Waist-pull Perturbations Research Articles

Does the effect of walking balance perturbations generalize across contexts?

Balance perturbations are used to study locomotor instability. However, these perturbations are designed to provoke a specific context of instability that may or may not generalize to a broader understanding of falls risk. The purpose of this study was to determine if the effect of balance perturbations on instability generalizes across contexts. 29 younger adults and 28 older adults completed four experimental trials, including unperturbed walking and walking while responding to three perturbation contexts: mediolateral optical flow, treadmill-induced slips, and lateral waist-pulls. We quantified the effect of perturbations as an absolute change in margin of stability from unperturbed walking. We found significant changes in mediolateral and anteroposterior margin of stability for all perturbations compared to unperturbed walking in both cohorts (p-values ≤ 0.042). In older adults, the mediolateral effects of lateral waist-pulls significantly correlated with those of optical flow perturbations and treadmill-induced slips (r ≥ 0.398, p-values ≤ 0.036). In younger adults but not in older adults, we found positive and significant correlations between the anteroposterior effect of waist-pull perturbations and optical flow perturbations, and the anteroposterior and mediolateral effect of treadmill-induced slips (r ≥ 0.428, p-values ≤ 0.021). We found no “goldilocks” perturbation paradigm to endorse that would support universal interpretations about locomotor instability. Building the most accurate patient profiles of instability likely requires a series of perturbation paradigms designed to emulate the variety of environmental contexts in which falls may occur.

Susceptibility to walking balance perturbations in young adults is largely unaffected by anticipation

Despite progress in understanding the mechanisms governing walking balance control, the number of falls in our older adult population is projected to increase. Falls prevention systems and strategies may benefit from understanding how anticipation of a balance perturbation affects the planning and execution of biomechanical responses to mitigate instability. However, the extent to which anticipation affects the proactive and reactive adjustments to perturbations has yet to be fully investigated, even in young adults. Our purpose was to investigate the effects of anticipation on susceptibility to two different mechanical balance perturbations – namely, treadmill-induced perturbations and impulsive waist-pull perturbations. Twenty young adults (mean ± standard deviation age: 22.8 ± 3.3 years) walked on a treadmill without perturbations and while responding to treadmill belt (200 ms, 6 m/s2) and waist-pull (100 ms, 6% body weight) perturbations delivered in the anterior and posterior directions. We used 3D motion capture to calculate susceptibility to perturbations during the perturbed and preceding strides via whole-body angular momentum (WBAM) and anterior-posterior margin of stability (MoSAP). Contrary to our hypotheses, anticipation did not affect young adults' susceptibility to walking balance challenges. Conversely, perturbation direction significantly affected walking instability. We also found that susceptibility to different perturbation contexts is dependent on the outcome measure chosen. We suggest that the absence of an effect of anticipation on susceptibility to walking balance perturbations in healthy young adults is a consequence of their having high confidence in their reactive balance integrity. These data provide a pivotal benchmark for the future identification of how anticipation of a balance challenge affects proactive and reactive balance control in populations at risk of falls.

Repeated Predictable Loading and Unpredictable Unloading Waist-Pull Perturbations to Elicit Protective Stepping Responses: Clinical Correlations with Fall History in Active Older Adults

Impairments in reactive stepping responses predispose older adults to fall-risk. Prior research has determined methods to assess reactive stepping responses, however those methods lack objectivity or feasibility for use in clinical settings.

How Does Lower Limb Respond to Unexpected Balance Perturbations? New Insights from Synchronized Human Kinetics, Kinematics, Muscle Electromyography (EMG) and Mechanomyography (MMG) Data.

