Better performance on both forward and backward walking tests was related to fewer reports of falls at 1 month (r = 0.515; P = .004, and r = 0.479; P = .009, respectively) and 6 months (r = 0.480; P = .008, and r = 0.430; P = .020, respectively). Similarly, better performance on simple and complex dual-tasks was strongly correlated with reports of fewer falls at 1 month (WWTT Simple: r = 0.472; P = .010, and WWTT Complex: r = 0.466; P = .011) and 6 months (WWTT Simple: r = 0.393; P = .035, and WWTT Complex: r = 0.430; P = .020). None of the walking tests were significantly associated with near-falls.
Better performance on forward and backward walking was significantly related to better physical quality of life on the MSQOL physical subscore (r = −0.467; P = .011, and r = −0.434; P = .019, respectively), but was not linked to mental quality of life on the MSQOL mental subscore, or pain severity or interference. Better performance on both simple and complex dual-tasks was strongly related to better physical (WWTT Simple: r = −0.593; P = .001, and WWTT Complex: r = −0.603; P = .001) and mental (WWTT Simple: r = −0.473; P = .010, and WWTT Complex: r = −0.509; P = .005) quality of life, but not pain severity or interference. Similarly, better balance confidence on the ABC was also linked with fewer falls at 1 month (r = −0.499; P = .006) and 6 months (r = −0.461; P = 0.012) as well as better walking performance (forward: r = −0.619; P < .001, and backward: r = −0.567; P = .001) and better dual-task performance (WWTT Simple: r = −0.634; P < .001, and WWTT Complex: r = −0.619; P < .001). Finally, better forward and backward walking performance was also strongly linked to better dual-task performance on both simple (forward: r = 0.884; P < .001, and backward: r = 0.886; P < .001) and complex (forward: r = 0.872; P < .001, and backward: r = 0.829; P < .001) tasks.
Better performance on the SDMT was associated with better performance on forward (r = −0.486; P = .035) and backward walking (r = −0.602; P = .006) as well as reports of fewer falls in the past month (r = −0.477; P = .039) and better balance confidence (r = 0.661; P = .002).
Better performance on Stroop Interference, while also related to better forward (r = −0.668; P = .002) and backward (r = −0.710; P = .001) walking, fewer reports of falls in the past month (r = −0.464; P = .045) and better balance confidence (r = 0.749; P < .001), was also related to better performance on both simple (r = −0.697; P = .001) and complex (r = −0.535; P = .018) dual-tasks. Consistent with whole-group analyses, only better backward walking performance and higher balance confidence on the ABC were related to better Stroop Color (backward: r = −0.528; P = .020, and ABC: r = 0.719; P = .001) and Stroop Word (backward: r = −0.505; p=0.027, and ABC: r = 0.612; P = .005) performance.
Fewer reports of falls at both 1 and 6 months were associated with better forward (1 month: r = 0.680; P = .001, and 6 months: r = 0.600; P = .007) and backward (1 month: r = 0.577; P = .010, and 6 months: r = 0.578; P = .010) walking, better performance on simple (1 month: r = 0.590; P = .008, and 6 months: r = 0.585; P = .009) and complex (1 month: r = 0.572; P = .011, and 6 months: r = 0.562; P = .012) dual-tasks, and better balance confidence (1 month: r = −0.714; P = .001, and 6 months: r = −0.603; P = .006). None of the walking tests were significantly related to near-falls.
We evaluated the DTE for the WWTT Simple and Complex within group. During the WWTT Simple, middle-aged individuals aligned with the whole-group data; the majority experienced mutual interference, while some prioritized gait and others prioritized cognition (Figure 2C). During the WWTT Complex, the middle-aged group followed the same pattern as the whole-group data (Figure 2B).
Poorer cognitive performance on the SDMT and Stroop Interference was associated with poorer performance on motor tests and poorer balance confidence ratings, but none of these relationships were significant in the older adult cohort.
In the older adult group, reported falls and near-falls were not significantly associated with walking, dual-task performance, or balance confidence ratings.
We evaluated the DTE for the WWTT Simple and Complex within group. During the WWTT Simple, older adults with MS demonstrated a different pattern from the whole-group results. No individuals prioritized gait during the dual-task condition; rather, they prioritized the cognitive task or experienced mutual facilitation (ie, a relative improvement in both gait speed and cognitive performance under dual-task conditions) (Figure 2D). During the WWTT Complex, the older adult group followed the same pattern as the whole-group data (Figure 2B).
