- Email: [email protected]
Conceptual challenges in linking physical activity and disability research
Conceptual Challenges in Linking Physical Activity and Disability Research Anita L. Stewart, PhD
R
educing the functional limitations and disability that accompany aging and disease are goals of both physical activity and disability research. Efforts to link the two fields, as reflected in this supplement to the American Journal of Preventive Medicine, should expand considerably our ability to achieve these goals because of the special perspectives and strengths that each discipline brings to this collaboration. Disability research focuses on epidemiologic pathways by which disease, injury, and other conditions ultimately lead to disability and on surveillance of population levels of disability. Through its numerous adaptations, the “disablement process” model has provided a longstanding, strong guiding framework for this work.1,2 More recently, research on health promotion for people with disabilities is being undertaken,3 with a special emphasis on physical activity as both a primary and secondary prevention approach.4,5 Physical activity researchers targeting older adults address a variety of issues in order to design optimal interventions to change physical activity behavior. Some of these include identifying numerous multilevel determinants of physical activity, examining characteristics of exercise interventions that increase physical activity and lead to improved outcomes in this age group, and exploring mechanisms by which improved outcomes occur.6 Physical activity research that is more closely aligned with disability research includes epidemiologic studies of how natural physical activity levels and changes in activity levels are associated with subsequent physical functioning and disability in older adults. The papers in this supplement1,2,7–17 reflect state-ofthe-art ideas from both disciplines. The physical activity experts describe exercise interventions in older adults in terms of fitness, functioning, and disability outcomes; they also summarize key system-level factors and policies that affect physical activity levels in older adults. The disability specialists describe the disablement process and suggest ways to improve measures of From the Institute for Health and Aging, University of California–San Francisco, San Francisco, California Address correspondence and reprint requests to: Anita L. Stewart, PhD, Institute for Health and Aging, University of California–San Francisco, 3333 California St, Ste 340, San Francisco CA 94118. E-mail: [email protected].
the steps in the process. Several physical activity researchers attempted to fit their discussions within the disablement process model; however, in many cases, the model was a poor fit with their concepts and research processes. To enable this visionary work to progress in a timely manner, a conceptual framework is needed that can be used easily by both fields. Such a unifying framework should begin with the disablement process model, given that the ultimate goal is to prevent or slow the onset and progression of the disablement process in older adults. Four basic ways in which the model could be improved for this particular collaboration follow: 1. Two key mechanisms by which disablement begins— physiologic aging and disuse—are not an explicit part of the model, although these are essential concepts in physical activity research in aging. 2. One important mechanism by which chronic disease affects functioning and disability—symptoms—is not part of the model, yet physical activity often improves symptoms. 3. All concepts along the pathway are negatively defined, making it problematic for exercise scientists to describe physical activity intervention outcomes. 4. The functional limitations category is too broadly defined and measured, limiting our ability to understand more subtle processes by which physical activity affects functioning and disability.
Two Additional Entry Points: Physiologic Aging and Disuse The physiologic aging process contributes to a portion of the decline in organ systems and functioning independent of pathology or chronic disease and injury18 and thus needs to be an explicit entry point. In this supplement, aging is discussed in terms of its effects on the onset of functional limitations and disability,15 loss of muscle mass and strength (impairments),9,11 balance,10,11 and mobility disability,10 which illustrate its relevance to topics of disability and exercise in older adults. Deconditioning19 and disuse20 also lead to numerous impairments (e.g., cardiovascular vulnerability, musculoskeletal fragility) and chronic disease, as well as to
Am J Prev Med 2003;25(3Sii) © 2003 American Journal of Preventive Medicine • Published by Elsevier Inc.
0749-3797/03/$–see front matter doi:10.1016/S0749-3797(03)00187-9
137
Figure 1. Suggested Conceptual Framework for Linking Physical Activity and Disability Research
premature aging. In their paper, Frank and Patla11 refer to this as “detuning.” The deconditioning concept originally pertained to adverse effects on a variety of organ systems of extended inactivity due to bed rest and space travel. As a risk factor for chronic disease and disability, disuse is typically defined in terms of sedentary behavior, the least active level along a physical activity continuum. The idea that disuse can be a determinant of impairments is not new, especially when considering disuse in terms of physical activity. In this supplement, Keysor8 presents a conceptual model in which physical activity precedes impairments (muscle strength), functional limitations, and disability. The modified model would thus have three entry points—pathology, physiologic aging, and disuse—all of which can, independently, begin the process of decline (see Figure 1). This modification is consistent with the definition of impairments by Carlson et al.4 as due to disuse, aging, disease, and trauma.