Making rapid and proper compensatory postural adjustments is vital to prevent falls and fall-related injuries. This study aimed to investigate how, especially how rapidly, the multiple lower-limb muscles and joints would respond to the unexpected standing balance perturbations. Unexpected waist-pull perturbations with small, medium and large magnitudes were delivered to twelve healthy young adults from the anterior, posterior, medial and lateral directions. Electromyographical (EMG) and mechanomyographical (MMG) responses of eight dominant-leg muscles (i.e., hip abductor/adductors, hip flexor/extensor, knee flexor/extensor, and ankle dorsiflexor/plantarflexors) together with the lower-limb joint angle, moment, and power data were recorded. The onset latencies, time to peak, peak values, and/or rate of change of these signals were analyzed. Statistical analysis revealed that: (1) agonist muscles resisting the delivered perturbation had faster activation than the antagonist muscles; (2) ankle muscles showed the largest rate of activation among eight muscles following both anteroposterior and mediolateral perturbations; (3) lower-limb joint moments that complied with the perturbation had faster increase; and (4) larger perturbation magnitude tended to evoke a faster response in muscle activities, but not necessarily in joint kinetics/kinematics. These findings provided insights regarding the underlying mechanism and lower-limb muscle activities to maintain reactive standing balance in healthy young adults.

482-P: Do Varied Off-Loading Modalities Have Differing Impacts Upon Users' Fall Risk?

Aim: Adherence to offloading devices for diabetic foot ulcers (DFU) is poor and postural instability influences adherence. Prior research suggests an ankle-high removable cast walker (RCW) in tandem with a contralateral shoe lift might increase users’ balance relative to a traditional knee-high RCW without a lift. This study compared compensatory responses to perturbations under both conditions. Methods: Twenty-five individuals (48% male; age: 68±8 years) with diabetic peripheral neuropathy participated. Participants completed trials under three conditions: bilateral standardized shoes, knee-high RCW, and ankle-high RCW with contralateral shoe lift. Stepping thresholds (ST) were assessed for two types of perturbations:1) From a stopped position, a treadmill rapidly accelerated at 3.5m/s² to reach a set peak-velocity and then decelerated and stopped. The smallest peak-velocity to elicit a step was identified as the ST. Participants were separately oriented on the treadmill to elicit anterior and lateral ST; 2) A waist mounted spring scale was pulled until a target load was reached, then the spring was rapidly released. ST was the lowest load necessary to force a step upon release. Anterior and posterior waist-pull perturbations were done.Participants also rated their stability in each footwear condition via a 10cm visual analog scale. Data analyses focused upon the primary comparison of the two offloading conditions. Results: The knee-high RCW resulted in significantly decreased ST for two of the four tests (treadmill anterior: mean difference .037m/s2, p=.048 and anterior waist-pull: mean difference 2.2lbs, p<.001) relative to the ankle-high RCW with a contralateral lift. Participants reported a large and statistically significant reduction in perceived stability with the knee-high RCW (mean difference= 0.93cm, p=0.030) . Conclusions: Future prospective studies should assesses whether the differences noted in this study translate into differences in RCW adherence by patients with DFU. Disclosure R.T.Crews: None. N.J.Rosenblatt: None. Funding National Institute of Diabetes and Digestive and Kidney Disease

Acute Effects of a Perturbation-Based Balance Training on Cognitive Performance in Healthy Older Adults: A Pilot Study.

