Stroke
often results in hemiparesis and gait asymmetries, such as spatial,
temporal, or kinematic differences between the paretic and nonparetic
legs [1,2,3]. Asymmetric gait post-stroke has been associated with slower walking speeds [4] and higher metabolic energy consumption [5, 6]
compared to unimpaired walking. Conventional gait retraining by
physical therapists can reduce gait asymmetries, particularly step
length asymmetry, and improve speed and energy economy, but sessions are
costly, limiting access. More automated rehabilitation techniques to
reduce step length asymmetry have been developed using split-belt
treadmills [7,8,9] or rehabilitation robots [10, 11].
These interventions, however, have not been more effective than
conventional physiotherapy for individuals with chronic stroke [8, 11,12,13,14,15].
A better understanding of the mechanisms driving step length asymmetry
in individuals with chronic stroke could allow for the development of
more targeted, effective, and accessible gait interventions.
Unimpaired individuals self-select many gait parameters, such as step frequency [16], step width [17], and even arm swinging characteristics [18],
to minimize their energy cost of walking, a strategy that might also
explain self-selected asymmetries in post-stroke gait. Deviations from
self-selected gait tend to lead to an increase in energy expenditure,
creating bowl-like relationships, or cost landscapes, between energy
cost and gait parameters, with the energy minimum at the self-selected
parameter value [19].
The relationship between step length asymmetry and metabolic rate has
not yet been characterized in unimpaired individuals, but studies that
enforce absolute differences in step length [20] or asymmetry in step time [21]
suggest that they self-select nearly symmetric step lengths that
correspond to a lower energy cost than asymmetric gait. Because stroke
often leads to physical asymmetries, such as paretic leg muscle weakness
[22], muscle spasticity [23], or reduced paretic-leg push-off force [24, 25],
an asymmetric gait could be metabolically optimal for individuals with
chronic stroke. On the other hand, factors other than effort
minimization, such as perceived effort, avoidance of fatigue, comfort,
or stability, could be primarily responsible for the observed step
length asymmetry in this population.
The effects of acutely
changing step length asymmetry may differ from those of slowly changing
step length asymmetry through the process of rehabilitation. Long-term
rehabilitation interventions that decrease gait asymmetry have shown
that cost of transport often improves concurrently [26]. However, other effects of long-term rehabilitation, such as increased muscle strength [27] or improved motor control [28],
may enable individuals to walk with reduced asymmetry more efficiently.
Therefore, acute reductions in gait asymmetry that are not accompanied
by the neuro-musculoskeletal changes often seen in long-term
rehabilitation may not correlate with improvements in walking economy.
Even for individuals who walk with highly asymmetric gaits, their
self-selected asymmetry may be the most energy efficient one, and acute
changes in gait asymmetry could still lead to increased energy
consumption.
Step length asymmetry is changed acutely during
split-belt walking, but this task change may also change the optimal
step length asymmetry. When belt speeds are matched immediately
following split-belt training, step length asymmetry is acutely changed,
but this washout effect does not persist long enough to collect
steady-state metabolic rate measurements [29].
To determine whether self-selected gait asymmetry minimizes energy
cost, gait asymmetry would need to be varied independently from other
factors affecting metabolic energy consumption, within an individual
participant, while metabolic rate is measured.
Previous studies
measured metabolic rate while individuals with chronic stroke acutely
altered step length asymmetry and step length difference using
biofeedback [20, 30].
These studies found no difference in metabolic rate between
self-selected and altered step length asymmetries in stroke survivors.
However, some uncertainty remains to be resolved. Each prior study
included participants with self-selected step length asymmetry values
close to zero. This makes differentiating between the potential
optimality of self-selected asymmetry and that of absolute symmetry
difficult. Prior studies also included some participants who were unable
to reliably alter step length asymmetry, so asymmetry values in
different conditions could have been similar. This can reduce the power
of a numerical analysis intended to identify an effect of step length
asymmetry on metabolic rate. In prior studies, participants were
instructed to hold onto the treadmill handrails to help with stability
and minimize fall risk. However, participants could have relied more
heavily on the handrails during more difficult conditions to improve
stability or to help maintain the correct speed. Both improved balance [31, 32] and handrail holding [33]
during treadmill walking have been shown to reduce metabolic energy
consumption. To minimize the amount of walking and number of
experimental sessions for participants with chronic stroke, participants
in these studies were familiarized with the biofeedback on the same day
as the data collection. However, motor learning can have an effect on
metabolic rate during a novel task; as individuals learn a new task,
metabolic power [34, 35] and muscle activity [36]
typically decrease, with steady state reached after hours or days of
practice. For example, Sánchez et al. recently showed that split-belt
treadmill training takes longer than originally thought [37], and training over multiple sessions can facilitate better learning because memory consolidation occurs during sleep [38].
The
purpose of this study was to characterize the relationship between step
length asymmetry and metabolic energy consumption during walking in
individuals with chronic stroke and unimpaired individuals. We screened
for individuals with chronic stroke who exhibited clinically meaningful
self-selected step length asymmetry, so as to differentiate between the
potential optimality of self-selected asymmetry and perfect symmetry.
Only participants who could substantially alter their asymmetry with
biofeedback were included, which ensured that the effects of changes in
asymmetry could be robustly analyzed. We disallowed participants from
using handrails during all conditions to avoid uncertainty related to
the potential benefits of improved balance or forward pulling during
more difficult conditions. Participants with chronic stroke received
training on the biofeedback system during the first session to
facilitate task learning and ensure that all participants could alter
their baseline asymmetry with biofeedback. Data were collected in three
subsequent sessions. During each collection session, conditions were
presented in a different order to avoid ordering effects, and the first
condition of the session was repeated to reduce within-session training
effects. We hypothesized that individuals with chronic stroke would
self-select the step length asymmetry that minimized their metabolic
energy consumption during walking, and that more symmetric or asymmetric
gaits would result in a higher metabolic cost. We hypothesized that
unimpaired individuals would self-select the step length asymmetry, near
symmetric, that minimized their metabolic cost, and more asymmetric
gaits would increase metabolic cost. The results from this study were
expected to improve our understanding of the mechanisms driving
self-selected step length asymmetry and influence the development of new
gait retraining techniques.