Symptoms as Mediators Physical and mental symptoms such as musculoskeletal pain, fatigue, shortness of breath, depression, and weakness are well-known determinants of functional limitations and disability.21–29 Symptoms have been found to mediate the effects of chronic disease on functioning and disability30 and the effect of physical activity on functional limitations.8,31 Symptoms are often improved by appropriate physical activity32; for example, Seguin and Nelson9 cite nine studies in which strength training reduced symptoms. Indeed, many physical activity interventions target persons with various symptoms (joint pain, back pain, sleep problems) with the aim of reducing them. Adding symptoms to the disablement process model would support disability researchers to study symptoms as potential mediators of the effect of disease on functional limitations and disability and would encourage physical activity researchers to explore the extent to which physical activity interventions improve functioning by way of symptom reduction, particularly for people with chronic conditions. Consistent with this suggestion, Fried and Guralnik33 and Leveille et al.26 have published hypothetical models in which symptoms (angina, dyspnea, low back pain) 138
precede impairments (e.g., muscle strength), suggesting that symptoms could be incorporated into the model between the entry points and impairments. However, because symptoms can be caused by chronic diseases, injuries, and disuse, they might better be incorporated at the same level as impairments.
Modify Definitions (and Measures) to Incorporate Positive Levels Despite the fact that they are often referring to the same concepts, the language of exercise scientists differs from that of disability researchers. In particular, the fact that all points in the disablement process model are defined negatively (e.g., impairments, limitations) limits its usefulness for physical activity research.
Impairments or Physiologic Fitness? In the current model,34 impairments (organ-level factors) are defined as abnormalities in specific body systems such as musculoskeletal, cardiovascular, and neurologic. Exercise scientists refer to the same organlevel systems in terms of physiologic fitness. Common components of fitness in a variety of organ systems include cardiovascular and muscular endurance, flexibility (range of motion), balance/coordination, muscle strength, reaction time, visual acuity, power, and body composition.2,9 Each can be defined on a broad continuum, ranging from very low to very high fitness levels; thus, they are concerned with the full range, not just impairments. One potential generic label for this category is physiologic function, which could accommodate concepts of impairments (problems) and positive physiologic fitness levels. An advantage of this modification is that it could formalize efforts by researchers from both disciplines to explore the effects of having high fitness levels or “reserve” on the disablement process. Noting that hospitalization and immobilization are associated with precipitous functional declines, Fried et al.33 suggest that physiologic reserve can help patients maintain functioning in the face of these events. Disability research could examine the effect of physiologic reserve on withstanding an injury, hospitalization, or a serious health problem without progressing to a functional limitation. Physical activity researchers could determine if increasing fitness levels (i.e., approaching the “reserve” end of the fitness continuum) could slow the progression to functional limitations.
Functional Limitations or Physical Functioning? A parallel problem exists with the functional limitations step in the pathway. The highest score one can typically
American Journal of Preventive Medicine, Volume 25, Number 3Sii
get on self-reported functional limitations measures is the absence of limitations,2 such as not having any difficulty walking. This could be why many exercise interventions conducted in community-based samples of older adults have been unable to detect effects of exercise on physical functioning, as these subgroups tend to have minimal limitations. Some researchers have attempted to extend the range of measurement of self-reported physical functioning into the positive side. Three such approaches include: (1) for those reporting no difficulty, asking how easy a function is35; (2) adding items about participation in various physical activities36; and (3) adding discretionary or advanced functions (walk 1 mile, run one-half mile).2,37 Performance-based measures of functional tasks were developed, in part, to assess a broader functioning range, such as obtaining more variation among those at the ceiling of self-reported physical limitations.38 For example, on a performance test of walking, the highest score can indicate a fairly high performance level (e.g., above-average walking speed). However, as Guralnik and Ferrucci2 articulate so clearly in this supplement, performance measures were not designed with the disablement model in mind and thus cannot easily be used to assess processes of disablement. Developers of performance-based measures were attempting to overcome several problems with self-report measures in older adults and tried to tap a broad range of functional tasks. Hence, as shown in Table 3 in Guralnik and Ferrucci,2 many available performance measures assess more than one point in the disablement process. This is also true of self-report function and disability measures.23 Regarding the need to extend the functional limitations category into more positive ranges, there are options for both self-report and performance measures to tap higher functioning levels. Thus, relabeling this category would help, perhaps as “physical functioning.” Although other functioning dimensions are part of the original model, such as cognitive and sensory functioning, to adapt this model for physical activity research, the physical dimension is probably the most crucial.
