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Impairment and disability in persons with MS: do functional performance or functional limitations matter? a
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Rachel E. Klaren , Lara A. Pilutti , Brian M. Sandroff & Robert W. Motl
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Department of Kinesiology & Community Health, University of Illinois, 233 Freer Hall, Urbana, IL 61801, USA Published online: 27 Mar 2015.
Click for updates To cite this article: Rachel E. Klaren, Lara A. Pilutti, Brian M. Sandroff & Robert W. Motl (2015): Impairment and disability in persons with MS: do functional performance or functional limitations matter?, Psychology, Health & Medicine, DOI: 10.1080/13548506.2015.1026359 To link to this article: http://dx.doi.org/10.1080/13548506.2015.1026359
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
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Psychology, Health & Medicine, 2015 http://dx.doi.org/10.1080/13548506.2015.1026359
Impairment and disability in persons with MS: do functional performance or functional limitations matter? Rachel E. Klaren, Lara A. Pilutti
, Brian M. Sandroff and Robert W. Motl*
Downloaded by [University of Nebraska, Lincoln] at 16:39 07 April 2015
Department of Kinesiology & Community Health, University of Illinois, 233 Freer Hall, Urbana, IL 61801, USA (Received 23 September 2014; accepted 2 March 2015) Persons with multiple sclerosis (MS) often demonstrate impairment in cardiorespiratory and musculoskeletal systems that may be associated with functional performance, functional limitations, and disability limitations. This study examined such relationships in persons with MS using Nagi’s disablement model and its subsequent conceptual modifications. The sample included 63 persons with MS (75% relapsing–remitting MS, median EDSS = 4.0, and mean MS duration = 13.1 years) who underwent measurements of aerobic fitness and muscular strength (i.e. impairment), functional performance, functional limitations, and disability limitations. The data were primarily examined using path analysis in Mplus 7.0. Our final model provided an excellent fit for the data (χ² = 0.67, df = 3, p = 0.88, SRMR = 0.01, and CFI = 1.00). The final model indicated that aerobic fitness and muscular strength were indirectly associated with disability limitations in persons with MS through a pathway that included functional limitations (indirect path coefficients of 0.29 and 0.20, respectively), but not functional performance. Aerobic fitness and muscular strength may be the important targets of exercise training interventions for improving perceived functional limitations and disability limitations in persons with MS. Keywords: multiple sclerosis; exercise; impairment; functional limitations; functional performance; disability
Introduction Multiple sclerosis (MS) is a neurological disease that results in impairments of cardiorespiratory and musculoskeletal systems, functional limitations, and disability. Researchers have not examined the relationships among those outcomes in persons with MS using a model that considers disability as a sociocultural construct. This can be accomplished through Nagi’s disablement model (Nagi, 1976) and its modification (Stewart, 2003). This disablement model defines a pathway between pathology (i.e. immune-mediated demyelination of axons), impairment (i.e. physiological deconditioning), functional limitations (i.e. perceived restrictions in basic physical actions such as walking), and disability (i.e. personal care). An additional step in this model involves functional performance (i.e. objective walking performance) between impairment and functional limitations (Stewart, 2003). This model provides a pathway that can inform decisions on the care and prevention of disability by identifying entry points for interventions such as physical activity (Guralnik & Ferrucci, 2003). *Corresponding author. Email: [email protected] © 2015 Taylor & Francis
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The disablement model has been adopted in research of older adults and persons with MS. One study of older adults demonstrated that lower aerobic fitness and muscular strength (i.e. impairment) were associated with more self-reported functional limitations (Morey, Pieper, & Cornoni-Huntley, 1998). Another study of older adults demonstrated that higher levels of physical activity were associated with better functional performance, fewer functional limitations, and less disability (McAuley et al., 2006). One other study identified physical activity as a correlate of functional limitations and disability in persons with MS (Motl, McAuley, & Suh, 2010). This study undertook the first complete examination of associations between aerobic and muscular fitness (i.e. impairment), functional performance, functional limitations, and disability limitations in persons with MS based on Nagi’s disablement model and its modifications. We