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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