Estimating common synaptic inputs to spinal motor neurons from motor unit spike trains using openhdemg
Common synaptic input is considered a fundamental principle of motor neuron control and represents the dominant component of the neural drive transmitted from the motor neurons to muscle. Recent advances in High-Density surface Electromyography (HDsEMG) and motor unit (MU) decomposition algorithms have enabled the concurrent identification of increasingly large populations of MUs and substantially expanded the possibility of estimating common synaptic input from MU spike trains, making this approach widely used to investigate the neural control of movement in humans. However, multiple analytical approaches are currently available, each relying on different physiological assumptions, mathematical formulations, and parameter choices. The lack of practical guidelines and open-source implementations has also limited the accessibility and reproducibility of these analyses. In this tutorial, we provide a practical, physiologically grounded guide to estimating common synaptic input from populations of MU spike trains using openhdemg, an open-source Python framework. We organize the available methods into three complementary categories: time-domain approaches applied to smoothed discharge rates, frequency-domain approaches based on coherence between cumulative spike trains, and a network-information approach based on nonlinear pairwise dependencies and graph theory. For each method, we describe its physiological interpretation, step-by-step estimation, and systematically examine how key parameter choices influence the resulting estimates, providing practical recommendations for their selection. Finally, we present a complete workflow from HDsEMG decomposition and MU cleaning to common synaptic input estimation, demonstrating that decomposition quality directly affects these estimates.