hypothesized that those with higher fitness levels would report less severe disability limitations and that the relationship would be indirect through functional performance and functional limitations (Figure 1). Methods Participants Participants were recruited through the North American Research Committee on MS registry and our database. The inclusion criteria were diagnosis of MS based on physician verification, relapse-free for the past 30 days, age between 18 and 64 years, ability to walk with or without an assistive device, willingness to complete in-person assessments, and physician approval for undertaking exercise testing. A total of 129 individuals made contact with the project coordinator and 85 underwent screening. Eighty-one persons met inclusion criteria and were scheduled for testing, and 18 canceled the appointment for reasons unrelated to testing. The final convenience sample included 63 persons with MS, and the sample size was not based on a power analysis. Measures We operationalized impairment as aerobic fitness (VO2peak ) in ml/kg/min and muscular strength (peak isometric torque of the knee extensors (KE)) in Nm (Sandroff, Pilutti, Benedict, & Motl, 2015). We operationalized functional performance as the timed 25-foot walk (T25FW) in seconds (s) (Fischer, Rudick, Cutter, & Reingold, 1999). We operationalized lower extremity functional limitations (LEFL) through the basic (BLEF) and advanced (ALEF) lower extremity function subscales of the abbreviated late Impairment
Functional performance
Functional limitations
Disability limitations
Timed 25-foot walk
LEFL, LL-FDI
Disability, LL-FDI
Aerobic fitness
Muscle Strength
Figure 1. Original hypothesized path model of impairment, functional performance, functional limitations, and disability limitations.
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life-function and disability inventory (LL-FDI) (Motl et al., 2010). We summed the scores from the BLEF and ALEF into a measure of LEFL that ranged between 10 and 50. Higher scores reflect fewer functional limitations. We operationalized disability limitations through the disability limitation subscale of the abbreviated LL-FDI (Motl et al., 2010). We summed the scores into a total disability limitations score that ranged between 8 and 40. Higher scores reflect less severe disability limitations.
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Procedure An institutional review board approved the safety of the procedures and participants provided written informed consent. Participants underwent a neurological evaluation by a Neurostatus-certified examiner for generation of an EDSS score; assessment of aerobic fitness and muscular strength; and completed the T25FW. Participants provided demographic/clinical information and completed the abbreviated LL-FDI. Participants were remunerated $150. Data analysis Descriptive statistics and correlations were performed using SPSS Version 22.0 (Chicago, IL), whereas path analysis was undertaken with Mplus version 7.0 (Muthen & Muthen). Descriptive statistics for demographic and clinical characteristics and the main outcome measures are presented as mean (SD), unless otherwise noted. Correlations between variables were examined using bivariate Pearson product-moment correlations (r) and interpreted by guidelines of 0.1, 0.3, and 0.5 as small, moderate, and large, respectively (Cohen, 1988). The path model in Figure 1 was tested using the robust maximum likelihood estimator in Mplus. The model included directional paths between (1) impairment (i.e. VO2peak and KEpeak ) and functional performance (i.e. T25FW); (2) functional performance and functional limitations (i.e. LEFL score from LL-FDI); and (3) functional limitations and disability limitations (i.e. disability limitations score from LL-FDI). We provided standardized path coefficients (β). Model fit was based on the chi-square (χ²), standardized root mean residual (SRMR < 0.06), and confirmatory fit index (CFI > 0.95) (Hu & Bentler, 1999). We examined modification indices (MIs) for path coefficients in the event of a poorly fitting model. We estimated the indirect paths between impairment and disability limitations using the INDIRECT option in Mplus. Results Participant characteristics Demographic and clinical characteristics are given in Table 1. The sample was primarily female (71.2%) with an average age of 52.0 (7.8) years. Forty-four participants had relapsing–remitting MS (74.6%) and the mean duration of MS was 13.1 (8.7) years. The median EDSS score was 4.0 (IQR = 2.5) indicating moderate neurologic disability. Main outcomes Descriptive statistics for impairment, functional performance and limitations, and disability limitations are given in Table 2. The means for VO2peak and KEpeak, T25FW, and abbreviated LL-FDI were comparable with other studies of MS (Fischer et al., 1999; Motl & Fernhall, 2012; Motl et al., 2010).
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Table 1.
Demographic and clinical characteristics.
