Effects of repeated walking in a perturbing environment: a 4-day

24 Previous studies have shown that when subjects repeatedly walk in a perturbing environment, initial 25 movement error becomes smaller, suggesting that retention of the adapted locomotor program occurred 26 (learning). It has been proposed that the newly learned locomotor program may be stored separately 27 from the baseline program. However, how locomotor performance evolves with repeated sessions of 28 walking with the perturbation is not yet known. To address this question, 10 healthy subjects walked on 29 a treadmill on 4 consecutive days. Each day, locomotor performance was measured using kinematics 30 and surface EMGs before, during and after exposure to a perturbation produced by an elastic tubing 31 that pulled the foot forward and up during swing, inducing a foot velocity error in the first strides. 32 Initial movement error decreased significantly between Days 1 and 2 and then remained stable. 33 Associated changes in Medial hamstring (MH) EMG activity stabilized only on Day 3, however. 34 Aftereffects were present after perturbation removal, suggesting that daily adaptation involved central 35 commands recalibration of the baseline program. Aftereffects gradually decreased across days but were 36 still visible on Day 4. Separation between the newly learned and baseline programs may take longer 37 than suggested by the daily improvement in initial performance in the perturbing environment, or may 38 never be complete. These results therefore suggest that reaching optimal performance in a perturbing 39 environment should not be used as the main indicator of a completed learning process, as central 40 reorganization of the motor commands continues days after initial performance has stabilized. 41 42