Movements learned in one set of conditions may not generalize to other conditions. For example, practicing walking on a split-belt treadmill subsequently changes coordination between the legs during normal ("tied-belt") treadmill walking; however, there is limited generalization of these aftereffects to natural walking over the ground. We hypothesized that generalization of split-belt treadmill adaptation to over-ground walking would be improved by maintaining consistency in other task variables, specifically gait speed. This hypothesis was based on our previous finding that treadmill aftereffect size was sensitive to gait speed: Aftereffects were largest when tested on tied-belts running at the same speed as the slower belt during split-belt adaptation. In the present study, healthy adults were assigned to a "slow" or "fast" over-ground walking group. Both groups adapted to split-belts (0.7:1.4 m/s), and treadmill aftereffects were tested on tied-belts at the slow (0.7 m/s) and fast (1.4 m/s) speeds. All participants were subsequently transferred to the over-ground environment. The slow and fast groups walked over-ground at 0.7 and 1.4 m/s, respectively. As in previous work, we found that the size of aftereffects during treadmill walking was speed-dependent, with larger aftereffects occurring at 0.7 m/s compared with 1.4 m/s. However, over-ground walking aftereffects were less sensitive to changes in gait speed. We also found that aftereffects in spatial coordination generalized more to over-ground walking than aftereffects in temporal coordination across all speeds of walking. This suggests that different factors influence aftereffect size in different walking environments and for different measures of coordination.

Understanding the mechanisms underlying locomotor learning helps researchers and clinicians optimize gait retraining as part of motor rehabilitation. However, studying human locomotor learning can be challenging. During infancy and childhood, the neuromuscular system is quite immature, and it is unlikely that locomotor learning during early stages of development is governed by the same mechanisms as in adulthood. By the time humans reach maturity, they are so proficient at walking that it is difficult to come up with a sufficiently novel task to study de novo locomotor learning. The split-belt treadmill, which has two belts that can drive each leg at a different speed, enables the study of both short- (i.e., immediate) and long-term (i.e., over minutes-days; a form of motor learning) gait modifications in response to a novel change in the walking environment. Individuals can easily be screened for previous exposure to the split-belt treadmill, thus ensuring that all experimental participants have no (or equivalent) prior experience. This paper describes a typical split-belt treadmill adaptation protocol that incorporates testing methods to quantify locomotor learning and generalization of this learning to other walking contexts. A discussion of important considerations for designing split-belt treadmill experiments follows, including factors like treadmill belt speeds, rest breaks, and distractors. Additionally, potential but understudied confounding variables (e.g., arm movements, prior experience) are considered in the discussion.

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