By Duangporn Suriyaamarit and Sujitra Boonyong

Sit-to-stand movements, where an individual rises from a chair to a standing position, are common daily functional actions. Previous research has demonstrated that the design and dimensions of the chair used can influence the performance of the sit-to-stand task. For instance, getting up from a chair with higher height and an anterior tilted seat surface is usually easier. No evidence has been presented to date, however, in children with cerebral palsy for whom sit-to-stand movements are a constant challenge. This research therefore sought to draw comparisons between performance levels in terms of mechanical work, movement time, kinematics, and kinetics while getting up from chairs with varying seat angles and heights, for children with cerebral palsy.

Experiments were carried out involving 12 children with cerebral palsy under three conditions. The control condition used a low and horizontal seat (low-flat). The other two conditions were a low seat with anterior inclination (low-tilted), and a high horizontal seat (high-flat). Under both low-tilted and high-flat conditions, there was a significant reduction in movement time and mechanical work during sit-to-stand, in comparison to the low-flat control condition. We also found that at the start of the sit-to-stand movement in the low-tilted, there was better trunk alignment, and reduced pelvic motion. Meanwhile, the high-flat showed a reduction in the range of movement for the knee, hip, and ankle joints, and in the maximal hip and knee extension moments in comparison to the low-flat.

Our findings indicate that both a high seat and anterior inclination can improve sit-to-stand performance in children with cerebral palsy, although these two chair types provide different benefits. The low-tilted chair improves trunk alignment with the pelvis as the sit-to-stand task commences. Anterior inclination should therefore be employed for children with cerebral palsy whose primary problem lies in trunk and pelvis alignment even though they have sufficient muscle strength in the lower legs to facilitate standing. We also found that the high-flat chair reduced hip, knee, and ankle joint excursion and lowered the maximal hip and knee extension moment. A higher chair may therefore be helpful for children with cerebral palsy whose primary problems concern the lower extremities, or who are still in the initial stages of training for sit-to-stand movements. The findings presented here may help clinicians determine which type of chair design is more appropriate for a specific child with cerebral palsy.

Figure: The three different chairs and corresponding movement time and mechanical work during the sit-to-stand task.

Publication

Suriyaamarit, D., & Boonyong, S. (2020). Comparison of the effects of chair height and anterior seat inclination on sit-to-stand ability in children with spastic diplegic cerebral palsy. Journal of Biomechanics, 113, 110098. https://doi.org/10.1016/j.jbiomech.2020.110098

About the Author

By Dr Tarun Arora, Dr Alison Oates and Dr Kristin Musselman

After someone has injured their spinal cord, the communication between their brain and body is disrupted, which can make movement challenging. In more than half of spinal cord injuries (SCI), the spinal cord is not completely damaged (i.e., an “incomplete” injury ). The majority of individuals with an incomplete SCI (iSCI) are able to walk. Unfortunately, about three-quarters of people with an iSCI report at least one fall per year. This risk of falling is similar or even higher than other populations who are prone to falls such as older adults (33%) or those living with stroke (73%) and Parkinson’s disease (68%). Research shows that individuals with iSCI walk slower, take shorter steps, and spend more time in double support which improves their mechanical stability. Thus, they are less likely to lose their balance during walking, but can they recover once their balance is perturbed? Our research studied reactive responses to an unexpected slip perturbation in people with an iSCI.

We brought 20 individuals with iSCI and 15 age-and-sex matched individuals without SCI (niSCI) to the Biomechanics of Balance and Movement Lab at the University of Saskatchewan. A slip device embedded into the floor becomes highly slippery (comparable to that of clean ice at 0 deg. celsius!) when unlocked (Figure 1). Without knowing when the slip device will be unlocked, participants walked for several “normal walking” trials. Then we unlocked the slip device for an “unexpected slip”without telling them  (click here for videos). The velocity of the slipping heel was used to categorize the severity of the slip as hazardous (>1m/s) or non-hazardous. Using 3-D motion capture, we measured stability at the first compensatory step after the slip. We also measured when and how much the muscles in the leg activated in reaction to slip.

Both groups had similar proportions of hazardous and non-hazardous slips suggesting a comparable slip severity. The iSCI group could not regain as much stability as the niSCI group using a compensatory step. Also, the iSCI group demonstrated lower calf muscle activation compared to the niSCI group.

Using the unexpected slip paradigm, we demonstrated some important limitations in the reactive balance of individuals with iSCI. This research highlights the importance of evaluating reactive balance during clinical balance evaluations in individuals with iSCI. Clinicians should consider evaluating reactive balance evaluation (e.g., mini-BESTest) in their practice.

