Cure my spasticity and I'll easily fix my walking asymmetry.
Can a passive unilateral hip exosuit diminish walking asymmetry? A randomized trial
Journal of NeuroEngineering and Rehabilitation volume 20, Article number: 88 (2023)
Abstract
Background
Asymmetric walking gait impairs activities of daily living in neurological patient populations, increases their fall risk, and leads to comorbidities. Accessible, long-term rehabilitation methods are needed to help neurological patients restore symmetrical walking patterns. This study aimed to determine if a passive unilateral hip exosuit can modify an induced asymmetric walking gait pattern. We hypothesized that a passive hip exosuit would diminish initial- and post-split-belt treadmill walking after-effects in healthy young adults.
Methods
We divided 15 healthy young adults evenly between three experimental groups that each completed a baseline trial, an adaptation period with different interventions for each group, and a post-adaptation trial. To isolate the contribution of the exosuit we compared a group adapting to the exosuit and split-belt treadmill (Exo-Sb) to groups adapting to exosuit-only (Exo-only) and split-belt only (Sb-only) conditions. The independent variables step length, stance time, and swing time symmetry were analyzed across five timepoints (baseline, early- and late adaptation, and early- and late post-adaptation) using a 3 × 5 mixed ANOVA.
Results
We found significant interaction and time effects on step length, stance time and swing time symmetry. Sb-only produced increased step length asymmetry at early adaptation compared to baseline (p < 0.0001) and an after-effect with increased asymmetry at early post-adaptation compared to baseline (p < 0.0001). Exo-only increased step length asymmetry (in the opposite direction as Sb-only) at early adaptation compared to baseline (p = 0.0392) but did not influence the participants sufficiently to result in a post-effect. Exo-Sb produced similar changes in step length asymmetry in the same direction as Sb-only (p = 0.0014). However, in contrast to Sb-only there was no significant after-effect between early post-adaptation and baseline (p = 0.0885).
Conclusion
The passive exosuit successfully diminished asymmetrical step length after-effects induced by the split-belt treadmill in Exo-Sb. These results support the passive exosuit’s ability to alter walking gait patterns.
Background
Over 8 million people in the United States live with lingering symptoms following stroke [1]. This disorder alters the functioning of the central nervous system (CNS), leading to impaired motor control (i.e., hemiparesis or partial paralysis) and the possibility of asymmetric walking patterns [2,3,4,5,6]. The ability to walk enables individuals to perform different home- or community-based activities and maintain healthy, active lifestyles. CNS motor and sensory pathways produce the framework for lower extremity muscles and joints to work in unison to move the body forward [7, 8]. Disruptions to neurological function can alter the symmetrical movement of the lower extremity joints and can sometimes lead to more inefficient asymmetric patterns. Interlimb asymmetry can take on temporal (e.g., stance or swing time) and/or spatial (e.g., step length) forms [2, 9, 10]. Consequently, reductions in preferred walking velocity and lower extremity range of motion may result from altered step length and modified stance duration (the degree of each varies on an individual level) [2,3,4, 9]. Altered mechanics limit mobility and increase effort, energy costs, and the risk of falls during ambulation in affected populations [5, 11,12,13,14]. This hemiparetic interference with daily activities may deteriorate overall health, which can lead to an increased risk for future medical issues in patient populations.
The utilization of novel perturbations to alter walking gait symmetry has produced short-term ambulation improvements. By utilizing these perturbations, the CNS can be trained to adapt to complex, unexplored environments through the integration of sensory feedback during ongoing movement [15,16,17]. To influence the asymmetric walking gait of patients following stroke, previous work explored perturbing ambulation through weighting the less-paretic limb, which is also known as constraint-induced movement therapy (CIMT) [18, 19]. After completing a 20-minute treadmill walking session with a weight attached to the less-paretic limb, participants increased their gait speed and step length from baseline to the follow-up [18]. This finding suggests a short-term walking gait improvement as a result of less paretic limb weighting. Long-term investigations of multiple CIMT training sessions found that participants developed improvements in stride length after completing treadmill walking with additional weight on their less-paretic limb [19]. Despite the improvements obtained using less-paretic limb weighting training, researchers found no significant differences compared to controls that completed treadmill walking training alone. The results suggest that treadmill training alone sufficiently improved walking ability. Additionally, adding weight at the ankle increases metabolic demands and destabilizes walking gait, which creates adverse issues for populations experiencing increased metabolic demands from abnormal gait [13, 20,21,22].
