Introduction
Multiple sclerosis (MS) is an autoimmune neurological disorder of the central nervous system, which affects over 2.3 million people—approximately twice as common in women than in men [
1]. MS can lead to lesions in the brain, spinal cord, or optic nerves, which causes a variety of symptoms, such as weakness, spasticity, fatigue, and changes in sensation, coordination, vision, and cognition. As a result, it is not surprising that MS negatively affects balance, gait function, and the risk of falling [
2].
The Six-Spot Step Test (SSST) is a measure of ambulation that captures complex elements of both gait function and dynamic balance [
3]. Additionally, the SSST measures more complex motor tasks than other commonly used tests of gait and functional mobility (e.g., the timed 25-foot walk (T25FW) and the timed “up and go” test (TUG)) [
4]. Fay Horak's theoretical framework of balance, which encompasses motor, sensory, and cognitive components, is commonly used to define the balance control system [
5]. In this context, the SSST includes fewer balance components compared to more comprehensive tests of balance capacity, such as the mini-Balance Evaluation Systems Test (mini-BESTest) [
6,
7]. However, the SSST is more time efficient than this more extensive measures of balance (i.e., mini-BESTest) and requires minimal equipment, space, and assessor training. These qualities make the SSST a convenient and easily implemented tool in clinical settings.
A recent systematic review of the psychometric properties of the SSST revealed that robust evidence supports the reliability and validity of assessing dynamic balance in older adults and people with neurological diseases, including those with Parkinson's disease, stroke, chronic inflammatory polyneuropathy, and MS [
8]. Furthermore, studies examining the SSST's validity found a strong to very strong correlation between the SSST and the T25FW and between the SSST and the TUG in people with MS [
9,
10]. Similar results have been found in other neurological diseases and older adults with balance problems [
11‐
14]. Only a few studies have investigated the correlation between the SSST and measures of balance capacity (e.g., mini-BESTest). Brincks et al. [
11] and Brincks and Callesen [
12] found a moderate correlation (−0.64 and −0.62, respectively) between the SSST and the mini-BESTest in both ambulatory people with Parkinson's disease and older adults with self-reported balance problems. However, the systematic review by Aakrann and Brincks [
8] of the SSST's psychometric properties did not yield sufficient evidence concerning its longitudinal validity (responsiveness, i.e., the ability of the test to detect meaningful change over time in the construct to be measured), as only two studies of low methodological quality assessed the SSST's responsiveness to intervention-induced changes. In addition, previous studies have only investigated the responsiveness of the SSST against walking-based outcomes (i.e., Multiple Sclerosis Walking Scale-12, the 2-min timed walk, and the T25FW) [
15,
16]. Hence, no previous study has evaluated the responsiveness of the SSST after a structured exercise intervention against measures of balance capacity (mini-BESTest) in people with MS. Examining the responsiveness of health-related outcome measures, such as the Six-Spot Step Test (SSST), is essential, as it reflects a measure’s capability to detect meaningful changes over time; an aspect not addressed by construct validity, which only captures relationships between variables at a single point in time.
Studies have found a positive effect of balance training when evaluated by the mini-BESTest and the T25FW [
17]. The magnitude of change after balance training in people with MS ranges from approximately 5% to 25% and from non-significant to approximately 10% for the mini-BESTest and the T25FW, respectively [
18‐
22]. Regarding resistance training, the magnitude of change after an intervention is approximately 10% for T25FW [
23], while the change in the mini-BESTest is unknown.
This study's primary objective was to examine the responsiveness of the SSST in people with MS after a 10-week intervention period involving resistance and balance control training, using the T25FW and the mini-BESTest as external criteria for gait speed and balance capacity, respectively. It was hypothesized that 1) changes in the SSST were moderately negatively correlated to changes in the mini-BESTest and the T25FW, representing a correlation coefficient between −0.3 and −0.5 in accordance with COSMIN guidelines [
24], and 2) a 10-week program involving resistance training and balance control exercises would result in an approximate 15% improvement in the SSST. This anticipated magnitude of improvement aligns with changes observed in the T25FW and the mini-BESTest after undergoing similar exercise treatments with known efficacy on the construct of interest (balance capacity and gait speed).
Discussion
To the best of our knowledge, this study is the first to investigate the responsiveness of the SSST concerning balance capacity, as measured by the mini-BESTest. Previous studies have only examined the responsiveness between the SSST to gait speed or straight-line walking [
15,
16]. Furthermore, including a clearly defined hypothesis regarding the expected relationships between a commonly used gait measure in MS rehabilitation (T25FW) and balance capacity enhances the evaluation of the SSST's psychometric properties, particularly its responsiveness, which has been insufficiently explored. As expected, a negative moderate correlation was found between the changes in the SSST and the changes in both the mini-BESTest (r
s = −0.33) and the T25FW (r
s = −0.37) after 10 weeks of combined resistance and balance control training in people with MS. Additionally, a notable improvement of 12.6% was found in the SSST, compared to 7.2% in the T25FW and 18.8% in the mini-BESTest.