Figure 1. Slip device and schematic lab setup for data collection (modified from Arora T et al., PM&R 2019)

Note: this study was a part of a larger research project that was funded by the Saskatchewan Health Research Foundation (awarded to AO and KEM).

Publication

Arora T, Musselman KE, Lanovaz JL, Linassi G, Arnold C, Milosavljevic S, Oates A. (2020) Reactive balance responses to an unexpected slip perturbation in individuals with incomplete spinal cord injury. Clin Biomech (Bristol, Avon), 78:105099. https//doi.org/10.1016/j.clinbiomech.2020.105099

About the Author

By Liana Tennant

Common sense tells us that flip-flops should never be worn in the chemistry lab or when cutting the grass, but could flip-flops increase injury risk in less obvious situations? We were approached by a forensic engineering company who wanted to determine the role flip-flops might play in a slip and fall incident. In addition to evaluating slip dynamics on wet and dry tile, the firm also wanted to know how slips change when the foot is also wet, a scenario that might be encountered on a rainy day. We expected that with a wet foot, it might move around inside the flip-flop during a slip. A slip within a slip if you will.

To answer these questions, we invoked slips from a standing posture by pulling one ankle forward using a cable and pulley device (Figure 1). Although the slip speeds using this method were higher than those reported during walking, it allowed us to control some of the variables that can affect how slips occur and progress. We tracked the foot and flip-flop separately using a 3D motion capture system. We found that there was minimal relative motion between the foot and flip-flop during slips, so long as the flip-flop stayed on the foot; however, in several instances the flip-flop slid forward off the participant’s heel, which we called ‘decoupling.’ Decoupling occurs when the friction between the foot and flip-flop is insufficient to halt the forward momentum of the flip-flop caused by the initial pull at the ankle. Sometimes the flip-flop came off entirely (see video Figure 2 )! Decoupling happened in at least 1 of 27 slips for 12 of the 17 participants. We saw decoupling less often on wet tile with a dry foot, and more often when both the tile and foot were either dry or wet.

This study highlighted that friction between the foot and flip-flop matters. When flip-flops are worn, two different yet important slips may occur: 1) your flip-flop can slip along the ground or 2) your foot can slip inside the flip-flop. If you lose your flip-flop during a slip, you could injure your unprotected foot, or it might make it harder for you to catch yourself when falling. The results also have implications for flip-flop design. As consumers, we likely focus on how comfortable our footwear is, but the materials and the surface texture of the flip-flop footbed could also be important from a safety perspective. Future work evaluating real-life scenarios like walking or going downstairs is needed to see if decoupling occurs as frequently in these situations as what we saw when we invoked a slip.

Figure1. The experimental setup. The participant stood with their right foot on a tile mounted on a force plate. Their ankle was connected to a cable and pulley machine loaded with 25% of the participant’s body weight (weight stack was hidden from view). An active motion capture system was used to track the foot and flip-flop. The participant wore a harness tethered to the ceiling to protect them in the event of a fall.

Figure 2. This video shows an example of the decoupling phenomenon we observed during some of the slips. Playback is at 0.5x speed. The flip-flop is represented by the white planar surface.

Publication

Tennant, L.M., Fok, D.J., Kingston, D.C., Winberg, T.B., Parkinson, R.J., Laing, A.C., Callaghan, J.P., 2021. Analysis of invoked slips while wearing flip-flops in wet and dry conditions: Does alternative footwear alter slip kinematics? Applied Ergonomics 92, 103318. https://doi.org/10.1016/j.apergo.2020.103318

About the Author

By Dr Keisuke Hirata

During walking, the front foot often slips when it comes in contact with a slippery surface (due to oil, water, ice, etc.). If the leading foot slips forward during walking, the body tends to rotate backward and a corrective response such as a backward step is adopted. The slip velocity relative to the walking velocity could be a determinant of falls. However, experimental studies using artificial slipping environments are still needed to clarify the exact relationship between slip velocity and walking velocity. This is because even though walking velocity can be unified among participants, slip velocity cannot, as it depends on the individual body weight. To overcome this problem, we used a double-belt treadmill built into the floor. The treadmill was programed to independently control the slip velocity and timing of each belt.