Characteristic
MS (n = 59)
Age, years Sex, % female Height, cm Weight, kg Marital status, % married Employment, % employed EDSS, mdn (IQR) Disease duration, years Disease course, % RRMS
52.0 (7.8) 71.2% 169.9 (10.4) 78.6 (19.8) 64.4% 45.8% 4.0 (2.5) 13.1 (8.7) 74.6%
Notes: EDSS = expanded disability status scale; IQR = interquartile range; mdn = median; RRMS = relapsing– remitting multiple sclerosis. Values are mean (SD), unless otherwise noted.
Table 2. Mean scores of VO2peak and KEpeak (impairment), T25FW (functional performance), LEFL (functional limitations), and disability limitations. Variable
MS (n = 59)
VO2peak, ml/kg/min KEpeak, Nm T25FW, s LEFL, LL-FDI Disability limitations, LL-FDI
19.8 (7.3) 139.3 (55.6) 6.9 (4.2) 33.5 (10.9) 32.2 (7.2)
Notes: VO2peak = aerobic fitness; KEpeak = knee extensor peak torque; T25FW = timed 25-foot walk; LEFL = lower extremity functional limitations; LL-FDI = late life-function and disability instrument. Values are mean (SD).
Correlations among main outcomes Bivariate correlations among aerobic fitness and muscular strength, T25FW, LEFL, and disability limitations are given in Table 3. VO2peak and KEpeak were significantly associated with T25FW, LEFL, and disability limitations. Of note, VO2peak and KEpeak were strongly correlated with disability limitations. T25FW was significantly associated with LEFL, and T25FW and LEFL were significantly associated with disability limitations.
Table 3. Correlations among VO2peak and KEpeak (impairment), T25FW (functional performance), LEFL (functional limitations), and disability limitations. Variable
VO2peak
KEpeak
T25FW
LEFL
VO2peak KEpeak T25FW LEFL Disability limitations
0.69 −0.53** 0.51** 0.73**
−0.59** 0.48** 0.67**
−0.43** −0.69**
0.65**
Disability limitations
Notes: Pearson product-moment correlation coefficient (r). **denotes significance at p < 0.01 level (2-tailed). VO2peak = aerobic fitness; KEpeak = knee extensor peak torque; T25FW = timed 25-foot walk; LEFL = lower extremity functional limitations.
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Path analysis The path model in Figure 1 provided a poor fit for the data (χ² = 71.85, df = 5, p < 0.0001, SRMR = 0.17, and CFI = 0.20). The MIs identified adding paths between both VO2peak and KEpeak with LEFL. This revised path model in Figure 2 provided an excellent fit (χ² = 0.67, df = 3, p = 0.88, SRMR = 0.01, and CFI = 1.00). The path coefficients indicated that (1) VO2peak and KEpeak had statistically significant direct associations with T25FW (β’s = −0.35 and −0.27, respectively) and LEFL (β’s = 0.46 and .31, respectively); (2) T25FW did not have a statistically significant direct association with LEFL (β = −0.09); and (3) LEFL had a statistically significant direct association with disability limitations (β = 0.65). The indirect path coefficients between measures of impairment and disability limitations were statistically significant (VO2peak: β = 0.29; KEpeak: β = 0.20) and accounted for by LEFL rather than T25FW. Discussion We provide the first examination of relationships among impairment, functional performance, functional limitations, and disability limitations in persons with MS based on Nagi’s disablement model (Nagi, 1976) and its modifications (Stewart, 2003; Verbrugge & Jette, 1994). Our results indicate that aerobic fitness and muscular strength (i.e. impairment) were significantly and indirectly associated with disability limitations by way of functional limitations, but not functional performance. T25FW (i.e. functional performance) was not directly associated with LEFL (i.e. functional limitations) or indirectly associated with disability limitations. This indicates that functional limitations, but not functional performance, might account for the association between impairment and disability limitations in persons with MS. Physical fitness (i.e. impairment) represents a target for modifying functional limitations and disability limitations in MS within clinical care and/or intervention. This would seemingly support the examination of exercise training as an intervention for reducing functional limitations, and, by extension, disability limitations in MS. This current study built upon previous research in MS that identified an association between physical activity and functional limitations and disability based on Nagi’s disablement model (Motl & McAuley, 2009). Our results provide novel support for the inclusion of functional limitations, but not functional performance, as a distinct, intermediate step in the disablement pathway in MS. The current results further extend upon previous findings by including aerobic fitness and muscular strength as entry points of impairment in the disablement pathway. Those physical fitness characteristics
Impairment
Functional performance
Aerobic fitness
Functional limitations
Disability limitations
.46 −.35 Timed 25-footwalk
−.09
LEFL, LL-FDI
.65
Disability, LL-FDI
−.27 Muscle Strength
.31
Figure 2. Revised path model of impairment, functional performance, functional limitations, and disability limitations.
