Gait & Posture 41 (2015) 222–227

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Further validation of the Six-Spot Step Test as a measure of ambulation in multiple sclerosis Brian M. Sandroff a, Robert W. Motl a,*, Jacob J. Sosnoff a, John H. Pula b a b

Department of Kinesiology and Community Health, University of Illinois at Urbana-Champaign, Urbana, IL, USA Department of Neurology, Northshore University Health System, Evanston, IL, USA

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 July 2014 Received in revised form 25 September 2014 Accepted 12 October 2014

Background: There is preliminary evidence regarding the validity of the Six-Spot Step Test (SSST) as a promising measure of ambulatory function in persons with multiple sclerosis (MS). To date, this assessment has not been subject to the same rigor and extent of psychometric evaluation as other widely-accepted measures of ambulatory (e.g., timed 25-foot walk (T25FW)). Objective: This study aimed to provide additional validity evidence for the SSST in 96 persons with MS, based on construct validity and precision. Construct validity involves examining the pattern of associations between the SSST and other measures, and precision involves comparing SSST performance relative to other valid measures of ambulation for differentiating between levels of disability status, MS clinical course, and fall risk based on balance confidence. Methods: All participants completed the SSST, T25FW, Timed Up-and-Go (TUG), six-minute walk, Multiple Sclerosis Walking Scale-12, Late-Life Function and Disability Inventory, Activities-specific Balance Confidence, and Paced Auditory Serial Addition Test. All participants further underwent a neurological examination for generating EDSS scores, and then wore an ActiGraph accelerometer for the waking hours of a 7-day period. Results: SSST performance was strongly associated with other valid measures of ambulation (jrj = .65– .90) and disability status (r = .73), moderately-to-strongly associated with balance confidence (r = .58), and weakly-to-moderately associated with cognitive processing speed and non-ambulatory measures (jrj = .35–.39). The SSST demonstrated stronger relative precision in discriminating between levels of disability status, MS clinical course, and fall risk based on balance confidence than the T25FW and TUG. Conclusions: We provide comprehensive validity evidence for the SSST that supports its consideration for inclusion alongside other highly-regarded objective measures of ambulatory function for clinical research and practice in persons with MS. ß 2014 Elsevier B.V. All rights reserved.

Keywords: Six-spot step test Multiple sclerosis Ambulation Timed 25 foot walk Validity

1. Introduction Ambulatory impairment is a hallmark consequence of multiple sclerosis (MS) and its progression [1]. There has been substantial interest in identifying reliable, valid, and precise measures of ambulatory function in this population [2]. One recent review examined the psychometric properties of ambulation assessments that are common in MS studies (e.g., timed 25-foot walk (T25FW); 10 m walk; 30 m walk; 100 m walk; 2-min walk; 6-min walk (6MW); Multiple Sclerosis Walking Scale-12 (MSWS-12)) [2], and

* Corresponding author at: University of Illinois at Urbana-Champaign, Department of Kinesiology and Community Health, 233 Freer Hall, 906 South Goodwin Ave., Urbana, IL 61801, USA. Tel.: +1 217 265 0886; fax: +1 217 244 7322. E-mail address: [email protected] (R.W. Motl). http://dx.doi.org/10.1016/j.gaitpost.2014.10.011 0966-6362/ß 2014 Elsevier B.V. All rights reserved.

identified the T25FW as the best-characterized objective test of ambulatory function based on its psychometric properties [2]. Importantly, that review did not consider other assessments of ambulation such as the Six-Spot Step Test (SSST) based on the limited evidence regarding its psychometrics. Nevertheless, the SSST might have advantages for measuring ambulatory function in MS compared with other measures because it presumably captures elements of coordination and balance that are important in freeliving walking behavior [3]. The SSST was developed based on the premise of better capturing lower-limb functioning than a straight-line walking test, such as the T25FW [3]. The SSST involves walking as quickly as possible across a rectangular course, while kicking blocks off of five separate marked areas on the ground. There are three published papers that provide preliminary psychometric data on the SSST in MS [3–5]. One study reported strong correlations between SSST