The participants walked onto the belt of the treadmill from overground walking (Figure 1A). The treadmill was programed to induce a slip (with a maximum slip velocity of 1.6 m/s) for a single foot (Figure 1B). The motion capture system recorded ten young male adults walking onto the belt under fast (≈ 1.6m/s) and slow (≈ 0.9m/s) walking velocity conditions. We classified the corrective responses based on heel marker distances as follows: (1) stop walking (taking-step strategy) and (2) keep walking (overcome the slip and continue the trial). The results showed that, in slow conditions, most participants took wide steps or stepped backward and then stopped walking (Figure 1C). Moreover, increased step length and hip flexion angle of the slipping leg was associated with effortless corrective response only in the slow walking condition.

In older people with balance-related problems who walk slowly, the unexpected slip perturbation velocity may be greater than the walking velocity. Considering the relationship between walking velocity and slip velocity, rehabilitation for falls prevention should focus on increasing the hip joint range of motion and on training at-risk people to take longer steps to ensure a stable base of support.

Figure 1 A: The experimental environment. B: Schematic of the situation wherein a slip occurs. C: The result of relationships between the corrective response (foot markers distance) and walking velocity.

Publication

Hirata K, Kokubun T, Miyazawa T, Hanawa H, Kubota K, Sonoo M, Fujino T, Kanemura N. 2020 Relationship Between the Walking Velocity Relative to the Slip Velocity and the Corrective Response. Journal of Medical and Biological Engineering. doi: https://doi.org/10.1007/s40846-020-00527-6

About the Author

By Prokopios Antonellis

Walking on level ground demands little effort, but walking on grades quickly becomes challenging. Is it possible to minimize energy expenditure with shoe outsoles that offset downhill or uphill grades? We investigated the interaction effects of outsole geometry and grade on the metabolic rate and biomechanics of walking.

We developed a modular shoe that allows for altering the inclination of the foot relative to the ground. Shoe height differences were between 3 to 6 cm between the heel and toe region. We tested the effect of these different shoe inclinations on the metabolic rate and biomechanics of downhill and uphill walking at different grades, including level walking. Each condition lasted 5 minutes. We expected that offsetting the downhill or uphill grade would minimize metabolic rate. Remarkably, shoes that exactly offset the grade did not minimize the metabolic rate. Instead, shoes that compensated for about half of the grade (by using a raised heel for uphill walking and a raised toe for downhill walking) proved to be optimal. Shoe inclination primarily influenced (distal) ankle joint parameters (e.g., soleus activity, ankle moment, and work rate), whereas grade influenced (whole-body) ground reaction force and center-of-mass parameters, as well as (distal) ankle joint parameters.

Walking on uneven terrain with uphill and downhill sections, the metabolic rate is mostly affected by the uphill portions. As such, it could be advantageous to use shoes with a slight downward shoe inclination in these situations (Figure). It could also be possible to design shoes that allow for changing the inclination depending on the grade of the terrain to avoid repetitive overstretching of the calf muscles. Our results could further explain some of the differences in metabolic rate between walking on stairs and ramps at an equivalent average grade. Current construction guidelines recommend the use of ramps for low grades and the use of stairs for higher grades. The benefit of shoes that partially offset the average grade, as observed in the current study, might, therefore, be seen as indirect evidence of the benefit of stairs. Overall, these modular shoes could be used as a research instrument to optimize parameters other than metabolic rate, such as minimizing joint loading to prevent injuries or assisting with walking on rolling terrain.

Figure: Modular shoe that allows for offsetting uphill and downhill grades. The shoe shown here has a downward outsole configuration for facilitating uphill walking. The figure evokes a potential outdoor application. The experiments were conducted during indoor walking between treadmill grades of -6° and 6°.

Publication

Antonellis, P., Frederick, C. M., Gonabadi, A. M., & Malcolm, P. (2020). Modular footwear that partially offsets downhill or uphill grades minimizes the metabolic cost of human walking. Royal Society open science, 7(2), 191527. https://doi.org/10.1098/rsos.191527

About the Author

By Dr Matthew King

Investigations of biomechanics are an ever-growing field of scientific research, with the assessment of “gait” being a popular choice to inform both clinical and experimental directions. However, what cannot be gleaned from the term “gait” is the type of locomotion that is being studied. Are the participants walking, running, crawling or jogging?

Initially, the study of human movement was so novel, that the term “gait” may have been all-encompassing. Over time, the term “gait” has been substituted to describe walking. However, “gait” is not mutually exclusive to walking, nor humans. Horses gallop and trot, fish swim, and humans run (just to name a few). What is not know is the task (or type of locomotion) being performed – as “gait” is the pattern produced during a mode of locomotion, not locomotion itself.

