The impact of quadriceps disuse atrophy on motor unit properties

Abstract

Disuse atrophy describes the reduction in skeletal muscle mass and strength accompanying periods of inactivity (1). This impairment is of interest in the lower limbs due to their high functional importance in activities of daily living. Similar to the age-related loss of muscle mass associated with sarcopenia, disuse atrophy has widespread impact throughout the muscle tissue, and whilst mechanisms of muscle atrophy are reasonably well described (2), data on adaptations of the peripheral neuromuscular system are scant. The aim of the current study was to explore neuromuscular changes in individual motor units (MUs) of the vastus lateralis (VL) following 15 days of whole-leg immobilisation. Intramuscular electromyography (iEMG) was performed in 8 healthy males (18-25 years) before and after 15 days of unilateral leg immobilisation. Individual MU potentials (MUPs) were sampled from near the VL motor point during isometric contractions held at 25% of maximum voluntary contraction (MVC). MUP complexity was quantified by the number of significant slope changes within the MUP template (turns), and MUP duration as the time in ms between MUP onset and offset. The non-immobilised limb served as a control. Multi-level mixed effects linear regression models were used to examine effects of the intervention, with leg and time as factors. Significance was accepted as p<0.05. MVC decreased by 30% in the immobilised leg (p<0.05) with no difference in the control leg (p=0.23). Analysis of the number of MUP turns revealed significant interaction effects between leg and time (p<0.05); the number of MUP turns increased in the immobilised leg (β = 0.442, 95% CI: 0.160 – 0.724, p<0.01), and did not differ in the control leg (p=0.861). Similarly, there was a significant interaction between leg and time point for MUP duration (p<0.05), which was greater post intervention in the immobilised leg (β = 1.349, 95% CI: 0.72 – 1.97, p<0.001), and did not differ in the control leg (p=0.183). The increased MUP complexity and duration in the immobilised leg can be caused by increased electrophysiological temporal dispersion across MU fibres, which in turn can be related to increased differences in conduction times along axonal branches and/or MU fibres. These findings therefore indicate that short-term immobilisation of large limb muscles can exert notable effects on MU morphology and electrophysiology. Clinical pre-/rehabilitation regimes targeting disuse atrophy should also aim to specifically target peripheral motor nerves to mitigate the MU changes shown to occur following periods of disuse, potentially facilitating enhanced improvements in functional outcomes. 1. Rudrappa SS et al, J. Front Physiol. 2016;7(AUG):1–10. 2. Brook MS et al, J. Curr Opin Clin Nutr Metab Care. 2017;20(6):433–9.