Unconstrained slip mechanics and stepping reactions depend on slip onset timing

The effect of arm restriction on dynamic stability and upper-body responses to lateral loss of balance during walking: an observational study

When losing balance, upper-body movements serve as mechanical aids to regain stability. However, it remains unclear how these movements contribute to dynamic stability during recovery from a lateral loss of balance while walking with arm restriction. We aimed to (i) quantify the effect of arm restriction on gait stability and upper-body velocities and (ii) characterize upper-body kinematic strategies in response to lateral surface translations under different arm restriction conditions. Healthy adults were exposed to lateral surface translations while walking on a computerized treadmill under three conditions: 'free arms', '1-arm restricted' and '2-arms restricted'. Dynamic stability and upper-body velocities for the first step after perturbation onset were extracted. We found decreased dynamic stability in the sagittal plane and increased trunk velocity in the '2-arms restricted' condition compared with the 'free arms' condition. Head and trunk movements in the medio-lateral plane were in opposite directions in 44.31% of responses. Additionally, significant trunk velocities were observed in the opposite direction to the perturbation-induced loss of balance. Our results support the contribution of increased upper-body velocities to balance responses following arm-restricted walking perturbations and suggest that the '2-arms restricted' condition may be utilized as a perturbation-based balance training, focusing on head and trunk responses.

Biomechanical strategies for mitigating unexpected slips: A review

Slips are the leading cause of falls, and understanding slip biomechanics is crucial for preventing falls and mitigating their negative consequences. This study analyses human biomechanical responses to slips, including kinetic, kinematic, spatiotemporal, and EMG variables. We reviewed 41 studies investigating slip-induced falls in lab settings, computational models, and training approaches. Our analysis focused on reactions and effects of factors like age, fatigue, strength, perturbation intensity, and gait speed. Trailing limbs' hip extension and knee flexion interrupt the swing phase earlier, increasing the support base. The slipping leg responds with two phases: hip extension and knee flexion, then hip flexion and knee extension. Furthermore, our analysis revealed that the medial hamstring muscles play an active role in slip recoveries. Their activation in the slipping limb allows for hip extension and knee flexion, while in the trailing limb, their activation results in the foot touching down. Additionally, successful slip recoveries were associated with co-contraction of the Tibialis Anterior (TA) and Medial Gastrocnemius (MG), which increases ankle joint stability and facilitates foot contact with the ground. Our review identifies various factors that influence biomechanical and muscular responses to slips, including age, perturbation intensity, gait speed, muscular fatigue, and muscular strength. These findings have important implications for designing interventions to prevent slip-related falls, including cutting-edge technology devices based on a deeper understanding of slip recoveries. Future research should explore the complex interplay between biomechanics, muscle activation patterns, and environmental factors to improve slip-fall prevention strategies.

Impact of standing perturbation intensities on fall and stability outcomes in healthy young adults

Motorized treadmills have been extensively used in investigating reactive balance control and developing perturbation-based interventions for fall prevention. However, the relationship between perturbation intensity and its outcome has not been quantified. The primary purpose of this study was to quantitatively analyze how the treadmill belt's peak velocity affects the perturbation outcome and other metrics related to the reactive balance in young adults while the total belt displacement is controlled at 0.36 m. Thirty-one healthy young adults were randomly assigned into three groups with different peak belt speeds: low (0.9 m/s), medium (1.2 m/s), and high (1.8 m/s). Protected by a safety harness, participants were exposed to a forward support surface translation while standing at an unexpected timing on an ActiveStep treadmill. The primary (perturbation outcome: fall vs. recovery) and secondary (dynamic stability, hip descent, belt distance at liftoff, and recovery step latency) outcome measures were compared among groups. Results revealed that a higher perturbation intensity is correlated with a greater faller rate (p < 0.001). Compared to the low- and medium-intensity groups, the high-intensity group was less stable (p < 0.001) with a larger hip descent (p < 0.001) and a longer belt distance (p < 0.001) at the recovery step liftoff. The results suggest that the increased perturbation intensity raises the risk of falling with larger instability and poorer reactive performance after a support surface translation-induced perturbation in healthy young adults. The findings could furnish preliminary guidance for us to design and select the optimal perturbation intensity that can maximize the effects of perturbation-based training protocols.

