LAUSR

Resistance training, involving repeated short-duration high-intensity exercises, evokes adaptive changes in muscle fibers and the electrophysiological properties of motoneurons (MNs). This study investigated whether weight-lifting exercises evoke changes in afferent synaptic transmission from muscle spindles to MNs. Male Wistar rats (n=15) performed 5 weeks of progressive weight-lifting training, while rats in the control group (n=15) were restricted to standard cage activity. In vivo intracellular recordings were obtained under general anesthesia from two pools of MNs, innervating either the medial gastrocnemius (MG; n=188) or lateral gastrocnemius and soleus (LG-Sol; n=187) muscles. Basic membrane properties of MNs were measured, and monosynaptic Ia excitatory postsynaptic potentials (EPSPs) were recorded after stimulation of homonymous or heteronymous afferents from synergistic muscles. A generalized linear mixed model was used to analyze differences between groups. Homonymous EPSP amplitudes were an average of 28% higher in the weight-trained group in comparison to controls for both MG and LG-Sol MNs. Heteronymous EPSPs were larger for fast-type MNs in the weight-lifting group compared to controls by 45% and 46% for MG and LG-Sol MNs, respectively. Enhanced synaptic transmission from muscle spindles to MNs after weight-lifting training suggests synaptic plasticity; adaptive changes in MN membrane properties and presynaptic modulation of Ia fibers are also discussed. Adaptations were predominantly observed in fast MNs, which reflects their potent recruitment during weight-lifting training.