Metabolic cost of lateral stabilization during walking in people with incomplete spinal cord injury
Introduction
During gait people with incomplete spinal cord injury (iSCI) have oxygen consumption rates ∼50–225% higher than non-disabled individuals [1], with metabolic cost of transport (COT) increasing with impairment level [2]. This energetically inefficient gait is associated with decreased social participation and quality of life [3], [4]. Thus, identifying specific factors that contribute to elevated COT could aid the development of targeted therapies to improve wellbeing.
Gait stability is crucial for community ambulation. Research suggests that human walking is passively unstable in the frontal plane and therefore requires active control [5]. An important strategy for maintaining lateral stability is step-to-step foot placement. Taking wider steps creates a larger lateral base of support (BOS) and may increase the threshold of perturbation before a corrective step is required to maintain balance [6]. However, increasing step width increases the mechanical work required to redirect the center of mass (COM) at each step [7]. A simple model suggests that COT will increase with the square of step width [8]. Thus, increasing lateral stability by increasing step width comes with a potentially severe energetic penalty.
People with iSCI may choose gait patterns that increase stability even if this action increases energetic cost. Selecting general strategies that increase passive stability every step (e.g. increasing step width) is desirable when sensory and motor impairment limit the ability to maintain lateral stability via step-to-step corrective foot placements. Individuals with iSCI exhibit significant step width variability [9] due to both corrective actions and poor motor control. Controlling step width variability may also impart an energetic cost [10]. While neuromuscular deficits limit the available locomotor strategies one can perform, there is evidence that people with iSCI can alter their gait patterns in response to varying environmental factors [11], [12] and task goals [13].
The energetic cost of maintaining lateral stability during walking can be quantified by measuring oxygen consumption when this requirement is reduced through external lateral stabilization [14], [15]. With external lateral stabilization, unimpaired individuals decrease step width and reduce their COT ∼3–7% [14], [15], [16]. Our purpose was to quantify the metabolic energy cost of maintaining lateral stability in people with iSCI. We hypothesized that people with iSCI would adapt to external lateral stabilization by decreasing step width, which in turn would result in a substantial decrease in COT. In addition, we hypothesized that standard clinical measures of function and balance would be related to the metabolic energy required for lateral stabilization.
Section snippets
Participants
Ten subjects with chronic motor iSCI participated in this study (9 male; age: 57 ± 10 years; all AIS D and >1 year post injury) (Table 1). Subjects gave written informed consent prior to participation. Northwestern University Institutional Review Board approved the protocol. With the exception of iSCI, subjects had no other neurological impairments. All subjects could walk without assistive devices for 5 min at their preferred speed. Subjects did not alter medications for this study; one subject
Results
All subjects except one decreased their COT with external lateral stabilization. Subject 6 did not appear comfortable walking with external lateral stabilization and exhibited a 36% increase in COT, which was 4.5 interquartile ranges from the median COT % change value. Thus, we treated this subject as an outlier and excluded the data from all analyses. For the remaining subjects, COT decreased 10% on average with external lateral stabilization, from 7.9 ± 1.5 to 7.0 ± 1.2 J/kg/m (p = 0.03) (Fig. 2a).
Discussion
When the demands of lateral stabilization were reduced, individuals with iSCI made significant decreases in step width, step width variability, and minimum MOS. These gait adaptations corresponded with a 10% decrease in COT. As functional impairment increased on the BBS and LEMS, the metabolic cost of lateral stabilization tended to increase. These findings support our hypotheses that people with iSCI will adapt their gait in response to changing demands placed on dynamic balance, and that the
Conclusion
People with iSCI devote a substantial amount of energy to maintaining lateral stability. The amount of metabolic energy individuals used for maintaining lateral stability was strongly correlated with clinical tests of balance and lower extremity strength. Individuals with the greatest impairment used the most metabolic energy for maintaining lateral stability. When provided with external lateral stabilization, subjects responded by decreasing their step width, step width variability, and
Conflict of interests
The authors declare no conflicts of interest.
Acknowledgments
This work was supported in part by Career Development Award #1 IK2 RX000717-01 from the United States Department of Veterans Affairs, Rehabilitation Research and Development Service and NIH training grant T32HD057845.
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2020, Gait and PostureCitation Excerpt :There is a possible tradeoff between gait economy and stability. In persons with an incomplete spinal cord injury, decreases in balance control were associated with increased metabolic energy required for lateral stabilization [16]. Further, in healthy adults, decreased variability in foot placement was accompanied by mild but significant decreases in metabolic cost.
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2017, Gait and PostureCitation Excerpt :Selecting narrower steps decreases mechanical work performed on the center of mass [17], and passive resistance to perturbations [25]. Our results are consistent with previous findings that non-impaired [18] and iSCI [16] populations choose narrower, less metabolic-costly steps when external stabilization reduces requirements to control frontal-plane stability. Both groups continued taking narrow steps when the Stabilization field was removed.