The results of this pilot study support the clinical effectiveness of unpredictable gait perturbations compared to walking-only treadmill training in improving dynamic balance and gait abilities in individuals with chronic stroke. Perturbation training had a significant and large effect on most variables in the Perturb group compared to no effect of walking-only in the NonPerturb group. In addition, large balance and gait improvements were also observed with perturbation training in the control (NonPerturb) group after the walking-only training program failed to produce improvements in balance and gait. The NonPerturb group results also underscore the superiority of the perturbation program, as the lack of improvement during the walking-only training program was not due to the inability of the control participants to improve (i.e. they were not at their maximum in their balance and gait abilities prior to the walking-only program).
To match variables across groups before training, we had to allocate the first nine participants to the Perturb group, which could have potentially increased the selection bias.(44) However, this did not deviate from the randomization method. This occurred because new potential participants did not match the characteristics of the participants previously included in the experimental group, and could thus not be allocated to the control group. Also, two participants allocated to the NonPerturb group could not participate in the control, walking-only, training program after randomization (the one participant had new medical issues, not related with the project and the other was concomitantly diagnosed with cancer). Therefore, despite no pre-training statistical difference between the groups, comfortable speed (.1 m/s) and RNLI (4%) were initially higher in the NonPerturb group, and dynamic balance was better (1.7/28) in the Perturb group. This likely affected between-group comparisons, mostly in favor of the walking-only training in the NonPerturb group. As a result, positive results presented here had to be strong to reach statistical significance.
Improvement in dynamic balance was the sole statistically significant difference, with a large effect size, between perturbation training in the Perturb group and walking-only training in the control, NonPerturb group. In comparison, walking-only training did not improve dynamic balance at group or individual levels. More specifically, perturbation training had a specific effect on reactive postural control and dynamic gait sub-systems, contrary to walking-only. Since reduced balance abilities is a major risk factor for falls,(10) these results may have an impact on the risk of falls; however, the number of falls post-training was not evaluated in the present study. Previous perturbation training programs among individuals with chronic and subacute stroke, consisted of 10-12 sessions during which perturbations were applied manually or by the antero-posterior or mediolateral translation of the support platform or treadmill in a standing position or during gait. (20-22, 45, 46) Improvements in reactive and proactive balance control after perturbation training(21, 45, 46), also observed in our results, and specifically for reactive activity and dynamic gait, likely explains better balance abilities, as determined by clinical evaluations such as the Berg Balance Scale (BBS) or the Timed Up and Go (TUG) test.(22) BBS was not used in this study because we targeted dynamic balance with the perturbation training and because of the ceiling effect of the BBS in individuals post-stroke.(3)
In addition to dynamic balance, gait speed also improved in the Perturb group. This improvement could be attributed to perturbation training since the speed on the treadmill was not increased in any group during training. Improvements after perturbation training post-crossover, compared to the absence of improvement post walking-only training in NonPerturb group, emphasize the beneficial effect of perturbation training on abilities in individuals with chronic stroke. Punt et al. also reported comfortable speed improvements, similar to our study (+0.16 m/s)(20). However, their training program included periods of gait at higher than comfortable gait speed,(19-22) which could have equally led to improved gait speed as much as did perturbations. It is therefore possible that the challenge posed by gait perturbations in the present study is a strong enough stimulus to improve the abilities required for both balance and gait. Contrary to our hypothesis, walking-only training did not improve gait abilities in the control group. This is likely explained by the good walking abilities of the participants pre-training (mean speed of 1.0m/s) and confirms the limited challenge that walking at a comfortable speed posed for this group.
Secondary outcomes also noticeably improved. Balance confidence increased with both types of training. Other studies found similar results between + 3.6 points and 10 points on the ABC scale, over 10 to 30 training sessions using moveable platform perturbations,(46) manual perturbation in a standing position,(19) and an agility exercise program designed to challenge dynamic balance. (14) However, this improvement may not be directly related to perturbation as balance confidence increased by similar amounts between perturbation- and walking-only training. The combination/duration of the two trainings periods that the NonPerturb group received may also have potentialized the effect on balance confidence. Longer periods of intervention indeed tend to provide greater improvement in balance confidence in older adults. (47) Lastly, maximal knee extensor strength improved in the Perturb group, but not during the secondary perturbation training in the NonPerturb group. However, dynamic balance improved in both groups after perturbation training. Higher maximal strength may thus not be a prerequisite for balance improvement, as underscored by the conflicting evidence of the effect of strength training on balance in a previous meta-analysis.(48)
Perturbation training in both groups led to an increased level of community reintegration as evaluated by the RNLI, with some sustained effect at 6 weeks. Improvement in RNLI scores after perturbation training supports the fact that improvements in balance abilities translated into better participation and mobility in the community. Previous studies that used progressive standing perturbations(19) or low-intensity gait perturbations(20) did not transfer to daily-life mobility. It is possible that the more intense and higher number of gait perturbations used in our study may have had a better effect on mobility and thus on community reintegration. These effects might be explained by the fact that these perturbations were applied during gait, at various and sometimes high intensities, in an unpredictable manner, and required adapted stepping reactions that could be used during loss of balance in daily life.(49) Such an effect in daily life may also explain the sustained effect at 6 weeks. It is to be noted that this result may also have been affected by the large variability of the pre-training mean score in the Perturb group. Also, both groups had an already good level of reintegration (i.e. low score) observed pre-training. Further studies are necessary to confirm increased community integration through objective measures and longer-term follow-up.
Despite the loss of balance induced by the perturbations and the intense postural reactions they triggered, a large majority of the participants felt in control during and between the perturbations and enjoyed the perturbation program, with very little discomfort. This might have been facilitated by the design of the program, with repeated perturbations followed by unpredictable perturbations, and by the possibility of producing small intensity perturbations at first, which then increased according to the participant’s comfort level. Such progression in the intensity of the perturbations was facilitated by the use of a treadmill. However, progression in level of difficulty might need to be more personalized as the perception of difficulty was reported by our participants as being between “Not difficult” and "Extremely difficult.” Despite this wide range of difficulty perception, most participants thought they were successful and improved their balance abilities during the training. The only other subjective evaluation found in the literature concerned the difficulty of the perturbation, which was rated as high as 7/10, with 10 representing a very difficult challenge. (21) In that study, only medio-lateral perturbations were used during gait, at the highest intensity possible without inducing a fall.