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Reliability of gait in multiple sclerosis over 6 months
Gait & Posture, Volume 41, Issue 3, March 2015, pages 860-862
- We estimated reliability of gait over 6 months in the absence of an intervention.
- Gait parameters of persons with MS had excellent reliability over 6 months.
- Experimentally determined reliability estimates will inform sample size estimates.
Gait impairment is ubiquitous in multiple sclerosis (MS) and is often characterized by alterations in spatiotemporal parameters of gait. There is limited information concerning reliability of spatiotemporal gait parameters over clinical timescales (e.g. 6 months). The current report provides novel evidence that gait parameters of 74 ambulatory persons with MS with mild-to-moderate disability are reliable over 6-months (ICC's for overall sample range from 0.56 to 0.91) in the absence of any intervention above and beyond standard care. Such data can inform clinical decision-making and power analyses for designing RCTs (i.e., sample size estimates) involving persons with MS.
Keywords: Multiple sclerosis, Gait, Locomotion, Reliability, Sample size estimates.
Gait impairment is ubiquitous in multiple sclerosis (MS) and represents an outcome of interest for research and clinical practice, , and . Gait can be indexed with various measures  , and pressure sensitive walkways are feasible, valid and capable of detecting gait impairment across the spectrum of disability in person with MS who are ambulatory, , and . Accordingly, pressure sensitive walkways could be incorporated into randomized control trials (RCTs)  and clinical practice. Such uses require estimates regarding the reliability of parameters from pressure sensitive walkways for quantifying gait – especially over longer durations (e.g. 6 months) – that are consistent with periods of outcome assessment in clinical practice and research. Reliability is critical for informing clinical decision-making and power analyses for designing RCTs (i.e., sample size estimates). This paper provides reliability estimates for gait parameters (i.e., behavior, not the equipment) over 6 months in persons with MS in the absence of any additional intervention above and beyond usual care.
Participants were 74 ambulatory persons with MS. Inclusion criteria were: (1) MS diagnosis; (2) relapse free for 30 days prior to both testing sessions; (3) ability to travel to the research facility twice over a 6-month period; (4) age between 18 and 64 years of age; and (5) ambulatory with or without aid.
The procedure was reviewed and approved by the local Institutional Review Board, and all participants provided written informed consent. Participants initially provided basic demographic information, including self-reported disability as indexed by the Patient Determined Disease Steps  (PDDS) scale. Participants then underwent gait assessment. Participants completed the gait assessment again 6-months (±1 week) later in the absence of any additional intervention above and beyond their usual care.
1.3. Gait assessment
Gait assessment utilized a 4.9 m GAITRite™ (CIR systems, Inc) pressure sensitive walkway. Participants walked at a comfortable pace starting from a standstill 1.5 m in front of the walkway and continue 1.5 m past the end of the walkway two times. The GAITRite™ provides an overall functional ambulatory performance (FAP) score and gait velocity, cadence, step time, step length, base of support and percentage of gait cycle spent in double support as spatial and temporal measures of gait. Values were averaged across trials.
1.4. Statistical analysis
Descriptive analyses were performed in SPSS version 22 (IBM, Inc). Values are mean ± standard deviation, unless otherwise noted. Reliability was based on the intra-class correlation coefficient (2,1 mixed model) and 95% confidence interval (CI) for FAP score, gait velocity, cadence, step time, step length, base of support and double support percentage. Standard error of measurement (SEM) was calculated using standardized calculation and indicates the amount of variability inherent in a measurement attributable to measurement error  . Stability was based on paired-samplet-tests. Significance was based onp-value of ≤0.05. We examined the reliability and stability in the overall sample and then by subgroups based on self-reported mobility disability (PDSS ≤ 2.0 (i.e., no walking impairment;n = 51) and PDSS ≥ 3.0 (i.e., walking impairment;n = 23)).
The overall sample had an average age of 49.2 ± 9.0 years and was primarily female (80%). The average duration since diagnosis of MS was 11.8 ± 8.2 years. 78% of the sample reported having relapse remitting MS, 14% reported secondary progressive and 8% reported primary progressive MS. The median PDDS score was 3.0 (IQR = 3.0) and ranged from 0 to 6.
Descriptive statistics for spatiotemporal gait parameters per assessment are presented in Table 1 . Gait velocity was significantly increased over time by 7.4 cm/s [t(1,73) = −5.1;p < .05] and this coincided with an increase in cadence [t(1,73) = −5.4;p < .05], and step length [t(1,73) = −3.7;p < .05], and a decrease in step time [t(1,73) = 3.1;p < .05]. There were no significant changes in FAP score, double support percentage or base of support [p's > 0.05]. The change in gait was consistent between self-reported disability groups.
