A Bilateral Adaptation during Locomotion Following a Unilaterally-

26 Human walking must be flexible enough to accommodate many contexts and goals. One 27 form of this flexibility is locomotor adaptation: a practice-dependent alteration to walking 28 occurring in response to some novel perturbing stimulus. Although studies have 29 examined locomotor adaptation and its storage by the central nervous system (CNS) in 30 humans, it remains unclear whether altered movements occurring in the leg contralateral 31 to a perturbation are caused by true practice-dependent adaptation or whether they are 32 generated via feedback corrective mechanisms. To test this, we recorded leg kinematics 33 and electromyography (EMG) from non-disabled adults as they walked on a treadmill 34 before, during and after a novel force was applied to one leg, which resisted its forward 35 movement during swing phase. The perturbation produced kinematic changes to 36 numerous walking parameters, including swing phase durations, step lengths and hip 37 angular excursions. Nearly all occurred bilaterally. Importantly, kinematic changes were 38 gradually adjusted over a period of exposure to the perturbation and were associated 39 with negative aftereffects upon its removal, suggesting they were adjusted through a true 40 motor adaptation process. In addition, increases in the EMG of both legs persisted even 41 after the perturbation was removed, providing further evidence that the CNS made and 42 stored changes to feedforward motor commands controlling each leg. Our results show 43 evidence for a feedforward adaptation of walking involving the leg opposite a 44 perturbation. This result may help support the application of locomotor adaptation 45 paradigms in clinical rehabilitation interventions targeting recovery of symmetric walking 46 patterns in a variety of patient populations. 47 48 49