2.3. Experimental protocol

Participants were instrumented with the 35 markers on the head, neck, shoulders, arms, trunk, knees, and feet. All wore an adjustable safety harness. All were instrumented with the C2 tactor on the skin over the left lower leg around the lateral fibularis longus area, a location chosen based on our previous finding of a consistent and immediate reactive response to gait perturbations regardless of the vibrotactile cueing's location (upper arm, trunk, and lower leg) (Lee et al., 2016, 2017a). Each participant self-selected a walking speed by adjusting the treadmill's speed. The average self-selected walking speed was 0.8 ± 0.2 m/s.

All participants completed 6 trials: 2 consecutive trials without vibrotactile cueing and with a slip perturbation (control trials); 3 trials with vibrotactile cueing [2 trials with the slip perturbation (cueing trial) and 1 trial without the slip perturbation (catch trial)]; and 1 trial without vibrotactile cueing and with a slip perturbation (post-control trial). The three trials with vibrotactile cueing were randomized. Based on our previous studies, we determined that six trials would eliminate the potential for learning effects (Lee et al., 2016, 2017a,b). The fNIRS device was calibrated for each participant with the fNIRS software to prevent signal saturation due to biological factors (e.g., skin and hair color, skin and skull thickness) (Kim et al., 2018). In particular, the output power of each laser source was controlled under 1 mW to balance the signal intensity of the 780-nm and 850-nm wavelengths for each channel. We adjusted the gain of each photodetector to maximize the signal intensity without reaching saturation for each channel.

Each trial consisted of a standing period (15 s quiet standing), a walking period (steady state walking at the self-selected speed), and a post-perturbation period (from the perturbation onset to the trial's end). The custom software increased the speed of the treadmill's two belts between the standing and walking periods and decreased it after the post-perturbation period at a rate of 0.2 m/s². During the control, cueing, and post-control trials, a randomized perturbation was applied to the left foot loading phase [10% of the gait cycle, corresponding to the initial double-limb support (Lee et al., 2016, 2017a)] between the 31st and 40th step by accelerating the left belt of the treadmill in the anterior direction at a rate of 10 m/s², to induce a backward slip. The accelerated belt returned to the pre-perturbation speed with the first heel strike of the right foot (i.e., the first step response of the non-slipping foot) because stepping is a common recovery response (Mcilroy and Maki, 1993; Jensen et al., 2001; Maki and Mcilroy, 2006). All trials ended 10 steps after the perturbation by considering the number of required steps to return to normal walking (Lee et al., 2016, 2017a) and the temporal delay (i.e., latency) between the cortical activity and the hemodynamic response of ~4 to 7 s (Herold et al., 2017). Normal walking resumed within three or four steps after the perturbation, corresponding to 4.2 ± 0.9 s. The average time of the post-perturbation period was 11.5 ± 1.1 s. During the cueing and catch trials, vibrotactile cueing was applied to the left lower leg 250 ms prior to the perturbation and was stopped with the first heel strike of the right foot (Lee et al., 2016, 2017a). A lead time for vibrotactile cueing (i.e., 250 ms prior to the perturbation) was chosen based on our previous finding of a consistent reactive response to gait perturbations regardless of the vibrotactile cueing's lead time (250 ms vs. 500 ms) (Lee et al., 2016, 2017a). The participants received no information about the onset of slip perturbations, the presence of vibrotactile cueing, or how they should respond. However, they were informed that vibrotactile cueing indicates the imminence of a slip perturbation.

To minimize head movements and side-to-side walking variations, participants walked on the split-belt treadmill at their self-selected walking speed while fixing their gaze on an “X” mark placed ~4.5 m ahead at eye level (Lee et al., 2016, 2017a). Consecutive trials were separated by a 20 s rest period so participants could briefly relax the torso and upper and lower extremities.