Aging is accompanied by an alteration in the capacity to ambulate, react to external balance perturbations, and resolve cognitive tasks. Perturbation-based balance training has been used to induce adaptations of gait stability and reduce fall risk. The compensatory reactions generated in response to external perturbations depend on the activation of specific neural structures. This suggests that training balance recovery reactions should show acute cognitive training effects. This study aims to investigate whether exposure to repeated balance perturbations while walking can produce acute aftereffects that improve proactive and reactive strategies to control gait stability and cognitive performance in healthy older adults. It is expected that an adaptation of the recovery reactions would be associated with increased selective attention and information processing speed. Twenty-eight healthy older adults were assigned to either an Experimental (EG) or a Control Group (CG). The protocol was divided in 2 days. During the first visit, all participants completed the Symbol Digit Modalities Test (SDMT) and the Trail Making Test (TMT). During the second visit, a cable-driven robot was used to apply waist-pull perturbations while walking on a treadmill. The EG was trained with multidirectional perturbations of increasing intensity. The CG walked for a comparable amount of time with cables on, but without experiencing perturbations. Before and after the training, all participants were exposed to diagonal waist-pull perturbations. Changes in gait stability were evaluated by comparing the distance between the heel of the leading leg and the extrapolated Center of Mass (Heel-XCoM Distance—HXD) at perturbation onset (PON) and first compensatory heel strike (CHS). Finally, the cables were removed, and participants completed the SDMT and the TMT again. Results showed that only the EG adapted the gait stability (p < 0.001) in reaction to diagonal perturbations and showed improved performance in the SDMT (p < 0.001). This study provides the first evidence that a single session of perturbation-based balance training produce acute aftereffects in terms of increased cognitive performance and gait stability in healthy older adults. Future studies will include measures of functional activation of the cerebral cortex and examine whether a multi-session training will demonstrate chronic effects.

Untangling biomechanical differences in perturbation-induced stepping strategies for lateral balance stability in older individuals

When recovering balance from a lateral perturbation, younger adults tend to stabilize balance with a single lateral sidestep while older adults often take multistep responses. Using multiple steps to recover balance is consistently associated with increased fall risk, altered body center of mass (CoM) control and instability. The aim of this study was to compare the spatio-temporal stepping characteristics and the margin of stability (MoS) of single lateral sidesteps (LSS1) with the first and second steps of a two-step protective step sequence. Two-step sequences begin with either a cross-over step to the front or back, or a medial step followed by a lateral sidestep. Seventy-one older adults received random lateral waist-pull perturbations to either side. We hypothesized that LSS1 would be more stable (larger MoS) than either step in a two-step sequence. With some exceptions, utilizing a two-step sequence was associated with a reduced CoM velocity and distance between the base of support and CoM and decreased stability in the frontal plane following limb loading of the first and second step. There were no differences in the time available to arrest the extrapolated CoM at the end of a single lateral sidestep or the final step of a two-step sequence. Two-step sequences involving a cross-over step include more complex stepping trajectories and also challeng stability in the sagittal plane requiring a multidimensional balance correction. These results indicate important step type differences in center of mass control in recovering balance with a single lateral sidestep as opposed to a two-step sequence among older adults.

Reactive Stepping Responses Mediated by Predictable Manual Waist-Pull Perturbations are Associated with Fall History in Older Adults

Background and Purpose The ability to respond effectively to perturbations is a key element of reactive postural control and is a crucial mediator of falls. Several studies exist in the literature that determine older adults’ responses to perturbations, however those studies typically involve procedures that lack objectivity or applicability outside a laboratory. A study involving waist-pull perturbations with a spring-scale (SS) is an exception. In that study, fall history was most accurately differentiated by a reactive stepping response to a perturbing force of 10% total body weight. Using data from that study, we retrospectively examined the association between fall history and the number of steps accompanying a SS perturbing force of 10% total body weight in older adults. For perspective, the association of fall history with Timed-Up-and-Go (TUG) and single limb stance (SLS) times was also determined. Methods Fifty-eight healthy older adults (mean age = 80.7 years) participated in the study. Their 2-year fall history (yes, no) was recorded. All participants underwent SS testing with one-pound incremental, horizontal sagittal plane manual waist-pull perturbations. The number of steps in response to perturbation with 10% total body weight was recorded; TUG and SLS tests were performed. Associations between variables were examined using Spearman (rank-biserial) correlations. Results The median number of steps for fallers was 5 in both anterior and posterior directions. For non-fallers, the median number of steps was 1 and 2 in the anterior and posterior directions, respectively. The significant correlations between fall status and number of steps were 0.772 and 0.813 for the anterior and posterior directions, respectively. Similarly, the significant correlations between fall status and balance tests were 0.722 and -0.456 for the TUG and the SLS, respectively. Conclusions The number of steps accompanying waist-pull perturbations with forces of 10% of body weight were highly explanatory of experiencing a fall during the preceding 2 years.

