This study finds that intramuscle synergies—patterns of motor unit recruitment at the spinal level—are selectively disrupted in essential tremor (reduced flexibility of motor unit recruitment) but are not altered in Parkinson's disease compared with controls.

The work identifies a potential spinal-level biomarker and mechanistic target for essential tremor interventions, but it offers limited direct actionable insight for Parkinson's therapeutic discovery.

Essential tremor (ET) is one of the most common movement disorders and is marked by centrally generated oscillations that interfere with stable hand function. The contribution of spinal circuitry to impaired movement stability remains largely unexplored. In this study, we assessed intramuscle synergies as a potential biomarker of spinal deficits during accurate cyclic force production in individuals with ET and compared them with those in patients with Parkinson's disease (PD), age-matched controls (AMC), and young adult controls. Motor unit activity was extracted through surface EMG decomposition of flexor digitorum superficialis, and force-stabilizing synergies were quantified using the uncontrolled manifold framework. Across groups, multifinger synergies stabilizing total force were similar, with ET participants not different from AMC and PD. In contrast, intramuscle synergies were selectively disrupted in ET. The reduction of the synergy index in ET compared with all other groups was driven primarily by markedly lower variance that did not affect force magnitude, while variance affecting force output was comparable across groups. We conclude that ET shows reduced flexibility in motor unit recruitment and firing. These findings identify a previously unrecognized alteration at the level of spinal circuitry in ET during voluntary force control. Such impaired organization of motor units may contribute directly to difficulties maintaining stable hand action in daily tasks. Intramuscle synergy metrics thus represent a promising biomarker of spinal-level dysfunction in ET and a potential mechanistic target for future therapeutic interventions.NEW & NOTEWORTHY This is the first study exploring the recently described phenomenon of intramuscle synergies in neurological patients. Patients with essential tremor showed reduced indices of such synergies during cyclical finger force production compared with patients with Parkinson's disease and two groups of controls. This reduction reflected the reduced flexibility in the recruitment of motor unit groups. Studies of intramuscle synergies are promising to provide a sensitive biomarker of spinal circuitry.