Trading Symmetry for Energy Cost During Walking in Healthy Adults and Persons Poststroke

Abstract

Background. Humans typically walk in ways that minimize energy cost. Recent work has found that healthy adults will even adopt new ways of walking when a new pattern costs less energy. This suggests potential for rehabilitation to drive changes in walking by altering the energy costs of walking patterns so that the desired pattern becomes energetically optimal (ie, costs least energy of all available patterns). Objective. We aimed to change gait symmetry in healthy adults and persons poststroke by creating environments where changing symmetry allowed the participants to save energy. Methods. Across 3 experiments, we tested healthy adults (n = 12 in experiment 1, n = 20 in experiment 2) and persons poststroke (n = 7 in experiment 3) in a novel treadmill environment that linked asymmetric stepping and gait speed-2 factors that influence energy cost-to create situations where walking with one's preferred gait symmetry (or asymmetry, in the case of the persons poststroke) was no longer the least energetically costly way to walk. Results. Across the 3 experiments, we found that most participants changed their gait when experiencing the new energy landscape. Healthy adults often adopted an asymmetric gait if it saved energy, and persons poststroke often began to step more symmetrically than they prefer to walk in daily life. Conclusions. We used a novel treadmill environment to show that people with and without stroke change clinically relevant features of walking to save energy. These findings suggest that rehabilitation approaches aimed at making symmetric walking energetically "easier" may promote gait symmetry after stroke.

Keywords: cost; feedback; gait; locomotion; rehabilitation; stroke.

Figure 1

A) Experimental setup for kinematic and metabolic testing. When the controller was activated, participants used the visual feedback to control their stepping patterns and manipulate the treadmill speed. B) An example asymmetric walking pattern (left) and corresponding visual display (right) that participants used to control foot placement. The green distance bar along the top of the display was visible only during Experiments 2 and 3. C) Day 1 protocol for Experiment 1 showing treadmill speeds (top, black) and foot placement asymmetry (i.e., difference in the targets stepped to with each foot; bottom, gray). Positive values of foot placement symmetry indicate that the participant stepped further with the left foot than the right (e.g., a value of 3 indicates that the left foot stepped three targets ahead of the right; a value of −3 indicates the opposite). Gray shading indicates that the treadmill speed was set by the experimenter. Costs of transport for D) symmetric walking at different speeds and E) different asymmetries at self-selected (SS) speed (mean±SEM).

Figure 2

Histograms of the proportion of strides within foot placement asymmetry bins for selfselected (SS) walking and foot placement asymmetry manipulations.

Figure 3

A) Day 2 protocol for Experiment 1. Line color conventions are consistent with Figure 1. Green text indicates that the controller was active. Gray shading indicates that the treadmill speed was set by the experimenter; green shading indicates that the speed was set by the controller. B) Costs of transport for asymmetry/speed combinations prescribed by the Experiment 1 controller (mean±SEM). *indicates difference compared to least costly condition with p<0.05. C) Proportion of participants picking each combination during the Experiment 1 test period. D) Protocol for Experiment 2. E) Costs of transport for asymmetry/speed combination prescribed by the Experiment 2 controller (mean±SEM). *indicates difference compared to least costly condition with p<0.05. F) Proportion of participants picking each combination during the Experiment 2 test period.

Figure 4

A) Relationships between Borg Rating of Perceived Exertion (RPE) and total cost of transport (top) and metabolic power (bottom) for each asymmetry/speed combination in Experiment 2. Cost of transport showed a weak negative association with Borg RPE while metabolic power showed a strong positive association with Borg RPE (both p<0.05). B) Asymmetry/speed combinations used by each participant during the Experiment 2 test period along the kilometer walked.

Figure 5

A) Day 1 protocol for Experiment 3. Green lines indicate treadmill speed, light dark green lines indicate foot placement, and dark green lines indicate step length asymmetry. Gray shading indicates that the treadmill speed was set by the experimenter. B) Representative participant data showing costs of transport for walking across all conditions. C) Costs of transport for walking with preferred asymmetry at different speeds. D) Costs of transport for foot placement asymmetry patterns at self-selected (SS) speed. Gray circles show individual data. E) Costs of transport for step length asymmetry patterns at SS speed. F) Foot placement asymmetry (mean±SEM) and G) step length asymmetry (mean±SEM) when the participants used online foot position feedback or step length feedback, respectively, to manipulate gait symmetry. H) Foot placement asymmetry (top) and step length asymmetry (bottom) over the last 30 strides of each condition. * indicates p<0.05. †indicates p=0.074. LS: less symmetric than preferred, pref: preferred asymmetry, MS: more symmetric than preferred. All bar graphs show mean±SEM.

Figure 6

A) Day 2 protocol for Experiment 3. Gray shading indicates that the treadmill speed was set by the experimenter; green shading indicates that the speed was set by the controller. B) Success rates for each of the combinations tested in minutes 4–16 of the Experiment 3 protocol. Here success rate was calculated as the proportion of strides during each four-minute period that participants hit the desired difference in targets (i.e., desired limping pattern; mean±SEM). C) Costs of transport for each foot placement asymmetry/speed combination prescribed by the Experiment 3 controller (mean±SEM). *indicates difference compared to the least costly condition with p<0.05 (p=0.101 for preferred asymmetry+slow speed vs. less symmetric+fast speed condition). D) Proportion of participants picking each combination during the Experiment 3 test period.