Lateral Perturbation-Induced and Voluntary Stepping in Fallers and Nonfallers After Stroke.

A loss of balance poststroke from externally induced perturbations or during voluntary movements is often recovered by stepping. The purpose of this study was to characterize stepping behavior during lateral induced waist-pull perturbations and voluntary steps in community-dwelling fallers and nonfallers with chronic stroke. This study used a cohort design. Thirty participants >6months poststroke were exposed to 24 externally triggered lateral waist-pull perturbations and 20 voluntary steps. Balance tolerance limit (BTL) (transition from single to multiple steps) and first step type were determined for the waist-pull perturbations. Step parameters of initiation time, velocity, first step length, and clearance were calculated at and above BTL and for the voluntary steps. Hip abductor/adductor torque, foot cutaneous sensation, and self-reported falls that occurred 6months prior were evaluated. Twelve participants were classified retrospectively as fallers and 18 as nonfallers. Fallers had a reduced BTL and took more medial first steps than nonfallers. Above BTL, no between-group differences were found in medial steps. At BTL, the nonparetic step clearance was reduced in fallers. Above BTL, fallers took longer to initiate a paretic and nonparetic step and had a reduced nonparetic step length and clearance compared with nonfallers. There was a between-group difference in step initiation time for voluntary stepping with the paretic leg (P<.05). Fallers had a reduced paretic abductor torque and impaired paretic foot cutaneous sensation. A high fall rate poststroke necessitates effective fall prevention strategies. Given that more differences were found during perturbation-induced stepping between fallers and nonfallers, further research assessing perturbation-induced training on reducing falls is needed. Falls assessments should include both externally induced perturbations along with voluntary movements in determining the fall risk.

Postural adjustments in anticipation of predictable perturbations allow elderly fallers to achieve a balance recovery performance equivalent to elderly non-fallers

BackgroundIn numerous laboratory-based perturbation experiments, differences in the balance recovery performance of elderly fallers and non-fallers are moderate or absent. This performance may be affected by the subjects adjusting their initial posture in anticipation of the perturbation.Research questions: Do elderly fallers and non-fallers adjust their posture in anticipation of externally-imposed perturbations in a laboratory setting? How does this impact their balance recovery performance? Methods21 elderly non-fallers, 18 age-matched elderly fallers and 11 young adults performed both a forward waist-pull perturbation task and a Choice Stepping Reaction Time (CSRT) task. Whole-body kinematics and ground reaction forces were recorded. For each group, we evaluated the balance recovery performance in the perturbation task, change in initial center of mass (CoM) position between the CSRT and the perturbation task, and the influence of initial CoM position on task performance. ResultsThe balance recovery performance of elderly fallers was equivalent to elderly non-fallers (p > 0.5 Kolmogorov-Smirnov test). All subject groups anticipated forward perturbations by shifting their CoM backward compared to the CSRT task (young: 2.1% of lower limb length, elderly non-fallers: 2.7%, elderly fallers: 2.2%, Hodges-Lehmann estimator, p < 0.001 Mann-Whitney U). This backward shift increases the probability of resisting the traction without taking a step. SignificanceThe ability to anticipate perturbations is preserved in elderly fallers and may explain their preserved balance recovery performance in laboratory-based perturbation tasks. Therefore, future fall risk prediction studies should carefully control for this postural strategy, by interleaving perturbations of different directions for example.

