Sarto, F. | bioRxiv Science

bioRxiv2025

Changes in motor unit conduction velocity after unilateral lower limb suspension and active recovery correlate with muscle ion channel gene expression

The effects of muscle disuse on the propagation of action potentials along muscle units, a key process for effective muscle activation and force production, remain poorly understood. This study aimed to investigate changes in action potential propagation and to identify biological factors influencing these changes following unilateral lower limb suspension (ULLS) and active recovery (AR). Eleven young male participants underwent 10 days of ULLS followed by 21 days of AR based on resistance exercise. Maximal force of the knee extensor muscles (MVC), High-Density surface EMG recordings and muscle biopsies of the vastus lateralis muscle were collected before ULLS, after ULLS and after AR. EMG recordings collected during submaximal isometric contractions were decomposed to estimate single motor unit conduction velocity (MU CV). Muscle biopsies were used to measure muscle fibre diameters via histochemical analysis and ion channel transcriptomic profiles via mRNA-sequencing. MVC decreased after ULLS by 29% and fully recovered after AR. MU CV decreased after ULLS and fully recovered, up to exceeding baseline values after AR. Muscle fibre diameters did not change across the interventions and showed no correlation with MU CV. Conversely, a feature importance analysis revealed that mRNA expression levels of specific ion channel genes, particularly those involved in K+ transport, correlate with MU CV at baseline and across the interventions. This study highlights the crucial role of K+ ion channels in influencing MU CV in humans, offering new insights into MU CV modulation and the mechanisms of muscle force changes after disuse and active recovery. Key pointsO_LIMuscle disuse, such as in unilateral lower limb suspension, leads to a decrease in motor unit conduction velocity (MU CV), a critical factor for muscle activation and force production. C_LIO_LIActive recovery through resistance exercise results in the full recovery of MU CV, even exceeding baseline levels. C_LIO_LIMuscle fibre diameters do not change significantly after limb suspension or active recovery and show no correlation with MU CV. C_LIO_LIConversely, ion channel mRNA expression, particularly of those related to K+ transport, correlates with MU CV and its changes following disuse and recovery. C_LIO_LIThese findings highlight K+ ion channels as a key factor in regulating MU CV in humans and provide new molecular determinants of the changes in muscle force after disuse and recovery. C_LI

physiology↗

bioRxiv2024

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.

physiology↗