Neural coupling between spinal motor neurons of the first dorsal interosseous muscle during individual index finger flexion and pinch tasks

Abstract

ObjectivePrecision grip tasks require complex coordination of intrinsic hand muscles, yet how common synaptic inputs to motor neurons are modulated during functionally different tasks remain unclear. This study investigated whether neural coupling between motor unit spike trains in the first dorsal interosseous (FDI) muscle differs between isolated index finger flexion and precision pinch tasks. ApproachSixteen healthy participants performed isolated index finger flexion and pinch tasks at 10% and 20% of maximal voluntary contraction while high-density surface electromyography was recorded from the FDI. Motor unit spike trains were decomposed and tracked across tasks. Neural coupling was assessed using complementary methods: coherence analysis and Proportion of Common Input (PCI) index to quantify linear common oscillations in delta (1-5 Hz), alpha (5-15 Hz), and beta (15-35 Hz) frequency bands, and mutual information-based network analysis to capture nonlinear interactions. Main results.Coherence analysis and PCI revealed no significant differences between tasks across all frequency bands. In contrast, network density derived from mutual information analysis showed significantly stronger nonlinear motor unit coupling during pinch compared to isolated finger flexion (p = 0.013), independent of force level. Significance.These findings demonstrate a dissociation between linear and nonlinear measures of motor unit coupling. In particular, precision pinch tasks appear to rely on stronger higher-order common inputs and distinct neural control strategies that are not fully captured by traditional linear coherence measures. This highlights that functionally relevant precision behaviors engage additional layers of nonlinear neural coupling, offering new insight into how the nervous system adaptively modulates motor unit coordination to meet complex task demands.