During walking each limb adapts independently to the environment, allowing for leg-specific responses to perturbations [23]. This concept is especially relevant during the use of a split-belt treadmill, a treadmill with separate belts for the left and right leg that can move at different velocities. Split-belt training has been used to perturb the walking environment of stroke patients to assess their ability to adapt to new locomotion patterns [24]. For example, participants following stroke altered their step length and stance times to accommodate different belt velocities on a split-belt treadmill [24]. The participants with asymmetries at baseline developed symmetrical step-length after-effects (adaptations to the perturbation) once the belts returned to a tied condition. This suggests that a damaged CNS does not restrict, short-term symmetrical walking adaptations [24]. Long-term investigations of the effects of split-belt walking in stroke populations found improved step length asymmetry compared to baseline initially after completing the protocol. However, the participants did not maintain improvements one and three months after the intervention [25]. Temporal walking symmetry improvements (i.e., stance or double support time) remained unchanged across all collection time points [25]. Although these studies provided the framework for short-term gait adaptations, split-belt training for long-term retention and rehabilitation is not particularly convenient (e.g., at home training interventions). The need to develop accessible rehabilitation techniques for patient populations is sizable and critical. One avenue that may improve access to long-term walking gait therapies involves the application of external wearable devices, such as exoskeletons (or exosuits).
Previously, researchers have used exoskeletons to manipulate spatiotemporal, kinematic, and kinetic movement characteristics. Robotic (active) exoskeletons use software and powered actuation systems to apply forces at specific times during a movement pattern, such as walking gait [26,27,28,29]. Newer designs significantly reduced the size of the devices and power actuation sources and improved the comfort of active exoskeletons [30, 31]. Passive exoskeletons consist of elastic elements, such as springs or mechanically triggered clutches that deform and return stored elastic energy at a different point during the movement [32, 33]. Unlike active exoskeletons, passive devices require no external power to apply resistance or assistance [34]. The simplicity of a passive elastic exoskeleton allows the individual operator to put them on in a few minutes, dramatically reduces the cost of materials, and permits device application outside of research or clinical rehabilitation settings [35,36,37]. Many exoskeleton designs focus on assisting the ankle. In the case of impaired patient populations, a hip device may provide further benefit because adding weight at the hip is less destabilizing and metabolically less expensive during locomotion compared to adding weight at the ankle [21, 22]. Furthermore, the hip joint plays a critical role in efficient limb advancement throughout walking by providing approximately 40–50% of the positive power required for forward progression during healthy gait [38,39,40]. From a musculotendon perspective, the hip extensors and flexors function as springs that store elastic energy during one phase of walking and impart the stored energy in another phase. Specifically, hip extensors (e.g., hamstrings, gluteus maximus) assist with the deceleration of the thigh during the swing phase of walking and accelerating at the beginning of stance; these muscles help stabilize the body to lower extremity forces [38, 41, 42]. The hip flexors (e.g., rectus femoris, iliopsoas, sartorius) actively progress the thigh forward during the swing phase and passively aid leg deceleration during the second half of stance [41, 43]. Due to the importance of the hip for walking, using a passive exoskeleton or exosuit to perturb the hip motion by adding a force that is not naturally produced by the body offers a promising avenue to induce adaptative changes.
Typically, wearable devices, such as exoskeletons, are used to provide assistance. However, they may also yield resistance to promote adaptations similar to those observed with split-belt perturbations. Recent studies have explored the use of exoskeletons [29, 44] and customized perturbation footwear [45] to achieve such adaptation effects. Two notable studies examined the effects of a powered unilateral ankle exosuit and a powered unilateral hip exoskeleton on healthy participants, with the goal of uncovering benefits that could ultimately be useful for post-stroke therapy [29, 44]. Both studies observed temporary increases in range of motion (plantarflexion in the ankle exosuit study and hip motion in the hip exoskeleton study), but neither reported significant step-length adaptation effects. The hip exoskeleton study highlighted common challenges in fitting rigid exoskeletons to the complex hip joint motion, supporting the idea of conducting similar research using a passive, soft hip exosuit.
The specific objective of our study was to determine if a passive unilateral hip exosuit can diminish asymmetric walking gait patterns in healthy participants. In order to induce walking asymmetry in healthy participants, we used a split-belt treadmill. Previous studies found that the split-belt paradigm leads to asymmetrical walking patterns in healthy young adults when initially introduced and asymmetrical after-effects upon return to a tied configuration [46,47,48,49]. We hypothesized that wearing the exosuit would reduce split-belt treadmill induced asymmetrical step length, stance time, and swing time after-effects in healthy individuals. This study’s findings could establish the proof-of-concept required for future research in neurologically afflicted patient populations and the foundation for an accessible community-based, long-term rehabilitation strategy to assist patients in their recovery.
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