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may be seen as quantifiable outcomes of physical activity and therefore represent distinct entry points relevant to the disablement pathway. This is important as Nagi’s disablement model includes impairment in a series of sequential steps for understanding the pathway between disease and disability (Guralnik & Ferrucci, 2009). Another exciting result of the current study was the association of aerobic fitness and muscular strength with T25FW. This is consistent with previous research demonstrating that aerobic capacity and muscular strength accounted for significant variance in T25FW (Sandroff, Sosnoff, & Motl, 2013). This suggests that exercise training interventions should address multiple domains of physiological fitness (i.e. aerobic fitness and muscular strength) for improving functional performance (Latimer-Cheung et al., 2013). There are limitations of this study. The primary limitation is the cross-sectional nature of this study, as it does not imply causation between variables. The sample was predominantly female with RRMS, and the results may not generalize among persons with MS. We did not analyze upper-extremity functional limitations. Conclusion This study provides novel data that aerobic fitness and muscular strength (i.e. impairment) are associated with disability limitations in persons with MS. Those associations are seemingly mediated through functional limitations, but not functional performance. Such results suggest that both impairment and functional limitations may be important targets of interventions and clinical practice for improving disability limitations in MS. Disclosure statement No potential conflict of interest was reported by the authors.
Funding This work was supported by National Multiple Sclerosis Society [grant number IL 0003].
ORCID Lara A. Pilutti
http://orcid.org/0000-0002-3074-4903
References Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ: Lawrence Erlbaum. Fischer, J. S., Rudick, R. A., Cutter, G. R., & Reingold, S. C. (1999). The multiple sclerosis functional composite measure (MSFC): An integrated approach to MS clinical outcome assessment. National MS society clinical outcomes assessment task force. Multiple Sclerosis, 5, 244–250. doi:10.1177/135245859900500409 Guralnik, J. M., & Ferrucci, L. (2003). Assessing the building blocks of function: Utilizing measures of functional limitation. American Journal of Preventive Medicine, 25, 112–121. doi:10.1016/S0749-3797(03)00174-0 Guralnik, J. M., & Ferrucci, L. (2009). The challenge of understanding the disablement process in older persons: Commentary responding to Jette AM. Toward a common language of disablement. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, 64A, 1169–1171. doi:10.1093/gerona/glp094
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Hu, L., & Bentler, P. M. (1999). Cutoff criteria for fit indexes in covariance structure analysis: Conventional criteria versus new alternatives. Structural Equation Modeling: A Multidisciplinary Journal, 6(1), 1–55. doi:10.1080/10705519909540118 Latimer-Cheung, A. E., Pilutti, L. A., Hicks, A. L., Martin Ginis, K. A., Fenuta, A. M., Mackibbon, K. A., & Motl, R. W. (2013). Effects of exercise training on fitness, mobility, fatigue, and health-related quality of life among adults with multiple sclerosis: A systematic review to inform guideline development. Archives of Physical Medicine and Rehabilitation, 94, 1800–1828. McAuley, E., Konopack, J. F., Morris, K. S., Motl, R. W., Hu, L., Doerksen, S. E., & Rosengren, K. (2006). Physical activity and functional limitations in older women: Influence of self-efficacy. The Journals of Gerontology Series B: Psychological Sciences and Social Sciences, 61, P270–P277. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/ Morey, M. C., Pieper, C. F., & Cornoni-Huntley, J. (1998). Physical fitness and functional limitations in community-dwelling older adults. Medicine & Science in Sports & Exercise, 30, 715–723. