All subjects were able to complete the walking task without difficulty, regardless of group assignment. We first made sure there was no difference in baseline walking across groups after the brief exposure to the split-belt perturbation. Analyzing the last 30 seconds of the second tied epoch we found no difference in step symmetry (F(3,24)=0.24, p=0.85), center of oscillation difference (F(3,24)=0.57, p=0.64), or phasing (F(3,24)=2.02, p=0.14). The mean baseline values from the last 30s of this second tied epoch were subtracted from all subsequent data on day 1, thus referencing all data to each subject’s initial baseline asymmetry. The standard error of the baseline period is shown by the first points in – (day 1 is blue; day 2 is red). The ‘No Washout’ group did not complete a tied period on day 2, so values from day 2 were subtracted from the day 1 baseline. For all other groups, the last thirty seconds of tied walking on day 2 were used as a second baseline period and the mean values were subtracted from subsequent values on day 2. We found no difference in the second baseline across groups in step symmetry (F(2,18)=1.02, p=0.38), center of oscillation difference (F(2,18)=0.88, p=0.43), or phasing (F(2,18)=1.67, p=0.22).
Figure 2 Comparison of step symmetry adaptation across groups. Baseline, first stride, adaptation curves (smoothed by 3 strides) for a portion of training epochs (2–190), and final plateau values are shown. A. No Washout. Mean step symmetry values subtracted (more ...)
Figure 4 Comparison of phasing adaptation across groups. Averaged data for the No Washout (A), No Switch (B), Switch (C), and Switch Control (D) groups are shown as in . Baseline, first stride, adaptation curves for a portion of training epochs (2–190), (more ...)
In order to compare groups on day 2, we first assessed adaptation behavior and extent on day 1 to ensure that it was similar across groups. To quantify the initial perturbation we analyzed the first stride. No differences were found across groups: step symmetry (F(3,24)=0.28 p=0.84); center of oscillation difference (F(3,24)=0.43, p=0.74); phasing (F(3,24)=1.12, p=0.36); therefore, all groups were similarly perturbed. To assess early adaptation, we measured the early change (i.e. average of strides 2–30). No differences were found across groups: step symmetry (F(3,24)=0.27, p=0.85); center of oscillation difference (F(3,24)=0.32, p=0.81); phasing (F(3,24)=0.76, p=0.53). This indicates that subjects were perturbed and adapted similarly, regardless of group assignment. To verify that subjects adapted to similar levels by the end of training on day 1, we measured the plateau value (i.e. last thirty strides). The mean and standard errors of plateau values for each exposure are shown by the error bars at the end of the adaptation curves (only first 190 strides are shown) in –. For the ‘No Washout’ and ‘No Switch’ groups, where training was continuous for 15 minutes, this meant that we took the last thirty strides from the Day 1, Exposure 1 (blue symbols). For the ‘Switch’ and ‘Switch Control’ groups, we compared behavior after the complete 15 minutes of training (i.e. last 30 steps of the third exposure, yellow symbols). We found no differences in plateau in any of the parameters: step symmetry (F(3,24)=0.40, p=0.75), center of oscillation difference (F(3,24)=1.20, p=0.33), phasing (F(3,24)=0.40, p=0.76). Since all groups adapted similarly to the perturbation, and adapted the same amount on the first day, we concluded that day 1 behavior was the same across groups. In verifying that day 1 behavior was the same, we could assume that our measures assessing differences across days were due to changes in day 2 behavior.
shows our primary measure of step symmetry. Group means, smoothed by three strides, ± standard error are shown. We saw typical adaptation behavior on the first day, where subjects in all groups were initially perturbed by the split-belt and returned towards symmetric walking with increasing stride number. The first 190 steps (approximately five minutes) of adaptation are shown (in blue for day 1 and in red for day 2), averaging every three strides, with the final plateau shown at the end.
When subjects were brought back 24 hours later and immediately exposed to the split-belts (i.e. No Washout group, ), the walking pattern was very well-retained (i.e. compare blue plateau with first stride of the red curve). Additionally, subjects were much less perturbed and adapted faster on the second day, seen by comparing blue and red curves in .