The mini-BESTest is a widely recognized and extensively validated tool for assessing balance capacity [
35], making it an appropriate choice for assessing the responsiveness of the SSST. The mini-BESTest includes tasks similar to those in the SSST, such as standing on one leg, adjusting gait speed, walking with pivot turns, and assessing ambulatory abilities. These similarities coincide with the findings of our subanalysis, which demonstrated a moderate correlation between changes in specific subdomains of the mini-BESTest, namely, dynamic gait and anticipatory postural adjustments, and changes in the SSST (r
s = −0.39 and −0.24, respectively). Other tasks bear little resemblance to the SSST, including compensatory stepping corrections in forward, backward, and lateral directions, standing on a firm surface, a foam surface, or an incline ramp, and walking with head turns. This aligns with our correlation analyses, which examined the relationship between changes in the subdomains, sensory orientation, postural responses (reactive strategies), and improvements in the SSST, yielding correlation coefficients of r
s = −0.04 and −0.08, respectively. Thus, it is plausible that the multifaceted nature of the mini-BESTest, where only a portion of tasks resembles the SSST, could explain why a moderate, rather than a strong, correlation was found between changes in the SSST and the mini-BESTest.
The SSST is a measure that largely consists of gait. However, only a moderate correlation was found between the SSST and a commonly used measure of gait (T25FW). The responsiveness of the SSST and the T25FW found in the present study is in accordance with previous studies [
15,
16]. A possible explanation could be found in the content of the SSST itself. While the SSST might seem like a simple test of gait, it is a much more complex motor task than the T25FW, which requires the ability to change direction and walking speed, walking agility, and balancing on one leg while simultaneously gently shoving blocks, none of which are found in the T25FW. It is, therefore, believed that the much more complex balance requirements of the SSST are the main reason for not finding a stronger correlation between the SSST and T25FW.
A 10-week program combining resistance training with balance exercises led to a 12.6% improvement in the SSST. Additionally, improvements of 7.2% in the T25FW and 18.8% in the mini-BESTest were observed. While there were some differences in the magnitude of change between the SSST, the T25FW, and the mini-BESTest, this was in accordance with the hypothesis. The magnitude of change for both the T25FW and the mini-BESTest found in the present study followed the changes observed in previous studies after resistance and/or balance training in people with MS [
1,
18‐
22]. The fact that the greatest improvement was found in the measure of balance capacity (i.e., mini-BESTest) is, therefore, not surprising, as the intervention consisted of resistance and balance control training —an intervention specifically aimed at improving balance. However, changes were reasonably similar across the SSST, the T25FW, and the mini-BESTest, suggesting that the responsiveness of the measures is quite comparable. Moreover, the effect sizes found after the 10-week intervention were also fairly similar across the SSST, the T25FW, and the mini-BESTest (i.e., 0.84, 0.89, and 0.71, respectively).80. 19, 2
The results of the present study inform about the psychometric properties of the SSST and provide necessary information on the responsiveness after commonly used training interventions. With the convenience, time efficiency, and implementation in clinical settings of the SSST, one could use the SSST to evaluate functional mobility after training interventions for people with MS in clinical practice. Furthermore, a recently conducted systematic review of the SSST, based on COSMIN guidelines, concluded that the SSST is a valid and reliable tool for individuals with Parkinson’s disease, multiple sclerosis, stroke, chronic inflammatory polyneuropathy, and older adults with perceived balance impairments [
8]. Although differences exist in diagnoses and symptoms, similarities in gait and balance impairments suggest that the SSST's responsiveness may extend to these populations, further enhancing the clinical applicability of this study’s findings. However, further research is necessary to validate this hypothesis.
When interpreting the study's results, some potential limitations should be considered. First, although comparing baseline data between participants and drops-outs indicated no differences between groups, the substantial drop-out rate (i.e., 23%) increases the risk of attrition bias. Second, the SSST was only compared to gait measures with limited balance requirements (i.e., T25FW). Other commonly used gait measures, such as the TUG, involve more complex balance demands, although not as intricate as those required by the SSST. In this context, the TUG could be an appropriate gait measure. Third, the correlations between the subdomains of the Mini-BESTest and the SSST should be interpreted cautiously, as the subdomain scores of the Mini-BESTest have not been formally validated as distinct measures of balance control [
36]. Fourth, responsiveness and the Minimal Clinically Important Difference (MCID) or Minimal Important Difference (MID) are often examined together conceptually for a more comprehensive insight into an outcome measure´s psychometric properties, typically through distribution-based and anchor-based approaches. Responsiveness represents an instrument’s ability to detect changes, whereas the MID/MCID denotes the smallest score or change in score that would likely be important from a patient’s or clinician’s perspective. Importantly, the findings of this study are limited to the responsiveness of the SSST, and further research is required to determine the MID/MCID for the SSST.
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