Synaptic inputs to motor neurons underlying muscle co-activation for functionally different tasks have different spectral characteristics.

The CNS may produce the same endpoint trajectory or torque profile with different muscle activation patterns. What differentiates these patterns is the presence of co-contraction, which does not contribute to effective torque generation but allows to modulate joints' mechanical stiffness. While it has been suggested that the generation of force and the modulation of stiffness rely on separate pathways, a characterization of the differences between the synaptic inputs to motor neurons (MNs) underlying these tasks is still missing. In this study, participants co-activated the same pair of upper-limb muscles, i.e., the biceps brachii and the triceps brachii, to perform two functionally different tasks: limb stiffness modulation or endpoint force generation. Spike trains of MNs were identified through decomposition of High-Density EMGs collected from the two muscles. Cross-correlogram showed a higher synchronization between MNs recruited to modulate stiffness, while cross-muscle coherence analysis revealed peaks in the beta-band, which is commonly ascribed to a cortical origin. These peaks did not appear during the co-activation for force generation, thus suggesting separate cortical inputs for stiffness modulation. Moreover, a within-muscle coherence analysis identified two subsets of MNs that were selectively recruited to generate force or regulate stiffness. This study is the first to highlight different characteristics, and probable different neural origins, of the synaptic inputs driving a pair of muscles under different functional conditions. We suggest that stiffness modulation is driven by cortical inputs which project to a separate set of MNs, supporting the existence of a separate pathway underlying the control of stiffness.