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/ Motl, R. W., & Fernhall, B. (2012). Accurate prediction of cardiorespiratory fitness using cycle ergometry in minimally disabled persons with relapsing-remitting multiple sclerosis. Archives of Physical Medicine & Rehabilitation, 93, 490–495. doi:10.1016/j.apmr.2011.08.025 Motl, R. W., & McAuley, E. (2009). Longitudinal analysis of physical activity and symptoms as predictors of change in functional limitations and disability in multiple sclerosis. Rehabilitation Psychology, 54, 204–210. doi:10.1037/a0015770 Motl, R. W., McAuley, E., & Suh, Y. (2010). Validity, invariance and responsiveness of a self-report measure of functional limitations and disability in multiple sclerosis. Disability & Rehabilitation, 32, 1260–1271. doi:10.3109/09638280903464463 Nagi, S. Z. (1976). An epidemiology of disability among adults in the United States. Milbank Memorial Fund Quarterly: Health & Society, 54, 439–467. Retrieved from http://www.ncbi. nlm.nih.gov/pubmed/ Sandroff, B. M., Pilutti, L. A., Benedict, R. H., & Motl, R. W. (2015). Association between physical fitness and cognitive function in multiple sclerosis: Does disability status matter? Neurorehabilitation and Neural Repair, 29, 214–223. doi:10.1177/1545968314541331 Sandroff, B. M., Sosnoff, J. J., & Motl, R. W. (2013). Physical fitness, walking performance, and gait in multiple sclerosis. Journal of the Neurological Sciences, 328, 70–76. doi:10.1016/ j.jns.2013.02.021 Stewart, A. L. (2003). Conceptual challenges in linking physical activity and disability research. American Journal of Preventive Medicine, 25, 137–140. doi:10.1016/S0749-3797(03)00187-9 Verbrugge, L. M., & Jette, A. M. (1994). The disablement process. Social Science & Medicine, 38(1), 1–14. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/
Psychology, Health & Medicine Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/cphm20
Impairment and disability in persons with MS: do functional performance or functional limitations matter? a
a
a
Rachel E. Klaren , Lara A. Pilutti , Brian M. Sandroff & Robert W. Motl
a
a
Department of Kinesiology & Community Health, University of Illinois, 233 Freer Hall, Urbana, IL 61801, USA Published online: 27 Mar 2015.
Click for updates To cite this article: Rachel E. Klaren, Lara A. Pilutti, Brian M. Sandroff & Robert W. Motl (2015): Impairment and disability in persons with MS: do functional performance or functional limitations matter?, Psychology, Health & Medicine, DOI: 10.1080/13548506.2015.1026359 To link to this article: http://dx.doi.org/10.1080/13548506.2015.1026359
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
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Psychology, Health & Medicine, 2015 http://dx.doi.org/10.1080/13548506.2015.1026359
Impairment and disability in persons with MS: do functional performance or functional limitations matter? Rachel E. Klaren, Lara A. Pilutti
, Brian M. Sandroff and Robert W. Motl*
Downloaded by [University of Nebraska, Lincoln] at 16:39 07 April 2015
Department of Kinesiology & Community Health, University of Illinois, 233 Freer Hall, Urbana, IL 61801, USA (Received 23 September 2014; accepted 2 March 2015) Persons with multiple sclerosis (MS) often demonstrate impairment in cardiorespiratory and musculoskeletal systems that may be associated with functional performance, functional limitations, and disability limitations. This study examined such relationships in persons with MS using Nagi’s disablement model and its subsequent conceptual modifications. The sample included 63 persons with MS (75% relapsing–remitting MS, median EDSS = 4.0, and mean MS duration = 13.1 years) who underwent measurements of aerobic fitness and muscular strength (i.e. impairment), functional performance, functional limitations, and disability limitations. The data were primarily examined using path analysis in Mplus 7.0. Our final model provided an excellent fit for the data (χ² = 0.67, df = 3, p = 0.88, SRMR = 0.01, and CFI = 1.00). The final model indicated that aerobic fitness and muscular strength were indirectly associated with disability limitations in persons with MS through a pathway that included functional limitations (indirect path coefficients of 0.29 and 0.20, respectively), but not functional performance. Aerobic fitness and muscular strength may be the important targets of exercise training interventions for improving perceived functional limitations and disability limitations in persons with MS. Keywords: multiple sclerosis; exercise; impairment; functional limitations; functional performance; disability
