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. Author manuscript; available in PMC: 2020 Oct 1.
Published in final edited form as: Clin Biomech (Bristol). 2019 Jul 26;69:205–214. doi: 10.1016/j.clinbiomech.2019.07.031

Anterior Fall-Recovery Training Applied to Individuals with Chronic Stroke.

Jamie Pigman 1, Darcy S Reisman 2, Ryan T Pohlig 3, John J Jeka 1, Tamara R Wright 2, Benjamin C Conner 1,4, Drew A Petersen 1,5, Jeremy R Crenshaw 1
PMCID: PMC6823156  NIHMSID: NIHMS1536500  PMID: 31382163

Abstract

Background:

To study the effects of the initial stepping limb on anterior fall-recovery performance and kinematics, as well as to determine the benefits of fall-recovery training on those outcomes in individuals with chronic stroke.

Methods:

Single-group intervention of 15 individuals with chronic stroke who performed up to six sessions of fall-recovery training. Each session consisted of two progressions of treadmill-induced perturbations to induce anterior falls from a standing position. Progressions focused on initial steps with the paretic or non-paretic limb. Fall-recovery performance (the highest disturbance level achieved and the proportion of successful recoveries), as well as step and trunk kinematics were compared between the initial stepping limbs on the first session. Limb-specific outcomes were also compared between the first and last training sessions.

Findings:

There were no between-limb differences in fall-recovery performance in the first session. With training, participants successfully recovered from a higher proportion of falls (p’s = 0.01, Cohen’s d’s > 0.7) and progressed to larger perturbation magnitudes (p’s < 0.06, d’s > 0.5). Initial steps with the paretic limb were wider and shorter relative to the center of mass (p’s < 0.06, d’s > 0.5). With training, initial paretic-limb steps became longer relative to the CoM (p = 0.03, d = 0.7). Trunk forward rotation was reduced when first stepping with the non-paretic limb (p = 0.03, d = 0.6).

Interpretation:

The initial stepping limb affects relevant step kinematics during anterior fall recovery. Fall-recovery training improved performance and select kinematic outcomes in individuals with chronic stroke.

Keywords: Balance, Falls, Postural Instability, Rehabilitation, Stroke, Perturbation Training

1. Introduction

Up to 75% of those living with stroke fall each year (Batchelor et al., 2012; Forster and Young, 1995; Gordon and Morris, 2008), and individuals with chronic stroke have a fall risk that is twice that of age and sex matched peers (Jorgensen and Jacosen, 2002). Approximately 84% of fractures in this population are due to accidental falls (Ramnemark et al., 1998). Up to 80% of chronic stroke survivors express a fear of falling (Andersson et al., 2008). Having reduced falls self-efficacy limits physical activity, engagement, and participation in the free-living environment after stroke (Davenport et al., 1996; Hellström et al., 2003; Pang et al., 2007; Robinson et al., 2011; Schmid et al., 2012). Therefore, an intervention that reduces the risk of falling is likely an important component to preventing injury and improving the independence, quality of life, and the health of those living with chronic stroke.

Survivors of stroke have an impaired fall-recovery response. They demonstrate lower anterior multiple-stepping thresholds, defined as the perturbation magnitude that elicits more than one recovery step (de Kam et al., 2017). In response to larger treadmill-induced falls, individuals with chronic stroke have a reduced ability to limit trunk rotation (Patel and Bhatt, 2018). When stepping to avoid a fall, those with chronic stroke prefer to step with their non-paretic limb (Honeycutt et al., 2016; Mansfield et al., 2012; Martinez et al., 2013). The inability to take a recovery step with the paretic limb has been prospectively related to falls in the free-living environment (Mansfield et al., 2015c). Despite this distinct and relevant influence of limb function asymmetry on fall-recovery after stroke, the effects of the stepping limb (i.e. paretic or non-paretic) on subsequent fall-recovery kinematics (e.g. trunk rotation, foot placement relative to the whole-body center of mass (CoM)) are not known. Such kinematics are important, as they are determinants of fall-recovery success (Crenshaw et al., 2012; Honeycutt et al., 2016; Owings et al., 2001; Pavol et al., 2001).