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and T25FW performance (r = .85), and between SSST performance and Expanded Disability Status Scale (EDSS) scores (r = .80) in 151 persons with relatively moderate MS disability (mean EDSS = 3.8, range = 0–6.5) [3]. Another study recently replicated those correlations between SSST and T25FW performance (r = .85) and SSST performance and EDSS scores (r = .88) in 82 persons with relatively mild MS disability (median EDSS = 2.5, range = 0–6.5) [4]. That study provided additional validity evidence for the SSST based on its significant associations with two-minute walk distance (r = .90), MSWS-12 scores (r = .76), and steps/day from an accelerometer(r = .65) [4]. The SSST further demonstrated strong test–retest reliability among 75 persons with MS and 91 healthy controls [5]. The preliminary validity evidence for the SSST is promising, but this assessment has not been subject to same rigor and extent of psychometric evaluation as other widely-accepted measures of ambulatory function. For example, the MSWS-12 underwent rigorous evaluation that demonstrated strong psychometric properties during multiple studies over nearly a decade [6– 8]. Researchers and clinicians now widely use the MSWS-12 as a patient-reported outcome measure of ambulatory function in MS. Such psychometric evaluation is warranted for the SSST, as evidence of validity is an ongoing, evolving process that evolves over continual examinations in different samples and conditions over time [9]. The current study aimed to provide additional validity evidence for the SSST as a measure of ambulatory function in MS. Such evidence is important for establishing the SSST as a plausible measure for research and clinical practice. Our framework for validation involved construct validity and precision. Construct validity (i.e., convergent and divergent validity) involves examining the pattern of associations between SSST performance and other measures. To first establish convergent validity, we examined the associations of SSST performance with other valid measures of ambulation (i.e., T25FW; 6MW; Timed Up-and-Go (TUG); MSWS-12; Late-Life Function and Disability Inventory (LLFDI-Lower Extremity); accelerometer-measured steps/day). To establish divergent validity, we then examined the associations of SSST performance with balance confidence (e.g., Activities-specific balance confidence (ABC)), disability status (i.e., EDSS scores; Functional Systems (FS) scores), cognitive processing speed (CPS; Paced Auditory Serial Addition Test (PASAT)), and other nonambulatory measures (i.e., age; disease duration; LLFDI-Upper Extremity). This is important as SSST performance might be associated with balance confidence, disability status, and CPS, given that those constructs have previously been associated with other measures of ambulation in MS [3,4,10–14]. Precision involves examining the degree to which SSST performance differentiates between levels of disability status, MS clinical course, and fall risk based on balance confidence relative to other valid measures of ambulation. Using this framework, we expected that the SSST would be strongly-associated with other valid measures of ambulation, moderately-associated with balance confidence, disability status, and CPS, and weakly-associated with non-ambulatory measures. We further hypothesized that the SSST would demonstrate better precision in defining levels of disability status, MS clinical course, and fall risk based on balance confidence than other valid measures of ambulatory function.

records. The inclusion criteria involved (a) having a clinicallydefinite diagnosis of MS and (b) being ambulatory either with or without use of an assistive device (i.e., unassisted, cane/crutch/ walker use, but not confined to wheelchair). We contacted 190 persons with MS, screened and scheduled 125; 65 of those we contacted were uninterested in the study and were not screened. Of the 125 who initially volunteered, 29 persons canceled the testing appointment and were not available to reschedule, resulting in a final sample of 96 persons with MS. 2.2. Measures SSST. The SSST was performed as an objective measure of ambulatory function [3]. The course was set up based on previously reported specifications (i.e., 5 m long and 1 m wide, with plastic cones positioned 1 and 3 m from the starting line on one side, and 2 and 4 m from the starting line on the other side, with a final cone positioned at the course’s midline, 5 m from the starting line; Fig. 1). Briefly, participants completed the course as quickly as possible, while kicking the cones off of their labeled position with one foot, alternating between medial and lateral sides of the foot. Each participant completed the SSST four times: twice using their dominant foot, and twice using their nondominant foot to kick the cones off of their demarcated position [3]. The primary outcome was the mean of the four trials in seconds. We further monitored slips, trips, and falls during the SSST in case of emergency. This information was not systematically collected for providing evidence of the safety of the SSST in persons with MS.

2. Methods 2.1. Participants We recruited persons with MS who resided within a 90-min drive of a Midwestern neurology practice. Recruitment was conducted via local media outlets, promotional flyers, and medical

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Fig. 1. Course set-up for the Six-Spot Step Test. Adapted from Nieuwenhuis et al. (2008).

