Task-dependent intermuscular coherence between postural muscles during voluntary upright reaching

Intermuscular coherence provides a window into the neural mechanisms coordinating posture and movement. This study investigated task-dependent modulation of coherence between postural muscles in healthy young adults performing upright forward and lateral reaching tasks. Bilateral electromyographic activity was recorded from trunk and ankle muscles from both the dominant and non-dominant reaching sides. Coherence was estimated in the delta, alpha, beta and low gamma frequency bands. During forward reaching, delta-band coherence was higher than in lateral reaching across bilateral homologous muscles and trunk-limb pairs within the posterior chain (all P < 0.001, g ≈ 1.765-3.712). Conversely, during lateral reaching, the non-dominant ankle antagonist pair exhibited higher delta-band coherence (P < 0.001, g ≈ 2.521-2.601) and increased beta/low gamma-band coherence (P < 0.05-0.001, g ≈ 0.860-1.040). In this pair, delta-band coherence of this antagonist pair correlated negatively with centre-of-pressure path length (r = -0.707, P = 0.0456). On the dominant side, delta- and beta-band coherence correlated positively with co-contraction (r ≈ 0.680-0.745, P ≈ 0.0319-0.00730). The ankle agonist pair exhibited greater delta-band coherence than antagonists (P < 0.001, g ≈ 1.583-3.064) and minimal variation in beta and low gamma bands, consistent with their synergistic role in postural control. These findings demonstrate that coherence organization adapts to postural demands: forward reaching engages bilateral and posterior-chain coupling for sagittal stability, whereas lateral reaching elicits asymmetric, limb-specific strategies combining automatic and voluntary components. This modulation highlights the adaptability of neural control processes that regulate muscle coordination under varying mechanical demands.