Fig. 1

Experimental setup and protocol. a Experimental setup. Participants wore a trunk harness attached to a passive overhead safety support system and walked on an oversized treadmill. Lateral forces were created using a pair of series-elastic linear motors and transmitted via cables to a pelvic harness. Load cells measured the applied forces, which varied with experimental trials. Cables were routed through trollies that allowed participants free fore-aft motion. b Protocol. Participants performed five trials in the following order: Null 1 – unassisted walking with no transitions between environments; Transition 1, Transition 2, and Transition 3 - a stabilizing velocity-dependent lateral force field was applied to the participant and then abruptly removed; and Null 2 – unassisted walking with no transitions. During each trial, participants completed 400 steps at their preferred speed. The first 100 steps established baseline unassisted walking to control for fatigue or carryover. The treadmill was then stopped. For Null trials, the treadmill was simply restarted and unassisted walking continued for 300 steps to mimic the treadmill stopping/starting during the Transition trials. For the Transition trials, the following sequence of events occurred: a stabilizing force field was applied during standing, the treadmill was then restarted, participants walked 200 steps with stabilization, forces were instantly removed on-the-fly without stopping the treadmill, and participants walked unassisted for another 100 steps