Successful recovery from a trip-induced fall is often dependent upon step placement and the ability to limit trunk forward rotation (Crenshaw et al., 2012; Honeycutt et al., 2016; Pavol et al., 2001). The extent to which the compensatory stepping response of those with chronic stroke can be improved with repetitive practice is not well understood. Individuals with subacute stroke improved their reactive balance response, as quantified by multiple-stepping thresholds, after performing twelve sessions of perturbation-based training (Handelzalts et al., 2019). With a six-week intervention that included external perturbations (e.g. manual pushes from a therapist), those with chronic stroke demonstrated higher scores on the reactive balance subset of the mini-BESTest compared to a control group receiving traditional therapy (Mansfield et al., 2018). In a separate study, individuals with chronic stroke improved their reactive step kinematics in all directions after ten sessions of perturbation-based training (Van Duijnhoven et al., 2018). With training, the step was placed further beyond the pelvis in response to lean-release perturbations, as well as in response to rapid surface translations. These studies suggest that such training can improve the stepping response stroke survivors. Although, the limb-specific adaptations with training (i.e. those specific to steps with the paretic or non-paretic limb) have not been evaluated.

The purpose of this study was to assess the effects of the initial stepping limb (i.e. paretic or non-paretic) on anterior fall-recovery performance and kinematics, as well as to determine the benefits of specific fall-recovery training on those outcomes for individuals with chronic stroke. We hypothesized that, after a standing perturbation, compensatory steps with the paretic limb would be associated with worse fall-recovery performance and kinematics. We also hypothesized that such aspects would improve with specific fall-recovery training.

2. Methods

2.1. Participants

We recruited eighteen participants with a unilateral chronic stroke from the University of Delaware’s Stroke Studies Registry. This study was approved by the University of Delaware’s Institutional Review Board, and all participants provided written informed consent prior to participation. Additionally, participants and research staff that appear in photographs and videos gave written consent. Fifteen (out of 18) of these participants who completed at least five of the six training sessions were included in this analysis (12 males, 3 females, Hemisphere of stroke: 12 right, 3 left) (Table 1). An a priori sample size was determined for this study for power (power = 0.80) to detect a large effect size (Cohen’s d > 0.77) that exceeded those observed from a preliminary sample of nine participants. Exclusion criteria included other neurologic disorders, musculoskeletal surgeries within the past year, recent cardiovascular events (past three months), or other conditions that precluded safe participation (i.e. insulin-dependent diabetes, pregnancy, use of an ostomy pouch). Participants had the self-reported ability to walk a city block without a gait aid such as a walker or cane. Those who were 50 years of age or older underwent a Dual-energy X-ray absorptiometry (DXA) screening to ensure that they were not osteoporotic (total hip or femoral neck bone mineral density t-score < −2.5) (Kanis et al., 2009). This screening criterion, which has been used previously in studies of older adults (Crenshaw et al., 2018), was conservatively in place to reduce the risk of fractures from the impact of fall-recovery steps or falls into the safety harness. No individuals were excluded from the study due to DXA screening. Of note, two participants wore articulating ankle foot orthosis during training that they typically wore on a day-to-day basis. We anticipated that removing the orthosis for training may have presented an unreasonable injury risk.

Table 1.