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T25FW. The T25FW was administered as a measure of walking speed and has been identified as the best-characterized objective measure of ambulation in MS based on its psychometric properties [2]. Each participant completed the T25FW twice, while walking as fast as possible. The primary outcome was the mean of the two walks in seconds. 6MW. The 6MW was administered as a measure of walking endurance that is valid and reliable in persons with MS [10]. Participants completed the 6MW as quickly as possible in a square hallway with four corridors, each 100 ft long, while performing 908 turns around corners. The primary outcome was total distance traveled in feet. TUG. The TUG was administered as an objective measure of ambulatory function that is valid and reliable in persons with MS [14]. The TUG involves two trials of being seated in a chair, followed by standing up, walking toward a cone placed 10 ft away, turning around the cone, walking back toward the chair, and then returning to a seated position, as quickly as possible without using the hands for support [14,15]. The primary outcome was the mean of the two trials in seconds. MSWS-12. The MSWS-12 is a 12-item patient-rated measure of the impact of MS on walking [6]. The 12 items on the MSWS-12 are rated on a scale ranging between 1 (Not at all) and 5 (Extremely). The total MSWS-12 score ranges between 0 and 100 and is computed by summing the individual item scores, subtracting the minimum possible score (12), dividing by the maximal score (48), and then multiplying the result by 100 [6]. Abbreviated Late-Life Function and Disability Inventory (LLFDI). The functional component of the abbreviated LLFDI was included as a patient-reported measure of functional limitations that has been validated in persons with MS [16]. This outcome contains 15 items that are broken down into 3 subscales (i.e., upperextremity function (UEF), basic lower-extremity function (BLEF), and advanced lower-extremity function (ALEF)). The 15-items were rated on a 5-point ordinal scale of 1 (none) to 5 (cannot do) and were reverse-scored. Scores were averaged to comprise composite UEF, BLEF, and ALEF measures. Scores for each fiveitem subscale range between 5 and 25, and higher scores reflect fewer functional limitations [16,17]. Free-living walking behavior. Free-living walking behavior was objectively-measured by ActiGraph (Health One Technology, Fort Walton Beach, FL) model GT3X accelerometers and expressed as average steps/day. Output from these accelerometers has been validated as a measure of walking behavior in persons with MS [18]. The data were retrieved from the accelerometer and then imported into ActiLife 5 for validity check (i.e., 10 h wear-time without periods exceeding 60-min of continuous zeroes per day) and processing of average steps/day. Balance confidence. The Activities-specific Balance Confidence (ABC) scale was used as a self-report measure of balance confidence and has been validated in persons with MS [19]. On the ABC, participants rate their confidence level, using a 0–100% scale, to complete 16 different activities. The primary outcome from the ABC is the total rating (i.e., between 0 and 1600) divided by 16. Scores below 63.92 are associated with an increased fall risk in persons with MS [20], and we operationalized this cut-off to examine the relative precision of the SSST, T25FW, and TUG in discriminating between high and low fall risk in the current sample. Disability status. All participants underwent a neurological examination by a neurologist (JHP) for generating FS and EDSS scores [21]. EDSS scoring includes testing and rating of visual, brainstem, pyramidal, cerebellar, sensory, bowel/bladder, and mental FS. The FS scores, along with ambulation based on 500 m walk, are combined into an EDSS step score that ranges from 0 (no disability) to 10 (death from MS). Based on the inclusion criterion

