however, Wakeling () found manipulating the mechanical demands of the task resulted in considerable regional differences in EMG amplitude in the lateral gastrocnemius (an anatomically compartmentalised muscle; Segal et al .…”

Reply from J. S. Blouin, H. J. Brown, M. E. Héroux, J. T. Inglis and G. P. Siegmund

“…We recently showed that low to medium threshold motor units in the human medial gastrocnemius muscle have territories that vary widely in size, from a few centimetres to the full length or width of the muscle (Héroux et al . ). In their Letter, Vieira et al .…”