Demographic and clinical assessment data

Measure Mean (SD), Range
Age (Years) 57 (12), 29 – 77
BMI (kg/m2) 28.6 (3.6), 22.0 – 33.9
Years after stroke 5 (3.5), 2 – 15
Fugl-Meyer LE 24 (6), 8 – 32
Activities Specific Balance Confidence Scale (ABC) 91 (8), 76 – 100
Functional Gait Assessment (FGA) 17 (6), 9 – 29
Berg Balance Scale (BBS) 50 (7), 36 – 56
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Note: Prior to starting fall-recovery training, descriptive measures of the Fugl-Meyer Lower Extremity assessment (Fugl Meyer et al., 1975), Activities-Specific Balance Confidence (ABC) scale (Powell and Myers, 1995), Berg-Balance Scale (Berg et al., 2009), and the Functionai Gait Assessment (Wrisley et al., 2004) were used to characterize our participants balance and mobility.

2.2. Training Protocol

Our training protocol was modified from previous interventions aimed at benefitting older adult women (Bieryla et al., 2007; Grabiner et al., 2014) and individuals with lower-extremity amputations (Crenshaw et al., 2013; Kaufman et al., 2014). The perturbations delivered within our training were designed to elicit the rapid, coordinated stepping response similar to that of trip recovery (Owings et al., 2001). All participants attempted to complete six sessions of the training protocol. The sessions included two progressions of treadmill belt perturbations that induced anterior falls (ActiveStep®, Simbex, Lebanon, NH). Progressions within a training session focused on initial steps with the non-paretic limb (Figure 1) or paretic limb. These progressions were limited to either 15 minutes or 36 perturbations, whichever occurred first, with rest periods lasting approximately five minutes between each progression. In addition, two progressions that focused on posterior fall recovery were delivered within each session. The results from these latter progressions will be reported in a subsequent paper (In Review). Progression durations were determined to reasonably limit fatigue and to keep training sessions within an hour. Six training sessions occurred over approximately three weeks. This number of sessions meets or exceeds that of many previous applications of fall-recovery training in older adults, those with lower-extremity amputations, and those with acute stroke (Bieryla et al., 2007; Crenshaw et al., 2013; Kaufman et al., 2014; Mansfield et al., 2018, 2015b; Rosenblatt et al., 2010).

Figure 1.

Figure 1.

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An individual with chronic stroke performs trip-recovery training. Treadmill-induced perturbations were applied to standing participants necessitating steps to prevent a fall into the safety harness. The top series of images is from the first training session and depict a failed trip recovery when initially stepping with the non-paretic limb at a perturbation magnitude of 4.25 m/s2. The bottom series of images is from the final training session and depict a successful trip recovery when initially stepping with the non-paretic limb at the same perturbation magnitude that previous caused a fall into the safety harness. Note the amount of forward trunk rotation in the failed recovery compared to the successful recovery.

Participants wore well-cushioned, closed-toe shoes with no elevated heels. They were outfitted with a full-body safety harness (Delta™, Capital Safety, Bloomington, MN) attached to a custom-built overhead rail system. The support straps were adjusted so that the participant’s hands and knees could not come into contact with the treadmill. The harness was instrumented with a force transducer (Dillon, Fairmont, MN), the peak forces of which were recorded for each trial.

When awaiting a perturbation, participants stood self-supported on the treadmill without the use of handrails (Figure 1). They placed their feet at a comfortable width, with their toes evenly positioned in the anteroposterior direction. The perturbation velocity waveforms consisted of an initial, 500 ms acceleration followed by a deceleration phase at 0.38 m/s2. Participants were instructed to “try not to fall” in response to these perturbations, and to specifically step with the targeted limb. No specific instructions or feedback pertaining to step kinematics were provided. The first perturbation of each progression had an initial acceleration of 0.5 m/s2, resulting in a displacement of 0.06 m. After a successful recovery, the subsequent perturbation had an initial acceleration 0.25 m/s2 greater than the previous perturbation (Crenshaw et al., 2013). After a failed recovery, the subsequent trial acceleration was reduced by 0.25 m/s2. Failures were defined as responses in which the force transducer attached to the harness recorded more than 20% body weight (Cyr and Smeesters, 2009), as well as responses in which the participant stepped with the wrong limb. Each perturbation was preceded by a 1 - 5 s delay to limit pre-planned timing of the response. Additionally, small perturbations (0.3 ms duration, 0.03 m displacement) resulting in a posterior fall were introduced approximately once every six trials to limit anticipatory adjustments. Participants were asked to inform research staff if the training intensity became too much for them to tolerate (i.e. minor muscle soreness, general fatigue, or uneasiness being on the treadmill). In such cases, we attempted to continue training at the highest perturbation magnitude tolerated for the remainder of the session. This approach was intended to maintain compliance with study participation while promoting practice repetitions.