of being ambulatory with or without assistance, all participants undertook a 500 m walk test. This took place in a square hallway with four corridors, each 100 feet in length, while performing 908 turns around corners; the 500 m walk was not timed. Cognitive processing speed. We measured CPS using the PASAT. This test has been validated in persons with MS and is included in the Multiple Sclerosis Functional Composite (e.g., [22]). Briefly, a series of single-digit numbers are presented at the rate of 1 every 3 s via audio-recording. Participants are explicitly instructed to orally report the sum of the last two numbers that were presented on the recording. The primary outcome was the total number of correct answers (i.e., raw score) [23]. 2.3. Protocol The procedure was approved by a University Institutional Review Board and all participants provided written informed consent. The data were collected during one session in a single clinical setting. There was no standardization of the exact ordering of tests as more than one participant underwent testing during a given time period. We did administer the tests in an order that minimized motor fatigue such that there was ample seated-rest between the administration of ‘physically-active’ outcomes (i.e., SSST; T25FW; 6MW; TUG) and ‘non-physically-active’ outcome (i.e., demographics questionnaire; MSWS-12; LLFDI; ABC; PASAT). Participants further underwent a neurological examination for generating EDSS/FS scores that included the 500 m walk test. After the testing session, participants were provided with an accelerometer, belt, log, and instructions for wearing the unit during the waking hours of the subsequent 7 days, along with a pre-stamped and pre-addressed envelope for its return. All participants received $20 remuneration upon return of the accelerometer. 2.4. Data analysis All data were analyzed in SPSS v.21 (SPSS Inc.; Chicago, IL). We report descriptive characteristics of all measures of ambulation (e.g., SSST; T25FW; 6MW; TUG; MSWS-12; LLFDI-ALEF; LLFDIBLEF; steps/day), ABC scores, EDSS/FS scores, PASAT scores, and non-ambulatory measures (e.g., age; disease duration; LLFDI-UEF) as mean (SD) throughout. Our analysis of construct validity involved performing bivariate Spearman rho (r) rank-order correlations between SSST performance and all other ambulatory and non-ambulatory measures in case outliers or non-linearity were biasing the correlations [24]. Values for correlation coefficients of .1, .3, and .5 were interpreted as small, moderate, and large, respectively [25]. We then were interested in the potential differences in precision of the SSST, T25FW, and TUG for defining subgroups based on disability status, MS clinical course, and fall risk based on balance confidence (i.e., cut-off scores from the ABC). To do this, we first examined those subgroup differences in SSST, T25FW, and TUG performance, respectively, using one-way ANOVA. To determine the relative precision of those tests in defining each subgroup, we then calculated a ratio of F-values (low/high) for each category, separately. This is consistent with previous research on ambulation in MS [10]. 3. Results 3.1. Descriptive characteristics The demographic/clinical characteristics of the overall sample are presented in Table 1. Briefly, the current sample demonstrated demographic and clinical characteristics that are similar to other samples of persons with relatively moderate MS disability (i.e., median EDSS = 4.5) (e.g., [3]). Descriptive data on the measures of ambulatory and non-ambulatory function characteristics of the current sample are presented in Table 2. Briefly, participants demonstrated similar performance on measures of ambulation (i.e., SSST; T25FW; 6MW; TUG; MSWS-12; LLFDI-ALEF; LLFDI-BLEF; steps/day), ABC scores, PASAT

B.M. Sandroff et al. / Gait & Posture 41 (2015) 222–227 Table 1 Demographic and clinical characteristics of 96 persons with MS. Variable

MS (N = 96)

Age (years) Sex (n, % female) MS type (n, %) Relapsing-remitting MS Progressive MS Unknown MS duration (years) Assistive device use (n, %) None Cane Walker

52.7 (11.1) 77/96 (80.2%)

62/96 (64.5%) 29/96 (30.2%) 5/96 (5.2%)

MS (N = 96)

Ambulation SSST (s) T25FW (s) 6MW (ft) TUG (s) MSWS-12 (score) LLFDI-ALEF (score) LLFDI-BLEF (score) Steps per day Balance confidence ABC (score) Disability status EDSS (median, range) FSS-Visual (median, range) FSS-Brainstem (median, range) FSS-Pyramidal (median, range) FSS-Cerebellar (median, range) FSS-Sensory (median, range) FSS-Bladder/Bowel (median, range) FSS-Mental (median, range) Cognitive processing speed PASAT (raw score) Upper-extremity function LLFDI-UEF (score)

Category

79/96 (82.3%) 13/96 (13.5%) 4/96 (4.2%) 11.8 (10.0)

Table 2 Descriptive characteristics of measures of ambulation, balance confidence, disability status, cognitive processing speed, and upper extremity function in 96 persons with MS. Variable

Table 3 Spearman rank-order correlations (r) between SSST performance and measures of ambulation, balance confidence, disability status, cognitive processing speed, and non-ambulatory measures in 96 persons with MS. Variable

SSST

T25FW (s) 6MW (ft) TUG (s) MSWS-12 (score) LLFDI-ALEF (score) LLFDI-BLEF (score) Steps per day

.90* .84* .86* .67* .65* .67* .65*

ABC (score)

.58*

Ambulation

Note: All data are presented as mean (SD), unless indicated otherwise.