Performance of Physical Functions—A Distinct Point in the Pathway? Much research suggests that performance measures of physical functioning are not simply an alternative to self-reported measures but might instead reflect a distinct step in the disablement process. Fried and Guralnik33 and Fried et al.39 have found that performancebased physical function tests identify a larger proportion of older adult samples than would be identified by self-report, particularly for ambulation (walking, climbing stairs). In a prospective study of older women with no mobility difficulty, they found
that lower mobility performance levels at baseline predicted declines in self-reported mobility difficulty.40 Another example is an exercise intervention by Cress et al.41 in which improved physical functioning was observed based on several performance measures, but not on self-reported measures. Last, self-reported walking difficulty and measured maximal walking speed were independently associated with self-rated health in older women.42 These and other findings that self-report and performance-based physical functioning measures behave differently are strong indications that they represent distinct concepts. It may be time to consider distinguishing performance of physical functions as a separate step in the pathway between impairments (physiologic fitness) and functional limitations; the latter category would then become “perceptions” of physical functioning or limitations rather than performance. This new performance step would not include performance of complex activities or disability. By incorporating performance as a potential distinct point in the sequential model, we can explore, for example, the extent to which physiologic fitness (impairments) is associated with subsequent performance of basic physical functions (e.g., whether fitness reserve slows decline in walking speed or the amount of weight one can lift), and in turn, how variations in performance of physical functions subsequently affect self-reported limitations (e.g., whether performance reserve such as fast walking speed slows the onset of self-reported walking difficulty).
Conclusions The articles in this supplement raise many issues that should facilitate the design and testing of physical activity interventions to affect the disablement process more specifically. Many issues have been raised regarding physical activity and disability3,5 that could be addressed by such collaborative research, including designing functional limitation–sensitive exercise guidelines, identifying types of exercise that prevent disability progression without exacerbating pain and musculoskeletal problems, and determining whether exercise equipment needs to be tailored for persons with disabilities. Exercise researchers can examine how interventions affect older adults who are at different points in the disablement process and whether interventions can “reverse” the process in addition to simply improving outcomes. Simultaneously, disability researchers can expand knowledge of specific transitions, including backward transitions (reversing the process). To continue to explore pathways to disability, and ways that physical activity interventions might affect them, interventions need to include good improved measures that assess distinctly each point in the process, as discussed in several articles in this supplement. An example of the benefit of including measures at all Am J Prev Med 2003;25(3Sii)
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levels was illustrated in the study by Cress et al.41; if they had included only self-report physical functioning measures, they may never have known that the intervention could improve performance; that is, true improvements may never have been detected. There are countless other ways in which the model could be modified to account for the many behavioral and psychosocial determinants of fitness, functioning, and disability; however, the few suggested here can provide a starting point for discussing how to expand this model to bridge the gaps between the disciplines and advance science in this area.