All trials were recorded with a 12 camera motion capture system operating at 120 Flz (Motion Analysis®, Santa Rosa, CA, replaced mid-study with Qualisys®, Göteborg, Sweden). The positions of thirty-five passive-reflective markers facilitated the definition of 13 body segments: head/neck, trunk, pelvis, upper arms, forearms, thighs, shanks, and feet. Marker trajectories were filtered via a fourth-order Butterworth filter with a low pass 6 Hz cutoff.

2.3. Analysis

Fall-recovery performance was quantified from the proportion of successful recoveries and the highest perturbation magnitude achieved within a session. To determine how lower-extremity impairment affected performance at baseline, we compared the separate progressions of stepping with the paretic or non-paretic limb within the first training session. To evaluate if these measures changed with training, we compared limb-specific outcomes on the first and last training sessions.

Custom LabView software (National Instruments, Austin, TX) was developed to calculate kinematic variables. We focused primarily on the anterior and lateral distances between the stepping limb toe marker and the whole-body CoM at step contact with the treadmill (“Step length CoM” and “Step width CoM”, respectively), as well as the maximum trunk forward rotation angle and angular velocity relative to the standing, starting position (positive values indicate forward rotation). These and additional step and trunk kinematic variables are defined and reported in the Appendix. To determine if stroke-related lower-extremity impairment affected these variables, we compared paretic and non-paretic limb stepping responses from the first training session. To remove the confounding effect of perturbation magnitude, we evaluated successful responses to the largest common disturbance magnitude across limbs. In addition, limb-specific kinematic outcomes were compared on the first and the last sessions to evaluate a training effect. Within each initial stepping limb, successful responses to the highest common disturbance magnitude across sessions were evaluated. Because a single researcher (JP) conducted all training in this study, the evaluator was not blinded to these outcomes. All comparisons were evaluated using paired t-tests and effect sizes (Cohen’s d for repeated measures). All statistics were evaluated using SPSS 25 (IBM, Armonk, NY). Alpha was set to 0.05, and assumptions of the t-test were tested and satisfied.

3. Results

Fourteen out of the eighteen participants successfully completed all six training sessions. One participant only completed five sessions due to an acute illness prior to the sixth session, and scheduling conflicts prevented the rescheduling of the sixth session in a timely manner (greater than 30 days). For this participant, the fifth training session was considered as the “last” session in our analysis. There were no serious adverse events. Two participants voluntarily withdrew from the study on the first training session, stating that they were not comfortable continuing with the training (Fugl-Meyer LE – 34, 32, ABC – 92, 73, FGA – 12, 13, BBS – 49, 46). A third participant performed three fall-recovery training sessions. This participant, however, withdrew from the study due to an unanticipated seizure that occurred outside of our study.

3.1. Performance-Based Outcomes

There were no between-limb differences in the proportion of successful responses, or for the highest disturbance magnitude achieved in the first training session (Table 2). Across sessions, participants successfully recovered from a higher proportion of falls, and progressed to significantly larger perturbation magnitudes when initially stepping with the paretic limb (all p’s < 0.06). During the first session, stepping with the wrong limb caused 24% and 12% of failures for paretic and non-paretic limbs respectively; this was reduced to 10% and 3% in the last session. Five participants experienced falls, fully engaging the safety harness. Observationally, by the last session these five participants successfully recovered from the same perturbation magnitudes that originally caused them to fall (Figure 1) (Video). The initial treadmill belt acceleration associated with these falls into the safety harness ranged from 1.5 m/s2 to 4.5 m/s2.