Since we found substantial retention the following day in the ‘No Washout’ group and were concerned about a ceiling effect, we included a washout prior to re-adaptation for the remaining groups so that we could assess improvements from training structure. That is, on day two we washed out the adaptation done on day one. This meant that all subjects would be in a baseline state, so that we could assess changes in re-adaptation. We used the average of the last thirty strides from this day 2 baseline period as the reference for day two behavior (i.e. red bars prior to adaptation, –). Following this wash-out, we then assessed re-adaptation to split-belts on the second day.
shows how subjects re-adapted to the perturbation in the ‘No Switch’ group. Subjects in this group showed a much smaller change across days, but still retained some memory from the previous day’s training, as evidenced by the difference between the blue and red curves. In the ‘Switch’ group, subjects were allowed to adapt and washout their walking pattern multiple times on the first day, so they would be forced to continue switching their walking pattern between the adapted and de-adapted state. Behavior for the ‘Switch’ group is shown in , with each exposure plotted. Note that these subjects were initially perturbed by the split-belts in each exposure (i.e. large errors in beginning of blue, green, and yellow curves in ). Using a repeated measures ANOVA, we compared the plateau of the prior exposure to the first stride on the next exposure (e.g. blue plateau symbols and green first stride) and found a significant change (F(1,13)=48.02, p<0.001). Subjects then adapted to the perturbation with a reduction in errors as stride number increased. Interestingly, the rate at which they adapted became progressively faster with each exposure to the split-belts (i.e. compare blue, green, and yellow curves in ). With a repeated measures ANOVA of early change values on day 1, we found an effect of exposure (Greenhouse-Geisser corrected: F(1.1,6.7)=12.97, p=0.008). When these subjects returned the second day, they adapted faster than in any of the exposures on the first day. To test if simply interspersing breaks, rather than wash-out periods, between split-belt exposures was sufficient to influence re-adaptation, we tested a ‘Switch Control’ group that sat for five minutes between adaptation epochs. We did not observe similar increases in errors from time alone between exposures as we did when split-belt periods were interspersed with tied-belt washout periods in the ‘Switch’ group (e.g., compare blue plateau to first point of green curve, ). We did not find a significant change between plateau of one exposure to the first stride on the next exposure (F(1,13)=0.25, p=0.62).
Rather, subjects in the ‘Switch Control’ group returned to the adapted pattern they ended with, even after sitting for five minutes. In addition, while adaptation on day 2 was faster than in the first exposure to split-belts on day 1, it was comparable to the ‘No Switch’ group, and re-learning in ‘Switch Control’ was not as fast as in ‘Switch’.
shows a comparison of behavior on day 2 across the four groups. Re-learning appears to proceed faster in the ‘No Washout’ and ‘Switch’ groups, compared to ‘No Switch’ and ‘Switch Control’. We quantified the changes in step symmetry recall (i.e. difference in the first stride), re-learning (i.e. differences in early change), and performance (i.e. difference in plateau) across days in .
Figure 5 Difference between day 1 and 2 for first stride (shown in A, D, G), early change (average of strides 2–30 strides – shown in B,E, H) and plateau (average of last 30 strides – shown in C, F, I). Bars represent group means ± (more ...)
First, we found an effect of day on recall of step symmetry (F(1,24)=25.46, p<0.001). Subjects were perturbed significantly less in the first stride across days, which means that they were able to immediately recall some of the split-belt pattern. The no washout group looks to have the best recall, though we did not find an interaction of group × day on the first stride across days (F(3,24)=1.01, p=0.41). This means that although subjects on a whole were able to immediately recall the pattern, group assignment did not affect the first stride. One contributing factor to this result may be the high variability of the first stride across subjects. As such, we looked at the first 3 or first 5 strides to see if they might be less variable, and still suitable for measuring recall. Unfortunately, the different groups changed a different amount within the first 3 or 5 strides. For example, from stride 1 to 3, the no washout group changed 23% of their initial perturbation, the washout group 5%, the switch group 11% and the switch control group 6%. If we analyze the mean of the first 3 strides as reflecting “recall,” the effect is largely driven by this early change rather than what subjects recall on the first stride. This is also true for the first 5 strides. Given this information, we think that the first stride on day 2 is the best indicator of recall, despite variability.
