Muscle Unloading Induced Sex Specific Neurophysiological and Myofiber Profile Adaptations

Muscle unloading affects a muscle’s ability to produce a contractile force and it affects the muscle’s endurance. The objective of this project was to investigate the differences between males and females and their neurophysiological adaptations to hindlimb suspension, an effective model of muscle unloading. Thirty nine young adult Wistar rats were divided into the following four groups: 1) male control, 2) female control, 3) male unloading, 4) female unloading. The unloading groups were subjected to a hindlimb suspension model. Soleus muscles were surgically removed to quantify neuromuscular function, and fluorescent fiber type staining was performed to quantify the cross-sectional area and fiber type composition. By using different stimulation protocols, muscle contraction was induced either directly or indirectly (via motor nerve terminals) and muscular force was quantified by a force transducer. Flourescent staining was used to image type I and type II fibers. The results showed that over a 5 minute stimulation protocol, muscle fatigue was greater during indirect stimulation than direct stimulation, indicating that the motor neuron fatigues at a faster rate than the muscle fibers it innervates. Hindlimb suspension affected the females more than the males whether the muscle was stimulated directly or by the nerve. Unloading significantly increased the neuromuscular block over the five minute fatigue train only in the females. There was significant atrophy in the unloaded groups, but no sex-specific significant differences and no fiber type transitions. In summary, the muscle fatigue is likely due to fatigue in the neuron’s ability to stimulate the muscle, and females are more affected by the hindlimb suspension than males. There was also unloading induced atrophy but it was not sex specific