Table 2.

Between-limb performance on the first day of training and changes with training between-sessions.

Anterior (Simulated Trips) Variable Initial Step Limb First Session p-value (Cohen’s d) Change w/training p-value (Cohen’s d)
% Successful Trials (%) Non-Paretic 88 (19) 0.20 (0.5) +9 (12) 0.01 (1.0)*
Paretic 76 (27) +13 (17) 0.01 (0.7)*
Largest Perturbation (m/s2) Non-Paretic 3.5 (1.0) 0.14 (0.5) +0.4 (0.8) 0.06 (0.5)
Paretic 2.9 (1.4) +0.4 (0.4) 0.001 (1.0)*
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Note: First session, and change with training data are displayed as mean (SD).

*

Significant (p < 0.05) between-session differences from the first session and the last sessions of training.

3.2. Kinematic Variables

In the first training session, participants demonstrated significantly wider steps relative to the whole-body CoM when initially stepping with the paretic limb (Table 3, Appendix Figure 1B and C). Although not significant, there was a trend of shorter initial step lengths relative to the whole-body CoM when stepping with the paretic limb (p = 0.06, d = 0.6) (Table 3, Appendix Figure 1A).

Table 3.

Between-limb kinematic variables on the first training session.

First Step Limb
Non-Paretic Paretic p-value (Cohen’s d)
Step length CoM (cm) 28.1 (5.4) 25.2 (6.2) 0.06 (0.6)
Step width CoM (cm) 11.5 (3.2) 14.4 (3.4) 0.01 (0.8)*
Second Step Limb
Paretic Non-Paretic p-value (Cohen’s d)
Step length CoM (cm) 24.5 (8.6) 26.4 (9.6) 0.51 (0.2)
Step width CoM (cm) 15.7 (5.0) 12.3 (3.8) 0.01 (0.7)*
Trunk Kinematics
Peak trunk forward rotation angle (deg) 24.6 (9.3) 31.4 (19.7) 0.09 (1.1)
Peak trunk forward rotation angular velocity (deg/s) 104.0 (38.8) 105.8 (30.6) 0.80 (0.1)
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Note: Non-paretic limb and paretic limb data are displayed as mean (SD).

*

Significant (p < 0.05) between-limb differences on the first session of training at a common perturbation magnitude between limbs.

Compensatory step kinematics, as well as trunk kinematics improved from the first to the last sessions at a highest common perturbation magnitude. First step lengths relative to the whole-body CoM became longer when stepping with the paretic limb (Table 4, Appendix Figure 3). Peak trunk forward rotation angles were reduced with training when initially stepping with the non-paretic limb (Table 4, Appendix Figure 4).

Table 4.

Between-session kinematic variables from the first and last sessions.

First Step: Non-Paretic Limb First Step: Paretic Limb
First Session Change w/Training p-value (Cohen’s d) First Session Change w/Training p-value (Cohen’s d)
Step length CoM (cm) 29.1 (6.9) −0.8 (5.5) 0.61 (0.1) 23.8 (6.1) +4.3 (6.7) 0.03 (0.7)*
Step width CoM (cm) 11.7 (3.4) −0.9 (3.2) 0.31 (0.3) 13.9 (4.7) −0.9 (4.3) 0.42 (0.2)
Second Step: Paretic Limb Second Step: Non-Paretic Limb
First Session Change w/Training p-value (Cohen’s d) First Session Change w/Training p-value (Cohen’s d)
Step length CoM (cm) 23.9 (9.2) +2.7 (7.2) 0.17 (0.4) 29.5 (6.8) +1.0 (7.2) 0.62 (0.2)
Step width CoM (cm) 13.7 (3.4) −1.2 (3.1) 0.16 (0.4) 11.6 (3.7) −0.1 (4.0) 0.89 (0.3)
Trunk Kinematics Trunk Kinematics
First Session Change w/Training p-value (Cohen’s d) First Session Change w/Training p-value (Cohen’s d)
Peak trunk forward rotation angle (deg) 25.2 (8.4) −2.5 (4.0) 0.03 (0.6)* 29.0 (12.6) −3.0 (8.3) 0.20 (0.4)
Peak trunk forward rotation angular velocity (deg/s) 114.5 (40.5) −15.2 (32.0) 0.08 (0.5) 108.1 (36.6) +0.4 (32.3) 0.96 (0.02)
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Note: First session, last session, and change with training data are displayed as mean (SD).