Intersegmental coordination in human slip perturbation responses

Intersegmental coordination (ISC) of lower limbs and planar covariation law (PCL) are important phenomena observed in biomechanics of human walking and other activities. Gait perturbations tend to cause deviation from the expected ISC pattern thus violating PCL. We used a data set of seven subjects, who experienced unexpected slips, to investigate and characterize the evolution of ISC during slip recoveries and falls. We have analyzed and presented the development of ISC patterns, encompassing the step preceding the slip initiation and duration of slip until it stops. The results show that the ISC patterns during slip recovery deviate considerably from the normal walking patterns. A newly proposed Euclidian distance-based metric (EDM) was used to quantify the deviation from the normal walking ISC pattern during four slip recoveries and three falls evaluated at gait events such as slip start, foot strike, and peak height of the swing foot. The timing of gait events after slip, pattern of EDM, placement of the feet after slip and temporal patterns of each limb angle have been presented. This initial investigation provides insight into the ISC during slip recovery which highlights the human natural recovery trajectories during such perturbations. The observed patterns of the ISC trajectories during slip can be used for the design of human-inspired controllers for exoskeleton devices that can provide external assistance to human subjects during balance recovery.

Curvilinear walking elevates fall risk and modulates slip and compensatory step attributes after unconstrained human slips

While much attention has been paid to understanding slip-related falls in humans, little has been focused on curvilinear paths despite their prevalence, distinct biomechanical demands and increased slipping threat. We determined the mechanics, compensatory stepping reactions and fall risk associated with slips during fixed-speed walking across ranges of path curvature, slipped foot and slip onset phase contexts possible in the community, which builds upon previous work by examining speed-independent effects of curvilinear walking. Twenty-one participants experienced 15 unconstrained slips induced by a wearable friction-reducing device as motion capture and harness load cell data were recorded. Falls were most likely after early stance slips to the inside foot and increased at tighter curvatures. Slip distance and peak velocity decreased as slips began later in stance phase, did not differ between feet, and accelerated on tighter paths. Slipping foot directions relative to heading transitioned from anterior (forward) to posterior (backward) as slips began later in stance, were ipsilateral (toward the slipping foot side) and contralateral (toward the opposite side) for the outside and inside foot, respectively, and became increasingly ipsilateral/contralateral on tighter curvatures. Compensatory steps were placed anteriorly and ipsilaterally after outside and inside foot slips, respectively, and lengthened at later onset phases for outside foot slips only. Our findings illustrate slip magnitude and fall risk relationships that suggest slip direction may influence the balance threat posed by a slip, imply that walking speed may modify slip likelihood, and indicate the most destabilizing curved walking contexts to target in future perturbation-based balance training approaches.

Strength in arms: empowering older adults against the risk of slipping and falling-a theoretical perspective

Background

Slips and falls are a serious health concern, particularly among older adults. Current physical therapy protocols strengthen the legs to improve balance. However, arm movements help maintain balance during a slip incident. Understanding how arm movements improve balance may help clinicians develop more comprehensive fall-prevention protocols to improve patient outcomes.

Clinical question

What limitations exist in current fall prevention protocols for reducing falls in older adults during slip incidents, and what new strategies can enhance these outcomes?

Key results

Slip incidents often result in a sideways loss of balance, leading to hip fractures in older adults. During a slip, the legs do not produce sideways motion and are less effective in regaining balance in this direction. Contrary, the arms produce 100 + degrees of abduction and this motion reduces falls by 200%+ during a slip incident. Notably, older adults exhibit 35.7% decreased arm abduction acceleration responses compared to younger adults during a slip incident. This delay may be attributed to age-related decreases in type II fibers of the deltoid. High-velocity and ballistic training have been shown to improve the proportion and size of type II fibers as well as improve fall outcomes when focused on the lower extremities.

Clinical application

Therefore, I propose incorporating arm abductor training, alongside leg exercises, as a cost-effective and low-risk intervention to enhance the slip responses in older adults. In light of its minimal risk and considerable potential benefits, starting arm abductor exercises with older adults is a sensible move.