To demonstrate the ambiguous use of the term “gait”, our recent publication reviewed the 319 papers published in Gait and Posture in 2019. In summary, approximately half of the papers that directly evaluated locomotion described the task as “gait” in the title with no quantifier (noun, verb, or gerund) for the actual task being performed (FIG).

Our primary goal for completing this paper and outlining this information, is to shed light on various, and often incorrect, use of the word “gait”. Together, as clinicians, academics, and scientists alike, we must rise to the challenge in being more literal in what we study and why. In particular with the use of the word “gait”. In doing so, search strategies will be streamlined, titles will provide insight into the task studied, and the ability for individuals to locate relevant research studies to further their evidence-based practice will be simplified.

Figure. Use of the word “gait” in the titles of papers published in Gait and Posture in 2019

Publication

Gill, N., Kean, C., & King, M. G. (2020). Let’s take the dog for a gait…. Gait & Posture, 79, pp. 1-2. doi: https://doi.org/10.1016/j.gaitpost.2020.03.018

About the Author

By Andreas Skiadopoulos

Recent investigations have demonstrated that step width variability increases with age, which could be a signal of developing deterioration in gait stability. In the literature, step width variability has been extensively examined in an attempt to find which exercise-based interventions improve gait stability in older adults. However, it is not known when step width variability values in older adults should be considered as excessive. This is an issue of great importance as this information could be used to enroll participants who have the potential to benefit most from an intervention, tailor the intervention dose to match participant’s level of walking stability, and monitor or evaluate intervention effects. Thus, the main purpose of this systematic review and meta-analysis was to compare the amount of step width variability (expressed as the standard deviation of step width) between young and older adults and to identify the boundaries of optimal reference range of step width variability.

After screening 1408 studies from 11 databases we found 10 studies that met our criteria. We conducted a meta-analysis to compare the step width variability between older and younger adults (304 older adults and 219 younger adults). Additionally, a two-decision method was used on the meta-analytic data to identify optimal thresholds of step width variability to discriminate age-related gait changes. The results showed that older adults have greater step width variability than young adults (Figure). Step width variability values in older adults above the upper threshold value of 2.50 cm were considered excessive, while step width variability values below the lower threshold value of 1.97 cm were considered within the optimal reference range. For our purposes, the step width variability values in healthy young adults set the optimal reference range.

Based on our systematic review and meta-analysis, older adults with excessive step width variability form a plausible target population for effective exercises in preventing falls and lowering fall risk. Moreover, optimal thresholds levels of step width variability could potentially impact rehabilitation technology design for devices targeting lateral instability during walking.

Figure. Forest plot of standardized mean difference (SMD) and 95% confidence intervals (CI) for the step width variability between older and younger adults.

Publication

Skiadopoulos, A., Moore, E.E., Sayles, H.R., Schmid, K.K., Stergiou, N. Step width variability as a discriminator of age-related gait changes. J NeuroEngineering Rehabil 17, 41 (2020). https://doi.org/10.1186/s12984-020-00671-9

About the Author

By Victoria Rapos

On a daily basis, individuals are constantly required to avoid another moving person in order to avoid a collision. Successfully avoiding a potential collision requires both walkers to mutually adapt their speed and orientation. The metric, Minimum Predicted Distance (MPD) has been used as a predictor variable in order to determine the risk of collision over time between two adult walkers. We were interested in determining whether MPD could be used to predict future risks of collisions between middle-aged children and adults.

Eighteen middle-aged children (mean±SD=10±1.5years) and eighteen adults (34±9.6years) walked at their normal pace, along a 12.6m pathway while avoiding another individual (child or adult). Three adults and three children were recruited per session. The study consisted of four obstructing walls (2.3m long), 90° to one another acting as barriers, such that participants were unaware who they were interacting with until they reached a steady walking speed. Each adult interacted with another adult 20 times, each child interacted with another child 20 times, and each adult interacted with a child 21 times. Motion capture of each participant’s head was recorded. The location of each participant’s head at each point in time was used to compute MPD and the walking speed of each participant. MPD(t) represents the progression of the distance between the two walkers if both walkers did not change their speed or path orientation at that instant in time. Trials were categorized as adult-adult, child-child, adult-child passing second, and child-adult passing second for statistical analysis.

