Effect of 10 days of unilateral lower limb suspension on knee extensors neuromuscular function and spinal excitability

The reduction in mechanical loading applied on the lower limb has numerous detrimental consequences on neuromuscular function. The current study aimed to investigate the changes in knee extensors strength and spinal excitability induced by unilateral lower limb suspension (ULLS), providing new insights into the neuromuscular adaptations to muscle hypoactivity. Ten young healthy males (19-28 years old) underwent 10 days of ULLS to simulate muscle disuse. Modulation by unloading of knee extensors function (muscle morphology and strength, activation capacity and contractile properties) and spinal reflexes were explored before and after the ULLS. The knee extensors anatomical cross-sectional area (-4%, p = 0.007), maximal strength (-27%, p < 0.001) and central activation ratio (-3%, p = 0.006) were reduced after 10 days of ULLS. Vastus medialis H-reflex amplitude was enhanced both at rest (+33%, p = 0.038) and during a low-intensity contraction set at 10% of maximal strength (+103%, p = 0.038). No changes in muscle contractility and nerve conduction velocity were observed after the ULLS. The present study suggests that neural impairments mainly contribute to the decrease in knee extensors strength induced by short-term ULLS. The decrease in muscle activation after a short period of ULLS was accompanied by an increase in spinal excitability. However, the latter adaptation did not counterbalance the reduction in activation capacity and thus in maximal strength resulting from ULLS. These adaptations to short-term ULLS should be considered when aiming at improving the neuromuscular function of people experiencing muscle hypoactivity. NEW & NOTEWORTHYThis study provides new insights into the effects of muscle hypoactivity on neuromuscular function and spinal excitability in the major antigravity muscle group of the lower limb. Neural impairments primarily contribute to maximal strength loss after short-term unilateral lower limb suspension, while spinal excitability increased. These findings are crucial as they offer valuable understanding for developing effective interventions to improve health outcomes for individuals experiencing muscle inactivity.