|Metric||PDDS||Baseline||6 Month||ICC||95% CI ICC||SEM|
|Gait velocity (cm/s)||Overall||104.8 (28.3)||112.2 (29.5) *||0.91||0.86–0.94||8.5|
|≤2.0||116.1 (20.4)||123.8 (21.3) *||0.83||0.72–0.90||9.6|
|≥3.0||79.5 (27.4)||86.2 (28.8) *||0.89||0.76–0.95||9.1|
|FAP||Overall||90.5 (11.6)||91.3 (12.1)||0.88||0.82–0.92||4.0|
|≤2.0||94.9 (4.8)||95.2 (5.5)||0.63||0.39–0.79||2.9|
|≥3.0||80.9 (15.7)||82.6 (17.4)||0.89||0.76–0.95||5.0|
|Step time (s)||Overall||0.61 (0.15)||0.57 (0.12) *||0.90||0.85–0.94||0.05|
|≤2.0||0.56 (0.01)||0.54 (0.01) *||0.80||0.68–0.88||0.004|
|≥3.0||0.69 (0.24)||0.66 (0.18)||0.90||0.77–0.95||0.01|
|Step length (cm)||Overall||60.0 (12.8)||62.3 (12.6) *||0.91||0.86–0.94||3.8|
|≤2.0||65.5 (9.8)||66.4 (9.5) *||0.88||0.79–0.93||4.2|
|≥3.0||49.9 (13.1)||53.0 (13.9) *||0.88||0.75–0.95||4.5|
|Cadence (steps/min)||Overall||102.6 (14.9)||106.5 (14.9)||0.91||0.86–0.94||4.5|
|≤2.0||107.9 (8.7)||112.1 (9.7) *||0.82||0.68–0.90||3.7|
|≥3.0||90.9 (17.8)||94.1 (16.9)||0.92||0.81–0.96||5.0|
|Base of support (cm)||Overall||11.3 (3.8)||10.7 (4.5)||0.56||0.39–0.70||2.5|
|≥3.0||12.9 (5.0)||11.7 (5.9)||0.35||0.06–0.66||4.0|
|Double support (%)||Overall||31.4 (6.2)||30.4 (7.2)||0.60||0.43–0.73||3.9|
|≤2.0||29.6 (4.3)||28.4 (6.6)||0.38||0.12–0.60||3.3|
|≥3.0||35.5 (7.7)||34.7 (6.9)||0.80||0.59–0.91||3.2|
* Significantly different from baseline (p < 0.05).
Table 1 contains ICCs and SEM for gait parameters. Overall, gait velocity, step time, step length, and cadence all had ICCs of 0.90 or larger (95% CIs = 0.85–0.94), whereas FAP score had an ICC of 0.88 (95% CI = 0.82–0.92). Base of support had the lowest ICC of 0.56 (95% CI = 0.39–0.70), and double support percentage had an ICC of 0.60 (95% CI = 0.43–0.73). Relatively to average parameters, base of support and double support had the largest SEM while FAP and cadence had the smallest.
The walking impairment group had larger reliability estimates than the no walking impairment group in all parameters, except base of support and double support percentage. The walking impairment group had slightly larger SEM in the majority of gait parameters, except for base of support
The current investigation provides estimates of reliability for common spatiotemporal parameters of gait in MS. Gait velocity, FAP score, step length, time and cadence were all reliable over 6-months. Double support percentage and base of support had lower reliability over 6-months. Such results generally confirm the reliability of this gait behavior over a clinically feasible time scale of 6-months. By comparison, other investigations have typically established reliability of various walking measures and technology in MS and other populations over much shorter time scales (e.g. trial to trial  , day to day  ) that may not be ideal for informing clinical practice and RCTs.
We provide evidence-based estimates of reliability that can be included in sample size estimates for RCTs targeting gait metrics  . For example, when powering a RCT based on a small effect size (f-value) of 0.1 for a condition by time interaction and an assumed 0.8 power, the default ICC of 0.5 included in power analysis software would yield an overall sample size of 200 participants. However, if an experimentally derived ICC for gait velocity of 0.90 was included in the power analysis, the estimate of sample size would include only 42 participants. This represents a five-fold decrease in sample size and would translate into significantly less financial cost of conducting the research and subject burden.
We further noted the influence of disability on reliability estimates with estimates being higher in those with walking impairment versus those who do not have walking impairment. This highlights the impact of sample characteristics on reliability estimates and can further inform power estimates. This further demonstrates the value of this technology for tracking gait impairment in those with onset of walking problems.
There was a slight increase (∼7%) in gait velocity and corresponding gait parameters over the 6-month observation period. This improvement was unexpected and the contributing factors are unclear. Although this change in gait velocity was statistically significant, it was below the threshold of clinically meaningful change (20%) in gait speed in persons of MS  . It was also noted that step time had the lowest relative measurement error as indicated by SEM calculations indicating that it is the most precise gait parameter examined.
This study is not without limitations. The biggest limitation is the inclusion of a self-report measure of disability status. This limits our understanding of clinically detectable change in disability over time for explaining changes in gait parameters and comparison with samples using the clinically determined EDSS.
Overall, we provide novel evidence that spatiotemporal gait parameters collected with a pressure sensitive walkway generally demonstrate t reliability over a 6-month period. We observed a significant, albeit small, increase in gait speed and related gait metrics over this period. Such data in the absence of an intervention can inform future gait assessments in clinical practice and research involving persons with MS.
This project was funded in part by the National Multiple Sclerosis Society (PP 1695).
Conflict of interest statement
The authors have no conflicts of interest to report.
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a Department of Kinesiology and Community Health, University of Illinois at Urbana-Champaign, IL, USA
b Department of Kinesiology, East Carolina University, USA
Corresponding author at: Department of Kinesiology and Community Health, University of Illinois at Urbana-Champaign, 906 South Goodwin Avenue, Urbana, IL 61801, USA. Tel.: +1 217 265 0886; fax: +1 217 244 7322.
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