Asymmetric Gait Patterns Alter the Reactive Control of Intersegmental Coordination Patterns during Walking

Recovery from perturbations during walking is primarily mediated by reactive control strategies that coordinate multiple body segments to maintain balance. Balance control is often impaired in clinical populations who walk with spatiotemporally asymmetric gait, and, as a result, rehabilitation efforts often seek to reduce asymmetries in these populations. Previous work has demonstrated that the presence of spatiotemporal asymmetries during walking does not impair the control of whole-body dynamics during perturbation recovery. However, it remains to be seen how the neuromotor system adjusts intersegmental coordination patterns to maintain invariant whole-body dynamics. Here, we determined if the neuromotor system generates stereotypical coordination patterns irrespective of the level of asymmetry or if the neuromotor system allows for variance in intersegmental coordination patterns to stabilize whole-body dynamics. Nineteen healthy participants walked on a dual-belt treadmill at a range of step length asymmetries, and they responded to unpredictable, slip-like perturbations. We used principal component analysis of segmental angular momenta to characterize intersegmental coordination patterns before, during, and after imposed perturbations. We found that two principal components were sufficient to explain ~ 95% of the variance in segmental angular momentum during both steading walking and responses to perturbations. Our results also revealed that walking with asymmetric step lengths led to changes in intersegmental coordination patterns during the perturbation and during subsequent recovery steps without affecting whole-body angular momentum. These results suggest that the nervous system allows for variance in segment-level coordination patterns to maintain invariant control of whole-body angular momentum during walking. Future studies exploring how these segmental coordination patterns change in individuals with asymmetries that result from neuromotor impairments can provide further insight into how the healthy and impaired nervous system regulates dynamic balance during walking.

160 first, we calculated the mean SLA of the four strides before each perturbation and then 161 distributed these mean values into five equally spaced bins centered at -15%, -10%, 0, 10%, 15% 162 with bin width equal to 5%. We used this achieved SLA instead of target SLA as the independent 163 variable in our statistical analyses. We categorized Baseline (BSL) steps as the two steps before 164 the perturbation occurred, perturbation (PTB) steps as the step during which the perturbation was 165 applied, and recovery (REC) steps as the steps that followed the perturbation. Since we did not

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( Here, m i is segmental mass, r CM-i is a vector from the segment's COM to the body's COM, 219 The included angle of the unit vectors was between 0° (parallel and identical) and 90° 220 (orthogonal and most dissimilar) [32].

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(3) We then determined if the included angle between perturbation steps and baseline steps 223 was outside the distribution of included angles observed during unperturbed baseline walking.
224 To this end, we performed a permutation test that randomly and repeatedly selected two groups

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We also determined if the differences in coordination observed during walking with 235 different levels of asymmetry were above the level of variance observed during symmetrical 236 walking. As described above, we obtained a reference distribution of included angles from 237 symmetric walking to determine if the included angle for each level of asymmetry was greater 238 than would be expected from natural, step-to-step variance.  As the magnitude of achieved asymmetry increased, we observed an increase in the 339 deviation of intersegmental coordination patterns from symmetrical walking ( Figure 5). Results 340 of log-likelihood ratio tests showed that random intercepts were required in the regression 341 models. One outlier was removed before fitting the linear mixed model for the perturbation step 342 for PC2 because it was more than three standard deviations higher than the median of the 343 included angles. Excluding the outlier did not change the statistical outcome. All included angles 344 differed from the permutated estimate of included angles (p<0.05), indicating that intersegmental 345 coordination at each level of asymmetry differed from the coordination pattern during 346 symmetrical walking. For all steps, we observed a significant main effect of asymmetry on the 347 included angle between the PCs from the asymmetric trials and the symmetric trial (Table 2).
348 Table 2 Statistical results from the ANOVA examining the effects of asymmetry and direction on 349 the included angle for each step type.
Step The included angle between the PCs extracted during asymmetric walking and symmetric 358 walking increased with the magnitude of achieved asymmetry ( Figure 5)

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Variations in coordination patterns during asymmetrical walking likely resulted from 378 changes in the momentum generated by the lower extremities to reach the target asymmetry.
379 Since the distal segments of the lower limbs are relatively far from the body's center of mass and 380 have a high velocity, they make the largest contribution to changes in intersegmental 381 coordination patterns. For example, to achieve a positive asymmetry, participants placed their