Neuromuscular Basis of Kinematic Adaptations During Bidirectional Walking.

Background: Human locomotion is a highly adaptive motor skill that adjusts to new environmental demands through learning. Split-belt treadmill paradigms have advanced our understanding of gait adaptation. Most studies have examined gait when the belts move at different speeds in the same direction. We are studying muscle activation patterns during an asymmetric gait, when the treadmill belts move at equal speed in opposite directions, i.e., bidirectional walking (BDW). Methods: Twelve healthy volunteers performed a single session on a split-belt treadmill. We simultaneously collected ground reaction forces via treadmill force plates, joint kinematics via motion capture, and surface electromyography (EMG) from bilateral soleus (SOL) and tibialis anterior (TA) muscles. Participants started with 2 min of forward walking (FW), followed with four 5-min blocks of BDW separated by 1-min standing rest intervals, and finished the session with 2 min of FW (washout). Results: All participants successfully completed the protocol. We analyzed EMG signals for temporal activation patterns (rhythm generation) and amplitude characteristics (pattern formation). EMG recordings revealed antiphasic activation of SOL and TA muscles bilaterally throughout all trials. During BDW, the backward-moving leg's TA showed prolonged activation patterns that persisted during washout FW, suggesting retention of adaptive changes. Burst-to-cycle duration ratios showed transient changes during early adaptation but remained relatively stable across conditions, demonstrating robust rhythm generation despite adaptive modulation of activation patterns during BDW. Discussion: These findings demonstrate that BDW induces asymmetric adjustments in muscle activation patterns. Rhythm generation (timing) did not significantly differ between BDW and FW. However, we did observe changes in pattern formation (i.e., EMG profiles) during FW pre- and post-BDW training. Burst-to-cycle duration ratios, as a measure of rhythm generation, showed changes during early adaptation, particularly the increase in right SOL and right TA during block 1, though these changes did not reach statistical significance and largely returned to baseline during washout. The underlying pattern formation structure, was maintained across all conditions, with selective amplitude modulations rather than fundamental reorganization of activation patterns. The substantial temporal adjustments in the backward-moving leg's SOL and phase shifts in TA provide the neuromuscular mechanism driving the bilateral step-length reduction, altered inter-limb phasing, and asymmetric double stance timing. These results extend our understanding of locomotor control by suggesting how the central nervous system (CNS) dynamically recalibrates muscle timing and amplitude to maintain satisfactory locomotion under new environmental demands.