PMC

Experiment 2 results. A: mean stride-by-stride step length difference for the treadmill portion of the split and march groups. B: step length difference for treadmill learning, over ground transfer, and treadmill washout for the 2 groups. C: over ground transfer and treadmill washout relative to the treadmill learning.

Experiment 1 results. A: mean (±SE) stride-to-stride step length difference for the SPLIT_TM and MARCH_TM groups. The step length difference is baseline subtracted on an individual basis. During adaptation, the step length difference is initially perturbed and then slowly adapted to near baseline values. During postadaptation, the belts are returned to the same speed and an aftereffect is observed in the opposite direction. B: analysis for initial adaptation, early adaptation, late adaptation, initial postadaptation, early postadaptation, and late postadaptation. *Significant differences (P < 0.05).

Model results, experiment 1. The step length difference, fast − slow, can be decomposed into spatial, temporal, and perturbation contributions. A: split-belt walking group averages ± SE. B: marching-walking hybrid group averages ± SE. The perturbation (largely negative) occurs when the treadmill belts are driven at different speeds. The temporal term adapts faster in both groups and is much greater in the split-belt group. The spatial term adapts slower and is greater in the marching-walking hybrid group. C–E: temporal, spatial, sum (temporal + spatial), perturbation, and step difference components were analyzed for initial adaptation (C), late adaptation (D), and initial postadaptation (E) of experiment 1.

Illustrations of step length and marker positions for split-belt walking (A) and the marching-walking hybrid (B). Step length is defined as the anterior-posterior distance between the ankle markers at heel strike. Fast (walk) step length occurs at heel strike on the fast (walk) belt, and slow (march) step length occurs at heel strike on the slow (march) belt. Infrared emitting markers were placed bilaterally on the fifth metatarsal head (MT), lateral malleolus (LM), lateral femoral epicondyle (LE), greater trochanter (GT), iliac crest (IC), and acromion process (AP).

Model relative contribution in late adaptation (last 30 strides after 15 min of adaptation). The temporal and spatial components each cancel some percentage of the perturbation. Perfect symmetry is represented with a black line, meaning that anywhere along the line a subject is walking symmetrically. These data indicate that the marching-walking hybrid subjects utilize a strategy that is mostly spatial whereas the split-belt training subjects use a strategy that is a mixture of both temporal and spatial. Relative contributions are shown for groups from both experiment 1 and experiment 2.

Model component derivation. The ankle position relative to the hip center for the fast and slow limbs is shown as a function of time. The step times t_s and t_f are the difference in times between heel strikes. Step length is defined as the distance between the 2 ankles as heel strike with the fast step length occurring with heel strike on the fast belt and the slow step length occurring with heel strike on the slow belt. The spatial variables α_f and α_s represent the position of the heel strike relative to the previous heel strike. Note that α_s is drawn as negative since the slow foot is placed behind the fast foot.

Experimental paradigms with belt speeds for split-belt walking (A) and the marching-walking hybrid (B). Experiments began with baseline walking at 3 different speeds for 2 min each. The subjects then walked at 0.5 m/s for 1 min, experienced a brief 10-s exposure (catch) to the belts moving at different speeds, and then walked again at 0.5 m/s for 2 min. This was followed by an adaptation phase with the belts moving at different speeds for an extended period. In experiment 1, subjects adapted for 15 min and then deadapted on the treadmill for 15 min. In experiment 2, subjects adapted for 15 min, experienced a 10-s tied-belt catch trial, adapted for 5 more minutes, walked over ground for ten 6-m walks, and then returned to the treadmill for washout. For all belt speeds at 0 m/s, the subjects were instructed to march; 10-s catch trials are indicated with an exclamation mark.