References 1. Jette AM. Assessing disability in studies on physical activity. Am J Prev Med 2003;25(suppl):122– 8. 2. Guralnik JM, Ferrucci L. Assessing the building blocks of function: utilizing measures of functional limitation. Am J Prev Med 2003;25(suppl):112–21. 3. Rimmer JH. Health promotion for people with disabilities: the emerging paradigm shift from disability prevention to prevention of secondary conditions. Phys Ther 1999;79:495–502. 4. Carlson JE, Ostir GV, Black SA, Markides KS, Rudkin L, Goodwin JS. Disability in older adults 2: physical activity as prevention. Behav Med 1999;24:157–68. 5. Rimmer JH, Braddock D, Pitetti KH. Research on physical activity and disability: an emerging national priority. Med Sci Sports Exer 1996;28: 1366 –72. 6. Bauman AE, Sallis JF, Dzewaltowski DA, Owen N. Toward a better understanding of the influences on physical activity. Am J Prev Med 2002; 23(suppl 2):5–14. 7. Rejeski WJ, Brawley LR, Haskell WL. The prevention challenge: an overview of this supplement. Am J Prev Med 2003;25(suppl):107–9. 8. Keysor JJ. Does late-life physical activity or exercise prevent or minimize disablement? A critical review of the scientific evidence. Am J Prev Med 2003;25(suppl):129 –36. 9. Seguin R, Nelson ME. The benefits of strength training for older adults. Am J Prev Med 2003;25(suppl):141–9. 10. Judge JO. Balance training to maintain mobility and prevent disability. Am J Prev Med 2003;25(suppl):150 – 6. 11. Frank JS, Patla AE. Balance and mobility challenges in older adults: implications for preserving community mobility. Am J Prev Med 2003; 25(suppl):157– 63. 12. Ory M, Kinney Hoffmann MK, Hawkins M, Sanner B, Mockenhaupt R. Challenging aging stereotypes: strategies for creating a more active society. Am J Prev Med 2003;25(suppl):164 –71. 13. Brawley LR, Rejeski WJ, King AC. Promoting physical activity for older adults: the challenges for changing behavior. Am J Prev Med 2003; 25(suppl):172– 83. 14. Satariano WA, McAuley E. Promoting physical activity among older adults: from ecology to the individual. Am J Prev Med 2003;25(suppl):184 –92. 15. Mihalko SL, Wickley KL. Active living for assisted living: promoting partnerships within a systems framework. Am J Prev Med 2003;25(suppl 3):193–203. 16. Sheppard L, Senior J, Park CH, Mockenhaupt R, Chodzko-Zajko W, Bazzarre T. The National Blueprint Consensus Conference summary report: strategic priorities for increasing physical activities among adults aged ⱖ50. Am J Prev Med 2003;25(suppl):209 –13. 17. Morey MC, Sullivan RJ Jr. Medical assessment for health advocacy and practical strategies for exercise initiation. Am J Prev Med 2003;25(suppl): 204 – 8.
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18. Bortz WM IV, Bortz WM II. How fast do we age? Exercise performance over time as a biomarker. J Gerontol Med Sci 1996;51:223–25. 19. Vorhies D, Riley BE. Deconditioning. Clin Geriatr Med 1993;9:745–63. 20. Bortz WM II. The disuse syndrome. West J Med 1984;141:691–4. 21. Strawbridge WJ, Cohen RD, Shema SJ, Kaplan GA. Successful aging: predictors and associated activities. Am J Epidemiol 1996;144:135–41. 22. Painter P, Stewart AL, Carey S. Physical functioning: definitions, measurement, and expectations. Adv Ren Replace Ther 1999;6:110 –23. 23. Stewart AL, Painter PL. Issues in measuring physical functioning and disability in arthritis patients. Arthritis Care Res 1997;10:395–405. 24. Caruso L, Silliman R, Demissie S, Greenfield S, Wagner E. What can we do to improve physical function in older persons with type 2 diabetes? J Gerontol Med Sci 2000;55:372–7. 25. Penninx BW, Guralnik JM, Simonsick EM, Kasper JD, Ferrucci L, Fried LP. Emotional vitality among disabled older women: the Women’s Health and Aging Study. J Am Geriatr Soc 1998;46:807–15. 26. Leveille SG, Guralnik JM, Hochberg M, et al. Low back pain and disability in older women: independent association with difficulty but not inability to perform daily activities. J Gerontol Med Sci 1999;54:487–93. 27. Clark DO, Stump TE, Hui SL, Wolinsky FD. Predictors of mobility and basic ADL difficulty among adults aged 70 years and older. J Aging Health 1998;10:422–40. 28. Leveille SG, Fried LP, Guralnik JM. Disabling symptoms: what do older women report? J Gen Intern Med 2002;17:766 –73. 29. Ettinger WH, Fried LP, Harris T, Shemanski L, Schulz R, Robbins J. Self-reported causes of physical disability in older people: the Cardiovascular Health Study. CHS Collaborative Research Group. J Am Geriatr Soc 1994;42:1035–44. 30. Bennett JA, Stewart AL, Kayser-Jones J, Glaser D. The mediating effect of pain and fatigue on level of functioning in older adults. Nurs Res 2002;51:252–65. 