Recovering whole-body angular momentum and margin of stability after treadmill-induced perturbations during sloped walking in healthy young adults

Although humans are well-adapted to negotiating sloped terrain, balance recovery after a disturbance on slopes is poorly understood. This study investigated how slope affects recovery from unanticipated simulated trips and slips. Eighteen healthy young adults walked on a split-belt treadmill at 1.25 m/s and three slope angles (downhill: - 8°; level: 0°; uphill: + 8°), with slip- and trip-like perturbations applied randomly at heel-strike. We evaluated balance recovery using whole-body angular momentum (WBAM) and perturbation response (PR), for which larger PR values indicate greater deviation of the margin of stability from baseline, therefore, greater destabilisation after perturbation. Overall, trips were more destabilising than slips, producing larger PR and greater range and integral of WBAM across all tested slopes, most significantly in the sagittal plane. Contrary to expectation, sagittal-plane PR post-trip was greatest for level walking and smallest for downhill walking. Heightened vigilance during downhill walking may explain this finding. Recovery strategy in both frontal and sagittal planes was consistent across all slopes and perturbation types, characterized by a wider and shorter first recovery step, with trips requiring the greatest step adjustment. Our findings advance understanding of the robustness of human locomotion and may offer insights into fall prevention interventions.

Reactive responses of the arms increase the Margins of Stability and decrease center of mass dynamics during a slip perturbation

Although reactive arm motions are important in recovering from a slip event, the biomechanical influences of upper extremity motions during slipping are not clear. The purpose of the current study was to determine whether reactive arm motions during slip recovery leads to increased margins of stability (MoS), and decreased center of mass (CoM) velocity and excursion. Thirty-two participants were randomized into 2 conditions: arms free and arms constrained. Participants traversed a 10-meter walkway and were exposed to an unexpected slip while wearing a protective harness. Anterior-posterior and medial-lateral MoS, as well as the CoM excursion and velocity during the slip perturbation was quantified using a three-dimensional motion capture system. In the frontal plane, individuals with their arms unconstrained demonstrated greater MoS (0.06 ± 0.03 vs -0.01 ± 0.02 m, p < 0.01), decreased CoM excursion (0.05 ± 0.02 vs 0.08 ± 0.01 m, p = 0.015), and a reduced CoM velocity (0.07 ± 0.03 vs. 0.14 ± 0.02 m/s, p < 0.01) compared to individuals with their arms constrained. In the sagittal plane, individuals with their arms unconstrained demonstrated, decreased CoM excursion (0.83 ± 0.13 vs 1.14 ± 0.20 m, p < 0.01) reduced CoM velocity (1.71 ± 0.08 vs. 1.79 ± 0.07 m/s, p = 0.02), but no differences in margins of stability (0.89 ± 0.13 vs 0.94 ± 0.10 m, p = 0.32). Our findings demonstrate that arm motions during a slip perturbation act to restore balance by minimizing displacement and velocity of the body CoM during a slip event in the frontal plane.

Comparison of over-ground and treadmill perturbations for simulation of real-world slips and trips: A systematic review

BACKGROUND

Trips and slips increase fall risk for young and older adults. To examine recovery responses, studies utilized treadmill and/or over-ground methods to simulate real-world perturbations. However, differences in the recovery response between treadmill and over-ground perturbations remain unexamined.

RESEARCH QUESTION

To assess the current literature on the reactive recovery responses between over-ground- and split-belt treadmill trips and slips as well as the effect of aging on these responses.

METHODS

PubMed, Medline, Web of Science, SCOPUS, and Cochrane databases were searched for publications examining trips and slips in healthy young, healthy older adults, and older adults who fall. Included articles were in English, full-text accessible, and biomechanically quantified the reactive recovery responses for slips and trips during either over-ground or split-belt treadmill protocols. The initial database search yielded 1075 articles and 31 articles were included after title, abstract, and full-text screening.

RESULTS

For slips, 7 articles utilized lubricated surfaces while 5 articles used treadmills. Further, 3 studies examined differences between older and younger adults. For trips, 9 articles utilized obstacles and 7 used treadmills. Further, 4 articles examined differences between older and young adults and 1 article only examined older adults during over-ground trips. For both perturbations, treadmill and over-ground protocols demonstrated similar anteroposterior destabilization on the center of mass. In the mediolateral direction, over-ground slips consistently found a lateral destabilization while treadmill articles did not examine this direction. Foot placement recovery responses varied less for both perturbation directions on a treadmill compared to over-ground.

SIGNIFICANCE

Although treadmill and over-ground perturbations destabilize the center of mass similarly, the recovery response to these perturbations were different on treadmills. Specifically, recovery responses were more consistent for both slips and trips on treadmills. As older adults have difficulty in perturbation recovery scaling, treadmills may be limited in their ability to investigate the variety of aging impairments on perturbation recovery responses.

