Motor Unit Behaviour in the Biceps Brachii During Asymmetric Bilateral Versus Unilateral Dynamic Force Tracking Across Force Levels and Rates of Force Development: A Preliminary Study.

Most bilateral actions in daily life are asymmetric: one limb sustains a steady postural load while the other performs a continuously adjusted force task. Whether the influence of such a sustained contraction on the contralateral limb is a fixed property of bilateral coordination or a strategy that higher centres rescale with task demand is unknown. In this exploratory study, we examined motor unit recruitment and common neural drive in the biceps brachii during asymmetric bilateral versus unilateral isometric force tracking, tested whether the interlimb effect depends on force level and rate of force development. Fifteen healthy adults tracked trapezoidal force profiles with the right biceps brachii at two force levels (10% and 30% of maximal voluntary contraction) and three rates of force development (5%, 10%, and 20% per second), performed unilaterally and bilaterally while the left biceps brachii held a constant 20% contraction. High-density surface electromyography was decomposed into individual motor units. Relative to unilateral tracking, the bilateral condition tended to raise recruitment threshold, lowered steady-state discharge rate, increased common-input variability, and reduced force stability, whereas action-potential amplitude and the variance explained by the common input (41-45%) were unchanged. No force- or rate-dependent interactions emerged. Although consistent with a spinal constraint, these findings cannot exclude cortical strategies and should be interpreted cautiously given the small sample.