Nerve-muscle interaction is poorly understood in the context of early-onset spasticity and hypertonia. In an animal model of early-onset spasticity, spa mice, we found a marked impairment of tibialis anterior neuromuscular transmission. This impairment is associated with an increased innervation ratio (mean number of muscle fibers innervated by a single motor neuron). These disturbances may underlie weakness and gait disturbances observed in individual with developmental hypertonia and spasticity.