*

Significant (p < 0.05) between-session differences from the first session and the last sessions of training at a common perturbation magnitude within each limb. First-session perturbation magnitudes considered here may not correspond with those considered in Table 3 (between-limb comparisons).

4. Discussion

The purpose of this study was to assess the effect of the initial stepping limb (i.e. paretic or non-paretic) on anterior fall-recovery performance and kinematics, and then determine the benefits of fall-recovery training on those outcomes. We hypothesized that compensatory steps with the paretic limb would be associated with worse fall-recovery performance and kinematics, but such aspects would improve with fall-recovery training. We found that between-limb differences in stepping were most pronounced in the frontal plane, with first and second paretic-limb steps being placed wider relative to the whole-body CoM. When stepping initially with the paretic limb, we observed training-based increases in anterior step placement relative to the whole-body CoM. When stepping initially with the non-paretic limb, we observed a reduction in trunk forward rotation. To our knowledge, this is the first report of kinematic adaptations in anterior fall recovery specific to initial steps with the paretic or non-paretic limb.

The initial stepping limb did not influence performance-based variables of fall-recovery (Table 2). This is a surprising result given the previous observation that survivors of stroke avoid stepping with their paretic limb (Honeycutt et al., 2016; Mansfield et al., 2012; Martinez et al., 2013) as well as the link between paretic-limb stepping and subsequent falls in the free-living environment (Mansfield et al., 2015c). Perhaps a lack of between-limb differences is due to our sample of relatively high-functioning participants (Table 1). Alternatively, the effects of the stepping limb on fall-recovery performance may have been diminished by practice that accompanies progressively difficult repetitions within one training session. Moreover, our protocol assessed paretic-limb stepping as part of a second progression after that focused on non-paretic limb stepping. Similar reasons may underlie the lack of significant between-limb differences in sagittal-plane kinematics (Table 3). However, large, yet non-significant between-limb effect sizes in these variables suggest that the detrimental effects of stepping with the paretic limb may exist, especially if considering a more impaired population.

We did observe significant effects of the initial stepping limb on frontal-plane kinematics. Participants demonstrated wider steps relative to the whole-body CoM when first stepping with the paretic limb (Table 3). This difference was observed despite no between-limb differences in absolute step width (p = 0.41, d = 0.2, Appendix Table 2). So, the observed differences in foot placement relative to the CoM are likely due to altered trajectories of the CoM itself. Previous studies have verified that stepping to recover from an anterior fall challenges lateral stability due to a limited postural adjustment before toe off (McIlroy and Maki, 1999). In a study of older adults, those with a history of falling demonstrated more lateral CoM motion and wider initial steps after an anterior waist pull compared to that of non-fallers and young adults (Rogers et al., 2001). In our study, the narrower distance between the CoM and non-paretic stepping foot may be influenced by slight asymmetric weightbearing before the disturbance, or the narrower foot placement may reflect an impaired ability of the paretic stance limb to generate frontal-plane moments that resist lateral motion. The wider paretic-limb steps may represent a conservative means to maintain stability when on the paretic limb due to its limited strength, control, and joint stability. We do not know if altering frontal plane kinematics improves fall-recovery success, nor do we know if this is a modifiable variable in those with chronic stroke.