Effects of repeated waist-pull perturbations on gait stability in subjects with cerebellar ataxia

BackgroundDamage to the cerebellum can affect neural structures involved in locomotion, causing gait and balance disorders. However, the integrity of cerebellum does not seem to be critical in managing sudden and unexpected environmental changes such as disturbances during walking. The cerebellum also plays a functional role in motor learning. Only a few effective therapies exist for individuals with cerebellar ataxia. With these in mind, we aimed at investigating: (1) corrective response of participants with cerebellar ataxia (CA) to unexpected gait perturbations; and (2) the effectiveness of a perturbation-based training to improve their dynamic stability during balance recovery responses and steady walking. Specifically, we hypothesized that: (1) CA group can show a corrective behavior similar to that of a healthy control group; (2) the exposure to a perturbation-based treatment can exploit residual learning capability, thus improving their dynamic stability during balance recovery responses and steady locomotion.MethodsTen participants with cerebellar ataxia and eight age-matched healthy adults were exposed to a single perturbation-based training session. The Active Tethered Pelvic Assist Device applied unexpected waist-pull perturbations while participants walked on a treadmill. Spatio-temporal parameters and dynamic stability were determined during corrective responses and steady locomotion, before and after the training. The ANalysis Of VAriance was the main statistical test used to assess the effects of group (healthy vs CA) and training (baseline vs post) on spatio-temporal parameters of the gait and margin of stability.ResultsData analysis revealed that individuals with cerebellar ataxia behaved differently from healthy volunteers: (1) they retained a wider base of support during corrective responses and steady gait both before and after the training; (2) due to the training, patients improved their anterior-posterior margin of stability during steady walking only.ConclusionsOur results revealed that participants with cerebellar ataxia could still rely on their learning capability to modify the gait towards a safer behavior. However, they could not take advantage from their residual learning capability while managing sudden and unexpected perturbations. Accordingly, the proposed training paradigm can be considered as a promising approach to improve balance control during steady walking in these individuals.

Perturbation-evoked lateral steps in older adults: Why take two steps when one will do?

BackgroundHip fractures in older adults often result from a fall in the lateral direction. While younger adults tend to recover balance from a lateral perturbation with a single lateral sidestep, older adults are prone to multistep responses which are associated with an increased fall risk. This study compared the stepping characteristics and stability of single and multistep responses to lateral perturbation in healthy older adults. MethodsEighty-four older adults received lateral waist-pull perturbations to either side. Spatio-temporal stepping characteristics and balance stability were quantified. FindingsFewer steps were taken to recover balance when the first step was a lateral sidestep. The stability margin of single lateral sidesteps was greater than medial sidesteps and cross-over steps to the back but not significantly different from single cross-over steps to the front at step termination. Single step responses were more stable than multistep responses at step termination and at step initiation for lateral sidesteps and cross-over steps to the front. The decreased stability of multistep responses was attributed to an increased center of mass velocity and a smaller distance between the center of mass and base-of-support at step termination. InterpretationAlthough lateral sidesteps result in fewer steps than cross-over steps to the front, the stability margin was not significantly different at step termination. These results suggest difficulty terminating center of mass motion and/or inefficient center of mass control differentiates single and multistep responses. Future studies should investigate perturbation training and/or hip abductor muscle conditioning as a means of improving compensatory stepping reactions.

Foot Placement for Balance with Waist-pull Perturbations Delivered while Standing

Foot Placement for Balance with Waist-pull Perturbations Delivered while Standing

Adaptation of Stability during Perturbed Walking in Parkinson\u2019s Disease

Gait and balance disorders are major problems that contribute to falls among subjects with Parkinson’s disease (PD). Strengthening the compensatory responses through the use of balance perturbations may improve balance in PD. To date, it is unclear how PD affects the ability to react and adapt to perturbations delivered while walking. This study aims to investigate how PD affects the ability to walk, respond to balance perturbations, and produce acute short-term effects to improve compensatory reactions and gait stability. A cable-driven robot was used to train nine patients with PD and nine age-matched controls with multidirectional waist-pull perturbations while walking on a treadmill. Margin of stability and base of support were evaluated while walking without cables and reacting to the perturbations. PD was associated with a reduced stability in the forward direction and the inability to produce proactive anticipatory adjustments. Both groups were able to improve the response to the disturbances and produce short-term aftereffects of increased gait stability once the cables were removed. A single session of perturbation-based balance training produced acute effects that ameliorated gait instability in PD. This result is encouraging for designing new therapeutic interventions that remediate falls risk.