Introduction Multiple sclerosis (MS) is a neurological disease that results in impairments of cardiorespiratory and musculoskeletal systems, functional limitations, and disability. Researchers have not examined the relationships among those outcomes in persons with MS using a model that considers disability as a sociocultural construct. This can be accomplished through Nagi’s disablement model (Nagi, 1976) and its modification (Stewart, 2003). This disablement model defines a pathway between pathology (i.e. immune-mediated demyelination of axons), impairment (i.e. physiological deconditioning), functional limitations (i.e. perceived restrictions in basic physical actions such as walking), and disability (i.e. personal care). An additional step in this model involves functional performance (i.e. objective walking performance) between impairment and functional limitations (Stewart, 2003). This model provides a pathway that can inform decisions on the care and prevention of disability by identifying entry points for interventions such as physical activity (Guralnik & Ferrucci, 2003). *Corresponding author. Email: [email protected] © 2015 Taylor & Francis
Downloaded by [University of Nebraska, Lincoln] at 16:39 07 April 2015
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The disablement model has been adopted in research of older adults and persons with MS. One study of older adults demonstrated that lower aerobic fitness and muscular strength (i.e. impairment) were associated with more self-reported functional limitations (Morey, Pieper, & Cornoni-Huntley, 1998). Another study of older adults demonstrated that higher levels of physical activity were associated with better functional performance, fewer functional limitations, and less disability (McAuley et al., 2006). One other study identified physical activity as a correlate of functional limitations and disability in persons with MS (Motl, McAuley, & Suh, 2010). This study undertook the first complete examination of associations between aerobic and muscular fitness (i.e. impairment), functional performance, functional limitations, and disability limitations in persons with MS based on Nagi’s disablement model and its modifications. We hypothesized that those with higher fitness levels would report less severe disability limitations and that the relationship would be indirect through functional performance and functional limitations (Figure 1). Methods Participants Participants were recruited through the North American Research Committee on MS registry and our database. The inclusion criteria were diagnosis of MS based on physician verification, relapse-free for the past 30 days, age between 18 and 64 years, ability to walk with or without an assistive device, willingness to complete in-person assessments, and physician approval for undertaking exercise testing. A total of 129 individuals made contact with the project coordinator and 85 underwent screening. Eighty-one persons met inclusion criteria and were scheduled for testing, and 18 canceled the appointment for reasons unrelated to testing. The final convenience sample included 63 persons with MS, and the sample size was not based on a power analysis. Measures We operationalized impairment as aerobic fitness (VO2peak ) in ml/kg/min and muscular strength (peak isometric torque of the knee extensors (KE)) in Nm (Sandroff, Pilutti, Benedict, & Motl, 2015). We operationalized functional performance as the timed 25-foot walk (T25FW) in seconds (s) (Fischer, Rudick, Cutter, & Reingold, 1999). We operationalized lower extremity functional limitations (LEFL) through the basic (BLEF) and advanced (ALEF) lower extremity function subscales of the abbreviated late Impairment
Functional performance
Functional limitations
Disability limitations
Timed 25-foot walk
LEFL, LL-FDI
Disability, LL-FDI
Aerobic fitness
Muscle Strength
Figure 1. Original hypothesized path model of impairment, functional performance, functional limitations, and disability limitations.
Psychology, Health & Medicine
3
life-function and disability inventory (LL-FDI) (Motl et al., 2010). We summed the scores from the BLEF and ALEF into a measure of LEFL that ranged between 10 and 50. Higher scores reflect fewer functional limitations. We operationalized disability limitations through the disability limitation subscale of the abbreviated LL-FDI (Motl et al., 2010). We summed the scores into a total disability limitations score that ranged between 8 and 40. Higher scores reflect less severe disability limitations.