Category

225

11.08 (5.38) 6.85 (3.09) 1335.53 (414.16) 9.20 (4.74) 33.57 (13.22) 12.99 (5.64) 19.48 (4.50) 4043 (2482) 66.89 (23.65) 4.5 1.0 2.0 2.0 1.0 2.0 1.0 2.0

(2.0–6.5) (0–3.0) (0–3.0) (0–3.0) (0–3.0) (0–3.0) (0–4.0) (0–3.0)

41.09 (13.05) 19.74 (3.96)

Note: All data are presented as mean (SD), unless indicated otherwise. SSST, Six-spot step test; T25FW, Timed 25-foot walk; 6MW, Six-minute walk; TUG, Timed Upand-Go; MSWS-12, Multiple Sclerosis Walking Scale-12; LLFDI-ALEF, Late-life Function and Disability Inventory-Advanced Lower Extremity Function; LLFDIBLEF, Late-life Function and Disability Inventory-Basic Lower Extremity Function; ABC, Activities-Specific Balance Confidence; EDSS, Expanded Disability Status Scale; FSS, Functional Systems Score; PASAT, Paced Auditory Serial Addition Test; LLFDIUEF, Late-life Function and Disability Inventory-Upper Extremity Function.

scores, and non-ambulatory measures (i.e., LLFDI-UEF) as other samples of persons with relatively moderate MS disability status (e.g., [3,4,18]).

Balance confidence Disability status EDSS (median, range) FSS-Visual (median, range) FSS-Brainstem (median, range) FSS-Pyramidal (median, range) FSS-Cerebellar (median, range) FSS-Sensory (median, range) FSS-Bladder/Bowel (median, range) FSS-Mental (median, range) Cognitive processing speed PASAT (raw score) Non-ambulatory measures LLFDI-UEF (score) Age (years) Disease duration (years)

.73* .36* .09 .47* .51* .23* .39* .18 .35* .35* .36* .20

Note: * Statistical significance at p < 0.05. SSST, Six-spot step test; T25FW, Timed 25foot walk; 6MW, Six-minute walk; TUG, Timed Up-and-Go; MSWS-12, Multiple Sclerosis Walking Scale-12; LLFDI-ALEF, Late-life Function and Disability InventoryAdvanced Lower Extremity Function; LLFDI-BLEF, Late-life Function and Disability Inventory-Basic Lower Extremity Function; ABC, Activities-Specific Balance Confidence; EDSS, Expanded Disability Status Scale; FSS, Functional Systems Score; PASAT, Paced Auditory Serial Addition Test; LLFDI-UEF, Late-life Function and Disability Inventory-Upper Extremity Function.

distance, less self-reported mobility impairment, better lower-limb function, and engaging in more free-living walking behavior. Divergent validity. SSST performance was associated with scores on the ABC (r = .58), such that those who demonstrated faster SSST performance had significantly greater balance confidence. SSST performance was strongly associated with EDSS scores (r = .73). SSST performance was moderate-to-strongly associated with FS-Visual, FS-Pyramidal, FS-Cerebellar, and FS-Bladder/Bowel scores (jrj = .35–.51). SSST performance was weakly associated with FS-Sensory scores (r = .23), and was not statistically significantly associated with FS-Brainstem and FS-Mental scores, respectively (Table 3). This indicates that faster SSST performance was associated with more mild disability status overall, as well as better visual, pyramidal, cerebellar, bladder/ bowel, and sensory function. SSST performance was moderately associated with PASAT scores (r = .35), such that those who demonstrated faster SSST performance had significantly faster CPS (i.e., higher PASAT scores). SSST performance was weakly-to-moderately-associated with some nonambulatory measures (jrj = .35–.39) (Table 3). This indicates that better SSST performance was weakly-to-moderately-associated with younger age and better upper-extremity function. There was a non-statistically significant association between SSST performance and disease duration (r = .09). 3.4. Relative precision of measures of ambulation

3.2. SSST administration All 96 participants were able to undertake the SSST without problem, regardless of disability status or type of assistive device used. The SSST was administered with standardized instructions and no participants reported difficulty in understanding the rules of the test. There further were no adverse events, including falls, during the SSST. 3.3. Construct validity The associations between SSST and all other outcomes are presented in Table 3. Convergent validity. As expected, SSST performance was strongly-associated with other valid measures of ambulation, and those associations were statistically significant (jrj = .65–.90). This indicates that faster SSST performance was stronglyassociated with faster T25FW performance, faster TUG performance, greater 6MW