Lateral Perturbation-Induced Stepping: Strategies and Predictors in Persons Poststroke.

Falls commonly occur as weight is transferred laterally, and impaired reactive stepping responses are associated with falls after stroke. The purpose of this study was to examine differences in and the determinants of mediolateral (M-L) protective stepping strategies when pulled off balance toward the paretic and nonparetic sides. Eighteen individuals more than 6 months poststroke were pulled in the M-L direction by a lateral waist-pull perturbation system. Step type (crossover, medial, and lateral) and count were recorded, along with first-step initiation time, length, and clearance. Sensorimotor variables including hip adductor/abductor and ankle plantar flexor/dorsiflexor peak isokinetic torques, paretic foot plantar cutaneous sensation, and motor recovery were used to predict step type by discriminant function analyses (DFAs). Regardless of pull direction, nearly 70% of trials required 2 or more recovery steps, with more frequent nonparetic leg first steps, 63.5%. The step type was significantly different for pull direction (P = 0.005), with a greater percentage of lateral steps when pulled toward the nonparetic side (45.1%) compared with the paretic side (17.5%). The M-L step length of the lateral step was increased (P < 0.001), with a reduced step clearance (P = 0.05), when pulled toward the paretic side compared with a pull toward the nonparetic side. DFAs revealed that nonparetic and paretic-side pulls could respectively classify step type 64% and 60% of the time, with foot cutaneous sensation discriminating for pull direction. Balance recovery initiated with the nonparetic leg occurred more frequently in response to M-L perturbations, and paretic foot cutaneous sensation was an important predictor of the stepping response regardless of the pull direction.Video Abstract available for more insights from the authors (see Video, Supplementary Digital Content 1, http://links.lww.com/JNPT/A190).

A single session of perturbation-based gait training with the A-TPAD improves dynamic stability in healthy young subjects.

Gait and balance disorders are among the most common causes of falls in older adults. Most falls occur as a result of unexpected hazards while walking. In order to improve the effectiveness of current fall-prevention programs, new balance training paradigms aim to strengthen the control of the compensatory responses required after external perturbations. The aim of this study was to analyze the adaptions of reactive and proactive strategies to control stability after repeated exposures to waist-pull perturbations delivered while walking. Eight healthy young subjects participated in a single training session with the Active Tethered Pelvic assisted Device (A-TPAD). Participants were exposed to repeated multi-directional perturbations of increasing intensity. The Antero-Posterior (AP) and Medio-Lateral (ML) Base of Support (BoS) and Margin of Stability (MoS) during the response to diagonal perturbations were compared before and after the training. Results showed that participants adapted both the reactive and proactive strategies to control walking balance by significantly increasing their pre- and post-perturbation stability. The changes were principally accounted for by an increment of the AP BoS and MoS and a reduction of ML BoS. This improved their ability to react to a diagonal perturbation. We envision that this system can be used to develop a perturbation-based gait training aimed at improving balance and control of stability during walking, thus reducing fall risk.

Relationship Between Head-Turn Gait Speed and Lateral Balance Function in Community-Dwelling Older Adults

