Biomechanical strategies to accommodate expected slips in different directions during walking.

The aim of the study was to verify whether heel kinematics, ground reaction forces and electromyography (EMG) during walking are affected when anticipating slips in anterior-posterior (AP) and medial-lateral directions (ML). Eight healthy men walked through a 7-m walkway, stepping on a robotic force platform. Initially, baseline (BASE) gait mechanics were assessed with the platform at rest. Subsequently, two sets of randomized perturbations (10-cm translations with at different platform movement velocities) in the AP and ML direction were applied. Perturbations were interspersed with unperturbed walking (i.e., catch-trials C-AP and C-ML). Heel accelerations, ground reaction forces and activities from the perturbed leg and trunk muscles were analyzed. EMG was analysed in four epochs: PRE (-100 ms to heel strike [HS]), EARLY (HS to 150 ms after HS), MID (150-300 ms after HS) and LATE (300 ms to toe-off). Comparisons were made between BASE, C-AP and C-ML. The first peak of the vertical force component (Fz) was decreased for C-AP and C-ML (p<0.05) but no changes were found for braking and propulsion impulses. EMG showed effects of expected slips on tibialis anterior, gastrocnemius lateralis, soleus and peroneus longus, especially for EARLY and MID epochs, with direction-specific increases in activity. In conclusion, expected slips in different directions determine only marginal changes in terms of kinetics and heel kinematics, but selective activation after HS indicates that direction-dependent strategies are adopted when anticipating perturbations.