There were also no significant differences between groups in cognitive performance, which aligns with literature showing that magnitude of cognitive impairments among individuals with MS does not vary across the lifespan.5 Our results show that poorer performance on cognitive tests of processing speed (ie, SDMT) was associated with poorer performance on forward and backward walking as well as greater reports of falls and poorer balance confidence. This builds upon work detailing the relationship among motor performance, cognitive performance, and fall risk in persons with MS14 and healthy older adults.6–12 We also show that better performance on the Stroop Interference test is related not only to better performance on forward and backward walking tests, fewer reports of falls, and better balance confidence, but also to better performance on simple and complex dual-tasks. The Stroop Interference test requires individuals to inhibit their immediate response and prioritize the color of the ink rather than reading the word itself, which is similar to dual-task performance where the individual must balance the demands of the walking and cognitive tasks simultaneously. The connection of Stroop Interference to clinical dual-task performance builds upon prior work highlighting the link among dual-task performance and fall-risk in persons with MS26–27 and healthy older adult fallers.54 The utility of examining motor and cognitive tests simultaneously is also highlighted by the DTE results shown in Figure 2. Differences in prioritization between the WWTT Simple and Complex conditions demonstrate the importance of including varying levels of cognitive complexity. The differences seen visually between middle-aged and older adults (Figures 2C and 2D) suggest that older adults may prioritize cognitive tasks during dual-tasks, which could lead to greater fall risk, though this should be examined further in a larger sample.
Clinical tests of walking and balance are often used to determine fall risk in healthy older adults and persons with MS. Indeed, backward walking has been linked with fall risk in healthy older adults.39 While backward walking performance has been shown to decline in individuals with MS compared with controls,55 our results show a significant correlation between poorer performance on backward walking and lower balance confidence on the ABC as well as more self-reported falls and near-falls. This indicates the potential significance of considering backward walking as a determinant of fall risk in persons with MS in future studies.
Although prior work suggests that older adults with MS exhibit greater declines in mental and physical quality of life related to greater physical and mental decline, our results did not show differences among middle-aged and older adults with MS. This is perhaps not surprising, given that we also did not find significant differences in motor or cognitive performance between groups. It is likely that these age-related declines are reduced in mildly disabled individuals with MS. In whole-group analysis, we were also surprised to find that cognitive performance on the SDMT and Stroop was not significantly related to quality of life on the MSQOL. This differs from our earlier findings30 in which poorer SDMT performance was highly correlated with both lower mental and physical MSQOL. This discrepancy is perhaps due to the wider range of disability levels and lower performance on the SDMT seen in our previous study. Our data also demonstrated a significant relationship between higher mental and physical quality of life and better performance on simple and complex dual-tasks. This is in agreement with recent work showing that the dual-task cost of standing balance is linked with quality of life in individuals with an EDSS less than 3.0.56
This study was limited by a small sample size as well as a smaller cohort of older adults with MS compared with middle-aged adults. Importantly, our convenience sample included individuals with low disability (indeed, 34.5% of individuals [10/29] ranked themselves as “no disability”), which limits the generalizability of our findings and impacts our ability to determine motor findings specifically attributable to aging. In addition to age and disease severity, there are other factors such as fatigue, depression, and spasticity that may impact motor, cognitive, and behavioral performance. As these factors were not evaluated, we are unable to determine their impact on the study results. We limited this study to ambulatory individuals with MS and are unable to generalize the findings to nonambulatory individuals. Our convenience sample included individuals with high education levels (the average was completion of college), which may also impact the results and limit generalization to all MS populations. Education level also contributes to cognitive reserve theory,57 which has been shown to moderate the effect of MS on cognitive functioning58 and may provide protection against disease-related declines in function. Whole-group findings of relationships among motor and cognitive performance persist in within-group analysis of the middle-aged group, but are not present in the older adult group. This may be a result of small sample sizes, or due to our older adult group being largely younger than 65 years, when age-related changes may not yet be apparent. We noted that the r values for correlations among motor and cognitive function were similar to the middle-aged and whole-group analysis, but did not reach significance. Future studies should focus on simultaneous comparison of young, middle-aged, and older adults with mild, moderate, and severe disability levels to assess age-related motor or cognitive decline across the disability spectrum. Once the age of first decline has been identified, additional studies may assess other factors for exacerbation or improvement of motor and cognitive decline in persons with MS.
In our cohort of mildly disabled (PDDS <2) middle-aged and older adults with MS, motor, cognitive, and behavioral performance remained stable and were not impacted by aging. Our data demonstrate that backward walking may be an important factor for understanding fall risk, while the Stroop Interference cognitive test is important for understanding dual-task deficits in persons with MS. Clinicians should consider including backward walking and Stroop Interference to their evaluations of persons with MS to assess fall risk and potential dual-task deficits. Future work to elucidate motor, cognitive, and behavioral declines across the disability and aging spectrum is needed to foster the development of novel rehabilitation paradigms for persons with MS.
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