ObjectiveTo determine and compare gait speed during head-forward and side-to-side head-turn walking in individuals with lower versus greater lateral balance. DesignCross-sectional study. SettingUniversity research laboratory. ParticipantsOlder adults (N=93; 42 men, 51 women; mean age ± SD, 73 ± 6.08y) who could walk independently. Main Outcome Measures(1) Balance tolerance limit (BTL), defined as the lowest perturbation intensity where a multistep balance recovery pattern was first evoked in response to randomized lateral waist-pull perturbations of standing balance to the left and right sides, at 6 different intensities (range from level 2: 4.5-cm displacement at 180cm/s2 acceleration, to level 7: 22.5-cm displacement at 900cm/s2 acceleration); (2) gait speed, determined using an instrumented gait mat; (3) balance, evaluated with the Activities-specific Balance Confidence Scale; and (4) mobility, determined with the Timed Up and Go (TUG). ResultsIndividuals with low versus high BTL had a slower self-selected head-forward gait speed and head-turn gait speed (P=.002 and P<.001, respectively); the magnitude of difference was greater in head-turn gait speed than head-forward gait speed (Cohen's d=1.0 vs 0.6). Head-turn gait speed best predicted BTL. BTL was moderately and positively related (P=.003) to the ABC Scale and negatively related (P=.017) to TUG. ConclusionsHead-turn gait speed is affected to a greater extent than head-forward gait speed in older individuals with poorer lateral balance and at greater risk of falls. Moreover, head-turn gait speed can be used to assess the interactions of limitations in lateral balance function and gait speed in relation to fall risk in older adults.

Evaluation of dynamic gait stability after a single-session of waist-pull perturbation training in Parkinson’s disease using an Active Tethered Pelvic Assist Device (A-TPAD). (P1.071)

Evaluation of dynamic gait stability after a single-session of waist-pull perturbation training in Parkinson’s disease using an Active Tethered Pelvic Assist Device (A-TPAD). (P1.071)

Single and multiple step balance recovery responses can be different at first step lift-off following lateral waist-pull perturbations in older adults

An inability to recover lateral balance with a single step is predictive of future falls in older adults. This study investigated if balance stability at first step lift-off (FSLO) would be different between multiple and single stepping responses to lateral perturbations. 54 healthy older adults received left and right waist-pulls at 5 different intensities (levels 1–5). Crossover stepping responses at and above intensity level 3 that induced both single and multiple steps were analyzed. Whole-body center of mass (COM) and center of pressure (COP) positions in the medio-lateral direction with respect to the base of support were calculated. An inverted pendulum model was used to define the lateral stability boundary, which was also adjusted using the COP position at FSLO (functional boundary). No significant differences were detected in the COP positions between the responses at FSLO (p≥0.075), indicating no difference in the functional boundaries between the responses. Significantly smaller stability margins were observed at first step landing for multiple steps at all levels (p≤0.024), while stability margins were also significantly smaller at FSLO for level 3 and 4 (p≤0.048). These findings indicate that although reduced stability at first foot contact would be associated with taking additional steps, stepping responses could also be attributable to the COM motion state as early as first step lift-off, preceding foot contact. Perturbation-based training interventions aimed at improving the reactive control of stability would reduce initial balance instability at first step lift-off and possibly the consequent need for multiple steps in response to balance perturbations.

Direction-Dependent Adaptation of Dynamic Gait Stability Following Waist-Pull Perturbations.

Balance recovery during an unexpected disturbance is a complex motor task, where part of the variability depends on the type of the perturbation itself. Despite of this, little is known to what extent adaptation mechanisms to repeated perturbations are dependent on the direction and the amplitude of the applied disturbances. Here, we used a modified version of the Active Tethered Pelvic Assist Device (A-TPAD) to apply unexpected force-controlled multidirectional waist-pull perturbations while subjects were walking. Healthy young subjects were divided into two groups and were exposed to a single training session. Each group received perturbations of different amplitudes along the Medio-Lateral (ML) or the Antero-Posterior (AP) direction. Dynamic stability was determined in both the AP and ML directions in terms of base of support (BoS) and margin of stability (MoS). Results showed: 1) an adaptation of the balance recovery reactions only for perturbations delivered along the AP directions; 2) aftereffects able to modify the control of stability during the post-training session of which type and extent depends on the direction of the perturbations previously applied; and 3) a directional and amplitude effect on the dynamic stability at the end of the balance recovery reactions.