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. 10 young, healthy male volunteers underwent 15 days of unilateral lower limb immobilisation. Intramuscular EMG (iEMG) was recorded from the vastus lateralis during knee extensor contractions normalised to maximal voluntary contraction (MVC) pre and post disuse-induced loss of function. Muscle cross-sectional area was determined by ultrasound. Individual MUs were sampled and analysed for changes in discharge characteristics and MU potential (MUP) shape and structure. Vastus lateralis 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 to the non-immobilised. Parameters of MUP size were largely reduced, while neuromuscular junction (NMJ) transmission instability increased at several contraction levels and MU firing rate reduced. 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.