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Procedure An institutional review board approved the safety of the procedures and participants provided written informed consent. Participants underwent a neurological evaluation by a Neurostatus-certified examiner for generation of an EDSS score; assessment of aerobic fitness and muscular strength; and completed the T25FW. Participants provided demographic/clinical information and completed the abbreviated LL-FDI. Participants were remunerated $150. Data analysis Descriptive statistics and correlations were performed using SPSS Version 22.0 (Chicago, IL), whereas path analysis was undertaken with Mplus version 7.0 (Muthen & Muthen). Descriptive statistics for demographic and clinical characteristics and the main outcome measures are presented as mean (SD), unless otherwise noted. Correlations between variables were examined using bivariate Pearson product-moment correlations (r) and interpreted by guidelines of 0.1, 0.3, and 0.5 as small, moderate, and large, respectively (Cohen, 1988). The path model in Figure 1 was tested using the robust maximum likelihood estimator in Mplus. The model included directional paths between (1) impairment (i.e. VO2peak and KEpeak ) and functional performance (i.e. T25FW); (2) functional performance and functional limitations (i.e. LEFL score from LL-FDI); and (3) functional limitations and disability limitations (i.e. disability limitations score from LL-FDI). We provided standardized path coefficients (β). Model fit was based on the chi-square (χ²), standardized root mean residual (SRMR < 0.06), and confirmatory fit index (CFI > 0.95) (Hu & Bentler, 1999). We examined modification indices (MIs) for path coefficients in the event of a poorly fitting model. We estimated the indirect paths between impairment and disability limitations using the INDIRECT option in Mplus. Results Participant characteristics Demographic and clinical characteristics are given in Table 1. The sample was primarily female (71.2%) with an average age of 52.0 (7.8) years. Forty-four participants had relapsing–remitting MS (74.6%) and the mean duration of MS was 13.1 (8.7) years. The median EDSS score was 4.0 (IQR = 2.5) indicating moderate neurologic disability. Main outcomes Descriptive statistics for impairment, functional performance and limitations, and disability limitations are given in Table 2. The means for VO2peak and KEpeak, T25FW, and abbreviated LL-FDI were comparable with other studies of MS (Fischer et al., 1999; Motl & Fernhall, 2012; Motl et al., 2010).
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Table 1.
Demographic and clinical characteristics.
Characteristic
MS (n = 59)
Age, years Sex, % female Height, cm Weight, kg Marital status, % married Employment, % employed EDSS, mdn (IQR) Disease duration, years Disease course, % RRMS
52.0 (7.8) 71.2% 169.9 (10.4) 78.6 (19.8) 64.4% 45.8% 4.0 (2.5) 13.1 (8.7) 74.6%
Notes: EDSS = expanded disability status scale; IQR = interquartile range; mdn = median; RRMS = relapsing– remitting multiple sclerosis. Values are mean (SD), unless otherwise noted.
Table 2. Mean scores of VO2peak and KEpeak (impairment), T25FW (functional performance), LEFL (functional limitations), and disability limitations. Variable
MS (n = 59)
VO2peak, ml/kg/min KEpeak, Nm T25FW, s LEFL, LL-FDI Disability limitations, LL-FDI
19.8 (7.3) 139.3 (55.6) 6.9 (4.2) 33.5 (10.9) 32.2 (7.2)
Notes: VO2peak = aerobic fitness; KEpeak = knee extensor peak torque; T25FW = timed 25-foot walk; LEFL = lower extremity functional limitations; LL-FDI = late life-function and disability instrument. Values are mean (SD).
Correlations among main outcomes Bivariate correlations among aerobic fitness and muscular strength, T25FW, LEFL, and disability limitations are given in Table 3. VO2peak and KEpeak were significantly associated with T25FW, LEFL, and disability limitations. Of note, VO2peak and KEpeak were strongly correlated with disability limitations. T25FW was significantly associated with LEFL, and T25FW and LEFL were significantly associated with disability limitations.
Table 3. Correlations among VO2peak and KEpeak (impairment), T25FW (functional performance), LEFL (functional limitations), and disability limitations. Variable
VO2peak
KEpeak
T25FW
LEFL
VO2peak KEpeak T25FW LEFL Disability limitations
0.69 −0.53** 0.51** 0.73**
−0.59** 0.48** 0.67**
−0.43** −0.69**
0.65**
Disability limitations
Notes: Pearson product-moment correlation coefficient (r). **denotes significance at p < 0.01 level (2-tailed). VO2peak = aerobic fitness; KEpeak = knee extensor peak torque; T25FW = timed 25-foot walk; LEFL = lower extremity functional limitations.
