Agility training enhances motor temporal precision by reweighting spinal phase-locked commissural inhibition

Agile motor action requires rapid switching between motor states while maintaining stability. Because motor output combines fast and slow muscle fibers with distinct kinetics, prolonged slow-fiber activation can broaden burst envelopes and blur within-cycle transitions, limiting temporal precision. Here, we show that agility training improves locomotor timing by selectively compressing the activity of slow motor neurons through enhancing commissural inhibition. In adult zebrafish, training increased locomotor stability and reshaped cycle structure in vivo, shortening the contraction phase while extending relaxation. Ex vivo motor-nerve recordings revealed sharper burst envelopes and reduced temporal dispersion after training, explained by a selective narrowing of slow, but not fast, motor neuron discharge within each cycle. Training enhanced phase-locked commissural inhibition during locomotion, consistent with an inhibitory gate aligned to burst offset. Finally, electrophysiology and single-cell transcriptomics associated this plasticity with increased glycinergic receptor expression in slow motor neurons. Together, our findings identify a circuit and a molecular substrate for training-induced gains in agility and suggest that motor precision can be improved by inhibitory reformatting of slow motor output rather than by uniformly increasing excitation.