With training, participants improved their fall-recovery performance. From the first to the last session, participants successfully recovered from a higher proportion of recoveries when stepping with either limb. From the first to the last session, participants progressed to larger perturbation magnitudes only when initially stepping with the paretic limb (Table 2). In this context, first-step lengths relative to the whole-body CoM became longer with practice. These results were accompanied by moderate increases in the absolute paretic-limb step length (p = 0.11, d = 0.4) and little change in the trunk orientation at first step contact (p = 0.74, d = 0.1; Appendix Table 3). So, the primary modification to the first step was in its length, an aspect that changed step position relative to the CoM and may have benefitted peak trunk rotation occurring later in the response. This benefit to peak trunk rotation, however, was small (−2.5 (4.0) degrees, Table 4). Previous studies that have compared treadmill-induced falls to successful recoveries in healthy adults demonstrated much larger differences in trunk angles («42 degrees) (Crenshaw et al., 2012). We cannot conclude that the reductions in trunk rotation observed for successful responses in our study are meaningful. However, we hypothesize that training-based improvements in trunk control would be more profound if we evaluated larger perturbations that could result in a failed response. Our observations of limb-specific adaptation are similar to those in which individuals with unilateral, lower-extremity amputations improved step placement when stepping with their prosthetic limb (Crenshaw et al., 2013). From our results across studies, it is our interpretation that the training adaptation likely occurs in the less-affected stance limb during the first step. Initial step length is an important factor, as it has discriminated fallers from non-fallers in older adults (Owings et al., 2001) as well as in those with chronic stroke (Honeycutt et al., 2016). Previous research has shown that, in response to anterior falls, those with stroke take longer compensatory steps to compensate for reduced trunk control (Patel and Bhatt, 2018). Of note, the disturbance magnitudes in this previous study were small (20 cm). We anticipated that, with larger disturbances that necessitate longer steps, the ability to compensate for poor trunk control may be diminished.

Against our hypothesis, initial step lengths relative to the CoM shortened with training when stepping with the non-paretic limb. Perhaps some, but not all participants adapted by reducing the initial step length and time. In a secondary analysis, 4 of 14 participants had a notable reduction (>5 cm) in initial absolute step length (5 – 14 cm change) and step time (25 – 116 ms change). This strategy seems to be beneficial, as evident by concurrent reductions in peak trunk rotation angles (1 – 8 degrees) and angular velocity (13 – 96 degrees/s). We do not know if this step-shortening strategy should be encouraged, as it may not always be viable in the free-living environment. Perhaps the use of a small obstacle in front of the stepping limb would curtail this type of adaptation.

With training, peak trunk forward rotation angles were reduced when initially stepping with the non-paretic limb. The ability to limit forward trunk rotation is critical to successful recovery from an anterior fall (Crenshaw et al., 2012; Honeycutt et al., 2016; Owings et al., 2001; Pavol et al., 2001). On the last training session, when initially stepping with the non-paretic limb, peak forward rotation occurred at 890 (296) ms after disturbance onset, much after initial step placement at 437 (41) ms. So, it is likely that the first as well as second steps played a substantial role in reducing trunk forward rotation. This evidence suggests that exercise interventions aimed to reduce anterior falls in this population should include perturbations that necessitate a multiple-stepping response.