Second, we found an effect of day on re-adaptation of step symmetry (F(1,24)=182.14, p<0.001). All groups showed a significant decrease in the early change across days (all p<0.01), signifying that all groups had improved re-learning on the second day. Third, there was a significant interaction effect found between groups for step symmetry early change across days (F(3,24)=10.95, p<0.001). As expected, we observed the fastest re-learning in the ‘No Washout’ group, since they immediately returned to split-belts on day 2. In post hoc analysis, the ‘No Washout’ group was significantly different from ‘No Switch’ (p<0.001) and ‘Switch Control’ (p<0.001), while there was a trend of a difference from ‘Switch’ (p=0.06). The ‘No Switch’ and ‘Switch Control’ groups were similar (p=0.97), thus time alone between exposures did not improve second day re-learning. The ‘Switch’ group was significantly different from both the ‘No Switch’ (p<0.01) and ‘Switch Control’ (p<0.01) groups. Finally, we found no differences in the changes in plateau values across groups (F(3,24)=1.55, p=0.23). Therefore, in sum we found that all groups recalled (i.e. first stride) the split pattern similarly. For the three groups that experienced a washout (‘No Switch’, ‘Switch’, and ‘Switch Control’), re-learning was best in the ‘Switch’ group, approaching similar early change values as what we observed when subjects were immediately re-exposed to the split-belts (i.e. ‘No Washout’ group). Despite this difference in re-learning (i.e. early change), all groups eventually converged to the same level of performance (i.e. plateau).
To ensure that our effect of re-learning wasn’t just due to a difference in the first stride, we used exponential curve fits to quantify step symmetry adaptation rates on day 2. A curve fit analysis which would assess re-learning rate without any influence from the starting point. The data with the fits overlaid are shown in . Time constants with 95% confidences intervals revealed that the ‘No Washout’ group re-learned the fastest (t=5.7 [3.36,8.04], r2=0.62), followed by ‘Switch’ (t=9.87 [8.40,11.37], r2=0.92), and then ‘No Switch’ (t=18.2 [15.2,21.1], r2=0.91), and ‘Switch Control’ (t=17.76 [15.03,20.52], r2=0.92).
Step symmetry can be influenced by changes in spatial (i.e. center of oscillation difference) and temporal (i.e. phasing) measures of coordination (see (Malone and Bastian 2010
)). Center of oscillation difference adaptation curves are shown in . Similar trends were noted where the ‘No Washout’ group demonstrated the fastest re-learning on day 2 (). In the three remaining groups, the ‘Switch’ group showed the fastest re-learning () and the slowest re-learning was observed in the ‘No Switch’ and ‘Switch Control’ groups (). We found a significant effect of day on the first stride (F(1,24)=24.62, p<0.001), but not a significant interaction effect across groups (F(3,24)=1.74, p=0.19). Additionally, there was a significant effect of day for early change (F(1,24)=73.48, p<0.001). Post hoc results showed a significant decrease across days for all groups (all p<0.05). When assessing differences in early change across days (), we also found a significant interaction effect across groups (F(3,24)=4.89, p<0.01). Post hoc analysis revealed a significant difference between ‘No Washout’ and ‘No Switch’ (p<0.01), and ‘No Washout’ and ‘Switch Control’ (p<0.01), but not between the ‘No Washout’ and ‘Switch’ groups (p=0.11). As with step symmetry, we found that re-learning was faster with ‘Switch’ training compared with ‘No Switch’ and ‘Switch Control’ (), but these differences did not reach significance (p=0.14 and 0.10, respectively). There was no difference found between ‘No Switch’ and ‘Switch Control’ (p=0.88). Additionally, we found no difference in the center of oscillation difference plateau across groups (F(3,24)=1.20, p=0.33) (). Overall, we saw trends in the spatial elements of limb motions that were similar to the findings in step symmetry.
Figure 3 Comparison of adaptation of center of oscillation difference across groups. Averaged data for the No Washout (A), No Switch (B), Switch (C), and Switch Control (D) groups are shown as in . Baseline, first stride, adaptation curves for a portion (more ...)
In contrast, we saw little effect of training in the temporal parameter, phasing. We did not find a significant effect of day (F(1,24)=0.25, p=0.62) or day × group (F(3,24)=0.61, p=0.61) on the first stride. However, we did find a day effect for early change (F(1,24)=33.76, p<0.001), but post hoc analysis only found a difference in the ‘No Washout’ (p<0.001) and ‘Switch Control’ (p=0.03) groups. The difference in early change values between day 1 and 2 did not reach significance for the ‘No Switch’ (p=0.23) and ‘Switch’ (p=0.15) groups. The ‘No Washout’ group re-adapts the temporal pattern much faster on the second day (), while the remaining three groups showed comparable re-learning (). For our early change measure, we found a significant effect across groups (F(3,24)=6.01, p<0.01), but this was due to the ‘No Washout’ group being significantly different from all other groups (all p<0.01) (). Similar to all the other parameters, we found no difference in the plateau for phasing (F(3,24)=0.40, p=0.76) (). Therefore, although subjects were able to retain the temporal pattern the second day (i.e. ‘No Washout’), the training method on the first day seemed to have little influence on phasing when subjects were washed out prior to re-adaptation.