The results of this study demonstrated that MPD(t) can be used to predict a future collision in children. When a child was involved in an interaction, MPD(t) was always lower compared to when two adults were interacting, with the lowest progression of MPD(t) being when two children interacted with one another. This is likely due to the differences in body size of the individuals. Since MPD(t) is an absolute measure, it does not consider body anthropometrics. Similar to previous collision avoidance research, the walker passing second, even when it is a child, contributes more to avoidance behaviour compared to the walker passing first. Therefore, the findings from the present study demonstrate that middle-aged children are capable of making adult-like decisions during a collision avoidance task involving two walkers. MPD is smaller in children compared to adults which may be due to person-specific characteristics or developmental changes. Body anthropometrics/characteristics should be considered when determining collision avoidance strategies between children and adults.

Figure. Mean evolution of minimum predicted distance (MPD(t)) over time for each group. In other words, the predicted distance two walkers would avoid one another if no adaptation to their behaviours occurred at each time point (i.e., 100% is the final crossing distance) Interactions were separated into adult-adult (AA), child-child (CC), adult-child passing second (AC), and child- adult passing second (CA).(Figure revised from Rapos et al., 2019).

Publication

Rapos, V., Cinelli, M., Snyder, N., Crétual, A., & Olivier, A-H. (2019). Minimum predicted distance: Applying a common metric to collision avoidance strategies between children and adult walkers. Gait & Posture, 72, 16-21. DOI: https://doi.org/10.1016/j.gaitpost.2019.05.016

About the Author

By Tom Buurke.

Humans are unique in their gait, as they walk on two legs instead of four. While two-legged gait is energy efficient, it comes with a higher chance of balance loss. The center of mass is higher and the base of support smaller in bi- versus quadrupedal gait, which is a challenge in balance control. However, this does not seem to pose a problem for healthy humans, as they effortlessly walk in ever-changing environmental circumstances (e.g. while twisting and turning or walking over uneven terrain). If gait control is impaired (e.g. due to injury), this adaptability declines. Knowledge on how people control their balance during constant spatiotemporal perturbations is still scarce. However, this knowledge may be crucial for gait rehabilitation. Therefore, we studied how people adapt their balance to a continuous perturbation as imposed by walking on a split-belt treadmill, where people walk faster with one leg than the other.

We assessed fourteen healthy young adults during fast (1.4 ms^-1) and slow (0.7 ms^-1) walking at baseline, during adaptation to a split-belt condition (1.4 : 0.7 ms^-1), and during a de-adaptation (0.7 ms^-1) phase. We calculated step width, mediolateral margins of stability, and mediolateral foot roll-off from force plate signals to quantify balance control. While step width did not change, the margins of stability and foot roll-off adapted to maintain dynamic balance during the split-belt perturbation. Furthermore, as Fig. 1A shows, the margins of stability and foot roll-off were strongly coupled during split-belt adaptation. This implies that if the margin of stability increased, the foot rolled off more inwards, and vice versa. In addition, this coupling adapted to split-belt gait, as observed in the change from the start (Early Adaptation) to the end of the adaptation phase (Late Adaptation) in both legs (Fig. 1B).

Relative foot placement and foot roll-off cooperate to maintain dynamic balance during perturbed walking. In addition, these coupled mechanisms show adaptation to split-belt gait. This implies that multiple spatiotemporal mechanisms are involved in maintaining dynamic stability. The finding that the margins of stability changed, while step width did not, implicates an influence of temporal control of gait on balance control through passive dynamics. Similar processes may induce altered balance control in persons with inherent spatiotemporal gait asymmetries, e.g. amputees and stroke survivors. Future research should determine whether changes in the margins of stability and foot roll-off are the result of an active balance control strategy, or the result of passive dynamics in gait.

Figure 1 – The relation between mediolateral Margin of Stability (MoS) and mediolateral foot roll-off (ΔCoP) during the split-belt adaptation phase. The shaded outer ellipses indicate standard error of the group’s mean (N=14). (A) The black ellipse shows the leg on the fast belt and the red ellipse the leg on the slow belt. (B) The blue ellipses show the fast (darker) or slow (lighter) leg during Early Adaptation (EA), the green ellipses during Late Adaptation (LA). This figure shows that a higher margin of stability is related to a more inward foot roll-off during stance, and vice versa. This relation shifts from a high MoS and inward foot roll-off to low MoS and neutral/outward foot roll-off from early to late adaptation.

Publication

Buurke T.J.W., Lamoth C.J.C., Vervoort D., van der Woude, L.H.V., den Otter R. Adaptive control of dynamic balance in human gait on a split-belt treadmill. (2018) Journal of Experimental Biology 221(13): jeb.174896 doi: 10.1242/jeb.174896  http://jeb.biologists.org/content/221/13/jeb174896

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