We don’t know if perturbation-based training can reduce falls in those with chronic stroke. Based on a review of 404 older adults and individuals with Parkinson’s disease, such training reduces the risk of falling by 30% (risk ratio 0.71, 95% CI 0.52 – 0.96) (Mansfield et al., 2015a). To our knowledge, one study has evaluated fall-recovery training effects in individuals with chronic stroke (Mansfield et al., 2018). In this randomized controlled trial, perturbation-based training and traditional balance-training groups did not differ in their post-training fall rates. However, as with our study, limitations included participants with relatively high baseline levels of function. Additionally, the therapist-induced perturbations of the previous study, although more feasible than the treadmill-induced presented here, were limited in intensity compared to our approach. We also showed kinematic adaptations to the second recovery step, an aspect which may not have been observed in the previous study which discouraged multistep responses. Therefore, further study is warranted on the benefits of fall-recovery training in this population. It appears that the benefit to fall-recovery may be specific to the fall directions applied during training. Older women who underwent training focused specifically on the trip-recovery response, similar to that reported in this study, reduced the rate of trip-related falling in the laboratory by 86% (Grabiner et al., 2012). This form of training also reduced trip-related falls in the free-living environment (rate ratio 0.54, 95% Cl 0.30 – 0.97) despite no effects on the number of stumbles experienced (rate ratio 0.82, 95% CI 0.62 – 1.11) (Rosenblatt et al., 2013). These results demonstrate that training directly improved the fall-recovery response from a trip, and not the awareness and avoidance of tripping hazards.

Our study was limited in that we did not conduct a controlled experiment, so we cannot conclude that observed changes were due to the training itself. It may be that these aspects improved due to confounding influences, such as interactions with study staff, familiarity with the treadmill, or the general benefits of more activity due to study participation. A controlled experiment with distinct pre- and post-training evaluations is needed to gain more insight on the benefits of this intervention relative to other forms of exercise. The use of single- and multiple-stepping thresholds (Crenshaw and Kaufman, 2014) or the mini-BESTest (Franchignoni et al., 2010) would provide objective, direction-specific measures of balance-reaction skill. Given that the effects of stroke are dependent on the injury location and severity, initial fitness of the person, and intensity of previous rehabilitation, this population presents with a wide range of function. Aspects such as lower-extremity impairment, or age may alter responsiveness to our training. These factors, then, would serve as ways to stratify groups in a controlled experiment. Further study is needed to identify such factors. Given the high-functioning status of our participants (Table 1), we must continue to evaluate the feasibility of this training with lower-functioning participants, particularly those with a high fear of falling, low falls self-efficacy, and those that rely on walking aids such as a cane or walker.

5. Conclusions

In conclusion, our study demonstrates the specific means by which anterior fall recovery can be modified with practice in those with chronic stroke. We investigated responses to large perturbations requiring multiple steps to regain stability. This aspect is important given the similarity of kinematics resulting from treadmill-induced falls and those resulting from an overground trip (Crenshaw et al., 2012; Honeycutt et al., 2016; Owings et al., 2001; Pavol et al., 2001), a common cause of falling in this population (Schmid et al., 2013). Training that can improve the ability of those with chronic stroke to respond to an anterior fall may serve as a means to prevent injury and enable independence. Further study is required to determine if this form of training is effective at reducing trip-related falls of stroke survivors in the free-living environment.

Supplementary Material

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Download video file (14.8MB, flv)
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Highlights.

  • The initial stepping limb did not influence performance-based variables of fall-recovery.

  • People with chronic stroke improved their fall-recovery performance with training.

  • The initial stepping limb influenced frontal plane step kinematics.

  • With training, trunk rotation was reduced and initial non-paretic steps were lengthened.

Acknowledgements

We thank all of the participants that volunteered for this novel study. We thank the Delaware Research Institute (DRI) for coordinating participant recruitment and scheduling. We would like to thank our Undergraduate Research Assistant Rebecca Peck who assisted with data collections.

Funding

Subject recruitment and scheduling was made possible with resources provided by the National Institutes of Health (P30 GM1033) and the Delaware Rehabilitation Institute/DRI. This project was supported by the University of Delaware Research Foundation, the American Society of Biomechanics Junior Faculty Research Award, and a grant from the National Institute of General Medical Sciences (2P20 GM103446) from the National Institutes of Health and the State of Delaware.

Footnotes

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