Slipping mechanics during walking along curved paths depend on the biomechanical context at slip onset

Curvilinear walking is common, causing limb- and radius-dependent asymmetries that distinguish it from straight walking and elevated friction demands that increase slip-and-fall risk. However, it is unclear how aspects of curvilinear walking influence the slip perturbations experienced. We cross-sectionally examined how three biomechanical slip contexts (slip onset phase, slipped foot relative to the path, path radius) influence slip direction, distance, and peak velocity. Eighteen young adults experienced unconstrained inside or outside foot slips during early, mid-, or late stance while following 1.0- or 2.0-m radius semicircular paths. We derived slip mechanics from motion-capture data and assessed their dependence on slip context using mixed-effects models. As slip onset phase progressed, slip directions exhibited an anterior-to-posterior transition, shortened mediolaterally, and accelerated anteroposteriorly. The slipped foot modified the direction transition, with inside and outside foot slips moving contralaterally and ipsilaterally, respectively. Inside foot slips were shorter and slower mediolaterally and longer anteroposteriorly than outside foot slips. Increasing path radius caused slips with greater mediolateral direction components. We show a range of context-dependent slips are possible, likely due to instantaneous magnitudes and orientations of shear ground reaction forces. Our results contribute to a comprehensive understanding of walking slips, which fall prevention methods can leverage.

Toward Safe Wearer-Prosthesis Interaction: Evaluation of Gait Stability and Human Compensation Strategy Under Faults in Robotic Transfemoral Prostheses

Although advanced wearable robots can assist human wearers, their internal faults (i.e., sensors or control errors) also pose a challenge. To ensure safe wearer-robot interactions, how internal errors by the prosthesis limb affect the stability of the user-prosthesis system, and how users react and compensate for the instability elicited by internal errors are imperative. The goals of this study were to 1) systematically investigate the biomechanics of a wearer-robot system reacting to internal errors induced by a powered knee prosthesis (PKP), and 2) quantify the error tolerable bound that does not affect the user's gait stability. Eight non-disabled participants and two unilateral transfemoral amputees walked on a pathway wearing a PKP, as the controller randomly switched the control parameters to disturbance parameters to mimic the errors caused by locomotion mode misrecognition. The size of prosthesis control errors was systematically varied to determine the error tolerable bound that disrupted gait stability. The effect of the error was quantified based on the 1) mechanical change described by the angular impulse applied by the PKP, and 2) overall gait instability quantified using human perception, angular momentum, and compensatory stepping. The results showed that the error tolerable bound is dependent on the gait phase and the direction of torque change. Two balance recovery strategies were also observed to allow participants to successful respond to the induced errors. The outcomes of this study may assist the future design of an auto-tuning algorithm, volitionally-controlled powered prosthetic legs, and training of gait stability.

Severity of Unconstrained Simultaneous Bilateral Slips: The Impact of Frontal Plane Feet Velocities Relative to the Center of Mass to Classify Slip-Related Falls and Recoveries

Targeted interventions to prevent slip-related falls may be informed by specific kinematic factors measured during the reactive response that accurately discriminate recoveries from falls. But reactive responses to diverse slipping conditions during unconstrained simultaneous bilateral slips, which are closely related to real-world slips, are currently unknown. It is challenging to identify these critical kinematic factors due to the wide variety of upper and lower body postural deviations that occur following the slip, which affect stability in both the sagittal and frontal planes. To explore the utility of kinematic measurements from each vertical plane to discriminate slip-related falls from recoveries, we compared the accuracy of four Linear Discriminant Analysis models informed by predetermined sagittal or frontal plane measurements from the lower body (feet velocities relative to the center of mass) or upper body (angular momentum of trunk and arms) during reactive responses after slip initiation. Unconstrained bilateral slips during over-ground walking were repeatedly administered using a wearable device to 10 younger (24.7 ± 3.2 years) and 10 older (72.4 ± 3.9 years) adults while whole-body kinematics were measured using motion capture. Falls (n = 20) and recoveries (n = 40) were classified by thresholding the dynamic tension forces measured in an overhead harness support system and verified through video observation. Frontal plane measurements of the peak feet velocities relative to the center of mass provided the best classification (classification accuracy = 73.3%), followed by sagittal plane measurements (classification accuracy = 68.3%). Measurements from the lower body resulted in higher accuracy models than those from the upper body, but the accuracy of all models was generally low compared to the null accuracy of 66.7% (i.e., predicting all trials as recoveries). Future work should investigate novel models that include potential interactions between kinematic factors. The performance of lower limb kinematics in the frontal plane in classifying slip-related falls demonstrates the importance of administering unconstrained slips and measuring kinematics outside the sagittal plane.