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Path analysis The path model in Figure 1 provided a poor fit for the data (χ² = 71.85, df = 5, p < 0.0001, SRMR = 0.17, and CFI = 0.20). The MIs identified adding paths between both VO2peak and KEpeak with LEFL. This revised path model in Figure 2 provided an excellent fit (χ² = 0.67, df = 3, p = 0.88, SRMR = 0.01, and CFI = 1.00). The path coefficients indicated that (1) VO2peak and KEpeak had statistically significant direct associations with T25FW (β’s = −0.35 and −0.27, respectively) and LEFL (β’s = 0.46 and .31, respectively); (2) T25FW did not have a statistically significant direct association with LEFL (β = −0.09); and (3) LEFL had a statistically significant direct association with disability limitations (β = 0.65). The indirect path coefficients between measures of impairment and disability limitations were statistically significant (VO2peak: β = 0.29; KEpeak: β = 0.20) and accounted for by LEFL rather than T25FW. Discussion We provide the first examination of relationships among impairment, functional performance, functional limitations, and disability limitations in persons with MS based on Nagi’s disablement model (Nagi, 1976) and its modifications (Stewart, 2003; Verbrugge & Jette, 1994). Our results indicate that aerobic fitness and muscular strength (i.e. impairment) were significantly and indirectly associated with disability limitations by way of functional limitations, but not functional performance. T25FW (i.e. functional performance) was not directly associated with LEFL (i.e. functional limitations) or indirectly associated with disability limitations. This indicates that functional limitations, but not functional performance, might account for the association between impairment and disability limitations in persons with MS. Physical fitness (i.e. impairment) represents a target for modifying functional limitations and disability limitations in MS within clinical care and/or intervention. This would seemingly support the examination of exercise training as an intervention for reducing functional limitations, and, by extension, disability limitations in MS. This current study built upon previous research in MS that identified an association between physical activity and functional limitations and disability based on Nagi’s disablement model (Motl & McAuley, 2009). Our results provide novel support for the inclusion of functional limitations, but not functional performance, as a distinct, intermediate step in the disablement pathway in MS. The current results further extend upon previous findings by including aerobic fitness and muscular strength as entry points of impairment in the disablement pathway. Those physical fitness characteristics
Impairment
Functional performance
Aerobic fitness
Functional limitations
Disability limitations
.46 −.35 Timed 25-footwalk
−.09
LEFL, LL-FDI
.65
Disability, LL-FDI
−.27 Muscle Strength
.31
Figure 2. Revised path model of impairment, functional performance, functional limitations, and disability limitations.
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R.E. Klaren et al.
may be seen as quantifiable outcomes of physical activity and therefore represent distinct entry points relevant to the disablement pathway. This is important as Nagi’s disablement model includes impairment in a series of sequential steps for understanding the pathway between disease and disability (Guralnik & Ferrucci, 2009). Another exciting result of the current study was the association of aerobic fitness and muscular strength with T25FW. This is consistent with previous research demonstrating that aerobic capacity and muscular strength accounted for significant variance in T25FW (Sandroff, Sosnoff, & Motl, 2013). This suggests that exercise training interventions should address multiple domains of physiological fitness (i.e. aerobic fitness and muscular strength) for improving functional performance (Latimer-Cheung et al., 2013). There are limitations of this study. The primary limitation is the cross-sectional nature of this study, as it does not imply causation between variables. The sample was predominantly female with RRMS, and the results may not generalize among persons with MS. We did not analyze upper-extremity functional limitations. Conclusion This study provides novel data that aerobic fitness and muscular strength (i.e. impairment) are associated with disability limitations in persons with MS. Those associations are seemingly mediated through functional limitations, but not functional performance. Such results suggest that both impairment and functional limitations may be important targets of interventions and clinical practice for improving disability limitations in MS. Disclosure statement No potential conflict of interest was reported by the authors.
Funding This work was supported by National Multiple Sclerosis Society [grant number IL 0003].
ORCID Lara A. Pilutti
http://orcid.org/0000-0002-3074-4903
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