Exercise-induced plasticity of Ia proprioceptive input to spinal motoneurons

The monosynaptic Ia afferent pathway provides direct and strong excitatory drive to spinal motoneurons (MNs), forming a critical substrate for reflex modulation, intermuscular coordination, and motor performance. Despite its central role in sensorimotor control, it has remained unclear how different forms of physical training reshape the Ia input to MNs. In this review, we summarize recent intracellular studies in rats revealing a pattern of MN-type specific plasticity. Whole‑body vibration and progressive weight‑lifting training, both characterized by the recruitment of high‑threshold motor units, produce robust potentiation of Ia excitatory postsynaptic potentials (EPSPs) in fast‑type MNs. In contrast, endurance treadmill running, which relies heavily on the recruitment of slow, fatigue‑resistant motor units, selectively strengthens Ia EPSPs in slow‑type MNs. These findings support a paradigm that Ia synaptic excitation is preferentially enhanced on the MNs most frequently and intensely recruited during a given training task. This selectivity provides mechanistic insight into how the spinal cord optimizes proprioceptive integration to support improvements in strength, motor coordination, and endurance, with important implications for training purposes and rehabilitation strategies.