Late appearance of obstacles in the walking swing phase demands simultaneous deceleration and change of direction in the subsequent step

To investigate the influence of the obstacle appearance timing relative to the swing phase on the deceleration and change of direction strategy. We hypothesize that the later the obstacle appearance in the swing phase of a specific step, the same leg of a subsequent step will be responsible for both deceleration and CoD.

Five healthy adults were asked to walk on the 5.5 m platform at 100 bpm cadence. If the visual obstacle (black circle of 25-cm radius) appeared on the platform in front of them while walking, they were required to change their walking direction toward left or right to avoid the obstacle. All the obstacles were controlled to appear during the swing phase of the dominant leg. The positions of the torso and toe markers were collected (360 Hz). The time between the obstacle appearance and peak torso deceleration was defined as the peak deceleration time. The time between the obstacle appearance and the onset of the left/rightward shift of the torso marker was defined as the CoD time. The obstacle appearance timing was quantified as the percentage of the swing phase from toe-off (0%) to heel contact (HC, 100%).

In all 227 trials, the peak decelerations occurred at either of the subsequent 3 HCs after the obstacle appearance. In 76.6% (n=174) trials, the peak deceleration occurred at the HC of the 2^nd next step after the obstacle appearance (2^nd step deceleration). 21.6% (n=49) of trials showed the peak deceleration at the HC of the 1^st step (1^st step deceleration). In most 2nd step deceleration, the obstacles appeared after 50% of the swing phase (late phase). In 70.7% (n=123/174) of 2^nd step deceleration, the peak deceleration and CoD occurred at the same step, while the peak deceleration and CoD were performed by different steps in 1^st step decelerations in most cases.

The deceleration and CoD strategy is influenced by the obstacle appearance timing. The later the obstacle appeared, the deceleration and CoD coincided with 2nd step.

Further studies should examine whether the temporal coincidence of peak deceleration and change of direction associated with late obstacle appearance increases the ACL stress.