31. Rejeski WJ, Ettinger WH, Martin K, Morgan T. Treating disability in knee osteoarthritis with exercise therapy: a central role for self-efficacy and pain. Arthritis Care Res 1998;11:94 –101. 32. Koltyn KF. Using physical activity to manage pain in older adults. J Aging Physical Activity 2002;10:226 –39. 33. Fried LP, Guralnik JM. Disability in older adults: Evidence regarding significance, etiology, and risk. J Am Geriatr Soc 1997;45:92–100. 34. Verbrugge LM, Jette AM. The disablement process. Soc Sci Med 1994;38: 1–14. 35. Simonsick EM, Newman AB, Nevitt MC, et al. Measuring higher level physical function in well-functioning older adults: expanding familiar approaches in the Health ABC study. J Gerontol Med Sci 2001;56:644 –9. 36. Guralnik JM, Kaplan GA. Predictors of healthy aging: prospective evidence from the Alameda County Study. Am J Public Health 1989;79:703–8. 37. Haley SM, Jette AM, Coster WJ, et al. Late Life Function and Disability Instrument II: development and evaluation of the function component. J Gerontol Med Sci 2002;57:217–22. 38. Seeman TE, Charpentier PA, Berkman LF, et al. Predicting changes in physical performance in a high-functioning elderly cohort: MacArthur studies of successful aging. J Gerontol A Biol Sci Med Sci 1994;49:97–108. 39. Fried LP, Bandeen-Roche K, Williamson JD, et al. Functional decline in older adults: expanding methods of ascertainment. J Gerontol A Biol Sci Med Sci 1996;51:206 –14. 40. Fried LP, Bandeen-Roche K, Chaves PH, Johnson BA. Preclinical mobility disability predicts incident mobility disability in older women. J Gerontol A Biol Sci Med Sci 2000;55:43–52. 41. Cress ME, Buchner DM, Questad KA, Esselman PC, deLateur BJ, Schwartz RS. Exercise: effects on physical functional performance in independent older adults. J Gerontol A Biol Sci Med Sci 1999;54:242–8. 42. Jylha M, Guralnik JM, Balfour J, Fried LP. Walking difficulty, walking speed, and age as predictors of self-rated health: the Women’s Health and Aging Study. J Gerontol A Biol Sci Med Sci 2001;56:609 –17.
American Journal of Preventive Medicine, Volume 25, Number 3Sii
R
educing the functional limitations and disability that accompany aging and disease are goals of both physical activity and disability research. Efforts to link the two fields, as reflected in this supplement to the American Journal of Preventive Medicine, should expand considerably our ability to achieve these goals because of the special perspectives and strengths that each discipline brings to this collaboration. Disability research focuses on epidemiologic pathways by which disease, injury, and other conditions ultimately lead to disability and on surveillance of population levels of disability. Through its numerous adaptations, the “disablement process” model has provided a longstanding, strong guiding framework for this work.1,2 More recently, research on health promotion for people with disabilities is being undertaken,3 with a special emphasis on physical activity as both a primary and secondary prevention approach.4,5 Physical activity researchers targeting older adults address a variety of issues in order to design optimal interventions to change physical activity behavior. Some of these include identifying numerous multilevel determinants of physical activity, examining characteristics of exercise interventions that increase physical activity and lead to improved outcomes in this age group, and exploring mechanisms by which improved outcomes occur.6 Physical activity research that is more closely aligned with disability research includes epidemiologic studies of how natural physical activity levels and changes in activity levels are associated with subsequent physical functioning and disability in older adults. The papers in this supplement1,2,7–17 reflect state-ofthe-art ideas from both disciplines. The physical activity experts describe exercise interventions in older adults in terms of fitness, functioning, and disability outcomes; they also summarize key system-level factors and policies that affect physical activity levels in older adults. The disability specialists describe the disablement process and suggest ways to improve measures of From the Institute for Health and Aging, University of California–San Francisco, San Francisco, California Address correspondence and reprint requests to: Anita L. Stewart, PhD, Institute for Health and Aging, University of California–San Francisco, 3333 California St, Ste 340, San Francisco CA 94118. E-mail: [email protected].
