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

Abstract

IntroductionThe reduction in mechanical loading applied on the lower limb has numerous detrimental consequences on neuromuscular function. While muscle mass and strength are known to be altered, changes in motoneuronal function remain poorly investigated.MethodsTen young healthy males (19-28 years old) underwent 10 days of unilateral lower limb suspension (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.ResultsThe knee extensors’ cross-sectional area (−4%,p= 0.007), maximal strength (−27%,p< 0.001) and activation capacity (−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 change in nerve conduction velocity was observed after the ULLS.ConclusionThe present study suggests that both muscular and neural impairments may contribute to the de crease in knee extensors’ function 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 was not sufficient to counteract the reduced activation capacity resulting from muscle disuse. These adaptations to short-term ULLS should be considered when aiming at improving the neuromuscular function of people experiencing muscle hypoactivity.