The F-values and values for relative precision (i.e., (1-ratio of F-values)) for the SSST, T25FW, and TUG based on levels of disability status, MS clinical course, and fall risk based on balance confidence are presented in Table 4. The mean scores (SD) for the SSST, T25FW, and TUG based on levels of disability status, MS clinical course, and fall risk based on balance confidence are presented in Table 5. Disability status. We stratified the overall sample into three groups based on mild (n = 30; EDSS = 2.0–3.5), moderate (n = 29; EDSS = 4.0–5.5), and severe (n = 37; EDSS = 6.0–6.5) disability; these groupings are consistent with benchmarks of disability accumulation in persons with MS [26]. Briefly, the ratio of F-values indicated that the SSST was approximately 27% more precise than the T25FW and 9% more precise than the TUG in discriminating between levels of disability. MS clinical course. We next dichotomized the overall sample based on persons with relapsing-remitting (n = 79) and progressive MS (n = 13); 4 persons with MS did not provide data on clinical course. Briefly, the ratio of F-values indicated that

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Table 4 F-values and relative precision of the SSST, T25FW, and TUG for defining subgroups of disability status, MS clinical course, and fall risk based on balance confidence in 96 persons with MS. Subgroup category

Disability status MS clinical course Fall risk

F-value

Relative precision (%)

SSST

T25FW

TUG

SSST vs. T25FW

SSST vs. TUG

33.639* 11.970* 29.461*

24.560* 6.277* 22.919*

30.800* 5.958* 19.693*

+27.0% +47.6% +22.2%

+8.5% +50.2% +33.2%

Note: * statistical significance at p < 0.05, F-values based on one-way ANOVA. SSST, Six-spot step test; T25FW, Timed 25-foot walk; TUG, Timed Up-and-Go; For relative precision, positive values indicate increased precision of the SSST and negative values indicate decreased precision of the SSST relative to the T25FW and TUG, respectively.

Table 5 Mean scores of the SSST, T25FW, and TUG based on levels of disability status, MS clinical course, and fall risk based on balance confidence in 96 persons with MS. Category Disability status Mild (EDSS = 0–3.0) Moderate (EDSS = 4.0–5.5) Severe (EDSS = 6.0–6.5) MS clinical course Relapsing-remitting Progressive Fall risk based on ABC Low Fall risk (ABC  69.32) High Fall risk (ABC < 69.32)

SSST (s)

T25FW (s)

TUG (s)

7.26 (2.13) 9.65 (3.45) 15.29 (5.60)

4.77 (1.02) 6.25 (1.81) 9.01 (3.62)

5.91 (1.56) 8.00 (2.08) 12.82 (5.50)

10.32 (4.56) 15.67 (8.06)

6.53 (2.59) 8.82 (5.12)

8.71 (4.18) 12.14 (7.20)

8.71 (3.61) 14.02 (5.86)

5.61 (1.92) 8.37 (3.60)

7.43 (3.47) 11.41 (5.25)

Note: Data are presented as mean (standard deviation). SSST, Six-spot step test; T25FW, Timed 25-foot walk; TUG, Timed Up-and-Go; EDSS, Expanded Disability Status Scale; ABC, Activities-specific Balance Confidence.

the SSST was approximately 48% more precise than the T25FW and 50% more precise than the TUG in discriminating between relapsing-remitting and progressive MS. Fall risk based on balance confidence. We lastly dichotomized the overall sample based on high (i.e., ABC scores < 69.32) (n = 42) and low fall risk (i.e., ABC scores 69.32) (n = 53) [20] using cut-off scores on the ABC; 1 participant did not complete the ABC. The ratio of F-values indicated that the SSST was approximately 22% more precise than the T25FW and 33% more precise than the TUG in discriminating between high and low fall risk based on balance confidence.

