A single low-dimensional neural component of spinal motor neuron activity explains force generation across repetitive isometric tasks

Low-dimensional control is thought to underlie spinal motor neuron activity, with low-frequency oscillations in common synaptic inputs serving as the primary determinant of muscle force production. Here, we used principal-component analysis and factor analysis to investigate the role of low-dimensional motor unit components in force production during repetitive isometric tasks with similar force profiles. In both individual and synergistic human muscles, the first motor unit component explained most of the variance in smoothed discharge rates and showed higher correlations with force oscillations than the second component. Additionally, the first component, but not the second, remained highly consistent across trials. A non-linear network-information framework further confirmed these findings, revealing high motor unit network density in the first component across all muscles. These results suggest that during isometric contractions, force oscillations are primarily driven by a single dominant shared synaptic input to spinal motor neuron activity.