Biology subjects

Stashuk, D. W.

Publications and source records attributed to Stashuk, D. W..

3 recordsLinked to original sources

bioRxiv2022

Motor unit dysregulation following 15 days of unilateral lower limb immobilisation

Disuse atrophy, caused by situations of unloading such as limb immobilisation, causes a rapid yet diverging reduction in skeletal muscle function compared to muscle mass. While mechanistic insight into the loss of mass is well studied, deterioration of muscle function with a focus towards the neural input to muscle remains underexplored. This study aimed to determine the role of motor unit adaptation in disuse-induced neuromuscular deficits. Ten young, healthy male volunteers underwent 15 days of unilateral lower limb immobilisation with intramuscular EMG (iEMG) recorded from the vastus lateralis during knee extensor contractions normalised to maximal voluntary contraction (MVC) pre and post disuse. Muscle cross-sectional area was determined by ultrasound. Individual MUs were sampled and analysed for changes in MU discharge and MU potential (MUP) characteristics. Vastus lateralis (VL) CSA was reduced by approximately 15% which was exceeded by a two-fold decrease of 31% in muscle strength in the immobilised limb, with no change in either parameter in the non-immobilised VL. Parameters of MUP size were largely reduced with immobilisation, while neuromuscular junction (NMJ) transmission instability increased, and MU firing rate decreased at several contraction levels. All adaptations were observed in the immobilised limb only. These findings highlight impaired neural input following immobilisation reflected by suppressed MU discharge rate and instability of transmission at the NMJ which may underpin the disproportionate reductions of strength relative to muscle size. Key pointsO_LIMuscle mass and function decline rapidly in situations of disuse such as bed rest and limb immobilisation. C_LIO_LIThe reduction in muscle function commonly exceeds that of muscle mass, which may be associated with the dysregulation of neural input to the muscle. C_LIO_LIWe have used intramuscular electromyography to sample individual motor unit and near fibre potentials from the vastus lateralis following 15 days of unilateral limb immobilisation. Following disuse, the disproportionate loss of muscle strength when compared to size was associated with suppressed motor unit firing rate and increased markers of neuromuscular junction transmission instability. C_LIO_LIThese central and peripheral motor unit adaptations were observed at multiple contraction levels and in the immobilised limb only. Our findings demonstrate neural dysregulation as a key component of functional loss following muscle disuse in humans. C_LI

physiology↗

bioRxiv2022

Training induced improvements in knee extensor force accuracy are associated with reduced vastus lateralis motor unit firing variability

BackgroundMuscle force output during sustained submaximal isometric contractions fluctuates around an average value and is known to be influenced by variation in motor unit (MU) firing rates. MU firing rate variability seemingly reduces following exercise training interventions, however, much less is known with respect to peripheral MU properties. We therefore investigated whether targeted force accuracy training could lead to improved muscle functional capacity and control, in addition to determining any alterations of individual MU features. MethodsTen healthy participants (7 females, 3 males, 27{+/-}6 years, 170{+/-}8 cm, 69{+/-}16kg) underwent a 4-week supervised, unilateral, force accuracy training intervention. The coefficient of variation for force (FORCECoV) and sinusoidal wave force tracking accuracy (FORCESinu) were determined at 25% maximal voluntary contraction (MVC) pre- and post-training. Intramuscular electromyography was utilised to record individual MU potentials from the vastus lateralis (VL) muscles at 25% MVC during sustained contractions, pre- and post-training. ResultsKnee extensor muscle strength remained unchanged following training, with no improvements in unilateral leg-balance. FORCECoV and FORCESinu significantly improved in only the trained knee extensors by ~13% (p=0.01) and ~30% (p<0.0001) respectively. MU firing rate variability significantly reduced in the trained VL by ~16% (n=8; p=0.001), with no further alterations to MU firing rate or neuromuscular junction transmission instability. ConclusionOur results suggest muscle force control and tracking accuracy is a trainable characteristic in the knee extensors, which is likely explained by the reduction in MU firing rate variability apparent in the trained limb only.

physiology↗

bioRxiv2021

Motor unit recruitment strategies of the vastus lateralis according to sex

AimDespite men exhibiting greater muscle strength and fatigibility than women, it remains unclear if there are sex-based differences in muscle recruitment strategies e.g. motor unit (MU) recruitment and modulation of firing rate (FR) at normalised forces and during progressive increases in force. MethodsTwenty-nine healthy male and thirty-one healthy female participants (18-35 years) were studied. Intramuscular electromyography was used to record individual motor unit potentials (MUPs) and near fibre MUPs from the vastus lateralis (VL) during 10% and 25% maximum isometric voluntary contractions (MVC), and spike-triggered averaging was used to obtain motor unit number estimates (MUNE) of the VL. Multilevel mixed-effects linear regression models were used to investigate the effects of sex at each contraction level. ResultsMen exhibited greater muscle strength (p<0.001) and size (p<0.001) than women, with no difference in force steadiness at 10% or 25% MVC. Women had smaller MUs and higher FR at 10% MVC (both p<0.02), similar to that at 25% MVC in MU size (p=0.062) and FR (p=0.031). However, both sexes showed similar increases in MU size and FR when moving from low-to mid-level contractions. There were no sex differences in any near fibre MUP parameters or in MUNE. ConclusionIn the vastus lateralis, women produce muscle force via different neuromuscular recruitment strategies to men which is characterised by smaller MUs discharging at higher rates. However, similar strategies are employed to increase force production from low to moderate contractions. These findings of similar proportional increases between sexes support the use of mixed sex cohorts in studies of this nature. Key pointsO_LIIncreases in muscle force production are mediated by motor unit (MU) recruitment, and MU firing rate (FR). C_LIO_LIWomen are underrepresented in studies of human neuromuscular research and markedly differ to men in a number of aspects of neuromuscular function, yet little is known of the recruitment strategies of each. C_LIO_LIHere we demonstrate men and women have similar vastus lateralis MU number estimates, yet women recruit smaller MUs with higher FR than men at normalised contraction levels. However, increases in force are achieved via similar trajectories of MU recruitment and MU FR in men and women. C_LIO_LIAlthough men and women exhibit divergent neuromuscular recruitment strategies to achieve normalised forces, increases in force are achived similarly and support the inclusion of mixed sex cohorts in studies of this nature. C_LI

physiology↗