4. Discussion The SSST has not been widely-adopted as an objective measure of ambulatory function in persons with MS, despite its conceptual distinction from other prominent measures of ambulatory function (e.g., T25FW). This likely reflects the lack of consistent and widely-available evidence for the psychometrics of the SSST in MS, much like what occurred with the MSWS-12. To that end, the current study provided (a) additional validity evidence for the SSST by replicating strong, statistically significant correlations between the SSST and T25FW, MSWS-12, EDSS, and average steps/day in a relatively large sample of persons with MS and (b) novel convergent validity evidence for the SSST based on strong, statistically significant correlations between the SSST and other validated measures of ambulatory function (i.e., 6MW, TUG, LLFDIALEF, and LLFDI-BLEF, respectively). We further provide novel evidence for the SSST having generally stronger precision than the T25FW and TUG in discriminating between levels of disability, MS clinical course, and fall risk based on balance confidence scores. Collectively, the current results offer additional and stronger psychometric data on the SSST that support its consideration for inclusion alongside other highly-regarded objective measures of ambulatory function for clinical research and practice in persons with MS. Another noteworthy aspect of the current study involves novel evidence of divergent validity for the SSST. This is partly based on moderate and smaller associations with measures of balance

confidence and functional systems (i.e., FS-Pyramidal, FS-Cerebellar) scores. This supports the consideration of the SSST as a unique measure of ambulatory function. As the SSST involves elements of balance and lower-limb coordination, we expected that it would be moderately-associated with measures of balance confidence and pyramidal/cerebellar function. However, as the primary emphasis of the SSST is on speed, we did not expect that SSST performance would purely reflect balance confidence/fall risk, and pyramidal/ cerebellar function. Further, the SSST was moderately associated with PASAT scores, and weakly-to-moderately associated with other functional systems scores and some non-ambulatory measures. Though the SSST is clearly not a measure of upperextremity function, visual/sensory function, or CPS, the current results suggest that the SSST might capture some degree of those functions. Indeed, there is evidence of cognitive-motor coupling in MS, based on a significant association between the T25FW and PASAT (r = .36) [11]. We report an almost identical example of cognitive-motor coupling, given the present moderate-sized correlation between SSST and PASAT performance (r = .35). Another novel feature of the current study is that we provide the first evidence of relative precision between the SSST and other validated measures of ambulatory function in MS. Although the SSST was strongly-correlated with T25FW and TUG performance, the SSST demonstrated increased precision in discriminating between levels of disability, MS clinical course, and fall risk based on ABC cut-off scores than the T25FW and TUG. This is important as there might be floor effects using the T25FW and TUG among persons with mild MS disability, whereas the SSST involves a more difficult task that might be more ecologically valid and less susceptible to floor effects [3]. One study limitation involved the lack of objective balance metrics (e.g., static posturography) to better characterize the SSST as a potential measure of balance function in MS. An additional limitation is that the current study did not provide reliability evidence for the SSST over time. To date, there is very limited evidence on the test-retest reliability of the SSST [5]; however, the current study represents a secondary data analysis from a study originally designed as an examination of objective ambulatory measures and optical coherence tomography [27]. Nevertheless, this afforded us the opportunity to provide important validity evidence for the utility of the SSST in clinical research and practice based on its pattern of associations with other validated measures of ambulatory and non-ambulatory function in persons with MS. Funding This study was funded by an investigator initiated grant from Biogen Idec. Acknowledgements This study was funded by an investigator initiated grant from Biogen Idec. Conflict of interest: All authors declare no conflicts of interest.

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References [1] Motl RW. Ambulation and multiple sclerosis. Phys Med Rehabil Clin N Am 2013;24(2):325–36. [2] Kieseier BC, Pozzilli C. Assessing walking disability in multiple sclerosis. Mult Scler 2012;18(7):914–24. [3] Niewenhuis MM, Van Tongeren H, Sorensen PS, Ravnborg M. The six spot step test: a new measurement for walking ability in multiple sclerosis. Mult Scler 2006;12:495–500. [4] Filipovic Grcic P, Matijaca M, Bilic I, Dzamonja G, Lusic I, Caljkusic K, et al. Correlation analysis of visual analogue scale and measures of walking ability in multiple sclerosis patients. Acta Neurol Belg 2013;113(4):397–402. [5] Pavan K, Tilbery Cp, Lianza S, Marangoni BEM. Validation of the six step spot test for gait among patients with multiple sclerosis in Brazil. Arq Neuropsiquiatr 2010;68(2):198–204. [6] Hobart JC, Riazi A, Lamping DL, Fitzpatrick R, Thompson AJ. Measuring the impact of MS on walking ability: the 12-item MS Walking Scale (MSWS-12). Neurology 2003;60:31–6. [7] Pilutti LA, Dlugonski D, Sandroff BM, Suh Y, Pula JH, Sosnoff JJ, et al. Further validation of multiple sclerosis walking scale-12 scores based on spatiotemporal gait parameters. Arch Phys Med Rehabil 2013;94(3):575–8. [8] Motl RW, Learmonth YC, Pilutti LA, Dlugonski D, Klaren R. Validity of minimal clinically important difference values for the multiple sclerosis walking scale12? Eur Neurol 2014;71(3–4):196–202. [9] Messick S. Standards of validity and the validity of standards in performance assessment. Educ Measure: Issues Practice 1995;14(4):5–8. [10] Goldman MD, Marrie RA, Cohen JA. Evaluation of the six-minute walk in multiple sclerosis subjects and healthy controls. Mult Scler 2008;14:383–90. [11] Benedict RH, Holtzer R, Motl RW, Foley FW, Kaur S, Hojnacki D, et al. Upper and lower extremity motor function and cognitive impairment in multiple sclerosis. J Int Neuropsychol Soc 2011;17(4):643–53. [12] Motl RW, Sosnoff JJ, Dlugonski D, Pilutti LA, Klaren R, Sandroff BM. Walking and cognition: but not symptoms, correlate with dual task cost of walking in multiple sclerosis. Gait Posture 2014;39(3):870–4. [13] Learmonth YC, Sandroff BM, Pilutti LA, Klaren RE, Ensari I, Riskin BJ, et al. Cognitive motor interference during walking in multiple sclerosis using an alternate-letter alphabet task. Arch Phys Med Rehabil 2014;95:1498–503.

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[14] Nilsagard Y, Lundholm C, Gunnarsson LG, Dcnison E. Clinical relevance using timed walk tests and ‘timed up and go’ testing in persons with multiple sclerosis. Physiother Res Int 2007;12(2):105–14. [15] Weikert M, Suh Y, Lane A, Sandroff B, Dlugonski D, Fernhall B, et al. Accelerometry is associated with walking mobility: not physical activity, in persons with multiple sclerosis. Med Eng Phys 2012;34(5):590–7. [16] Motl RW, McAuley E, Suh Y. Validity: invariance and responsiveness of a selfreport measure of functional limitations and disability in multiple sclerosis. Disabil Rehabil 2010;32(15):1260–71. [17] Learmonth YC, Motl RW, Sandroff BM, Pula JH, Cadavid D. Validation of patient determined disease steps (PDDS) scale scores in persons with multiple sclerosis. BMC Neurol 2013;13(1):37–44. [18] Motl RW, Pilutti L, Sandroff BM, Dlugonski D, Sosnoff JJ, Pula JH. Accelerometry as a measure of walking behavior in multiple sclerosis. Acta Neurol Scand 2013;127:384–90. [19] Nilsagard Y, Carling A, Forsberg A. Activities-specific balance confidence in people with multiple sclerosis. Mult Scler Int 2012;2012:613925. [20] Dibble LE, Lopez-Lennon C, Lake W, Hoffmeister C, Gappmaier E. Utility of disease-specific measures and clinical balance tests in prediction of falls in persons with multiple sclerosis. J Neurol Phys Ther 2013;37(3):99–104. [21] Kurtzke JF. Rating neurologic impairment in multiple sclerosis: an expanded disability status scale (EDSS). Neurology 1983;33:1444–52. [22] Fisher JS, Jak AJ, Kniker JE, Rudick RA, Cutter G. Multiple sclerosis functional composite (MSFC): administration and scoring manual; 2001;p. 1–44. [23] Gronwall DMA. Paced auditory serial-addition task: a measure of recovery from concussion. Percept Mot Skills 1977;44:367–73. [24] Rousselet GA, Pernet CR. Improving standards in brain-behavior correlation analyses. Front Hum Neurosci 2012;6:119. [25] Cohen J. Statistical power analysis for the behavioral sciences. second edition, Hillsdale, NJ: Lawrence Erlbaum Associates; 1988. [26] Confavreux C, Vukusic S. Natural history of multiple sclerosis: a unifying concept. Brain 2006;129:606–16. [27] Balantrapu S, Sandroff BM, Pula JH, Motl RW. Integrity of the anterior visual pathway and its association with ambulatory performance in multiple sclerosis. Mult Scler Int 2013;2013:481035.