the steps in the process. Several physical activity researchers attempted to fit their discussions within the disablement process model; however, in many cases, the model was a poor fit with their concepts and research processes. To enable this visionary work to progress in a timely manner, a conceptual framework is needed that can be used easily by both fields. Such a unifying framework should begin with the disablement process model, given that the ultimate goal is to prevent or slow the onset and progression of the disablement process in older adults. Four basic ways in which the model could be improved for this particular collaboration follow: 1. Two key mechanisms by which disablement begins— physiologic aging and disuse—are not an explicit part of the model, although these are essential concepts in physical activity research in aging. 2. One important mechanism by which chronic disease affects functioning and disability—symptoms—is not part of the model, yet physical activity often improves symptoms. 3. All concepts along the pathway are negatively defined, making it problematic for exercise scientists to describe physical activity intervention outcomes. 4. The functional limitations category is too broadly defined and measured, limiting our ability to understand more subtle processes by which physical activity affects functioning and disability.
Two Additional Entry Points: Physiologic Aging and Disuse The physiologic aging process contributes to a portion of the decline in organ systems and functioning independent of pathology or chronic disease and injury18 and thus needs to be an explicit entry point. In this supplement, aging is discussed in terms of its effects on the onset of functional limitations and disability,15 loss of muscle mass and strength (impairments),9,11 balance,10,11 and mobility disability,10 which illustrate its relevance to topics of disability and exercise in older adults. Deconditioning19 and disuse20 also lead to numerous impairments (e.g., cardiovascular vulnerability, musculoskeletal fragility) and chronic disease, as well as to
Am J Prev Med 2003;25(3Sii) © 2003 American Journal of Preventive Medicine • Published by Elsevier Inc.
0749-3797/03/$–see front matter doi:10.1016/S0749-3797(03)00187-9
137
Figure 1. Suggested Conceptual Framework for Linking Physical Activity and Disability Research
premature aging. In their paper, Frank and Patla11 refer to this as “detuning.” The deconditioning concept originally pertained to adverse effects on a variety of organ systems of extended inactivity due to bed rest and space travel. As a risk factor for chronic disease and disability, disuse is typically defined in terms of sedentary behavior, the least active level along a physical activity continuum. The idea that disuse can be a determinant of impairments is not new, especially when considering disuse in terms of physical activity. In this supplement, Keysor8 presents a conceptual model in which physical activity precedes impairments (muscle strength), functional limitations, and disability. The modified model would thus have three entry points—pathology, physiologic aging, and disuse—all of which can, independently, begin the process of decline (see Figure 1). This modification is consistent with the definition of impairments by Carlson et al.4 as due to disuse, aging, disease, and trauma.
Symptoms as Mediators Physical and mental symptoms such as musculoskeletal pain, fatigue, shortness of breath, depression, and weakness are well-known determinants of functional limitations and disability.21–29 Symptoms have been found to mediate the effects of chronic disease on functioning and disability30 and the effect of physical activity on functional limitations.8,31 Symptoms are often improved by appropriate physical activity32; for example, Seguin and Nelson9 cite nine studies in which strength training reduced symptoms. Indeed, many physical activity interventions target persons with various symptoms (joint pain, back pain, sleep problems) with the aim of reducing them. Adding symptoms to the disablement process model would support disability researchers to study symptoms as potential mediators of the effect of disease on functional limitations and disability and would encourage physical activity researchers to explore the extent to which physical activity interventions improve functioning by way of symptom reduction, particularly for people with chronic conditions. Consistent with this suggestion, Fried and Guralnik33 and Leveille et al.26 have published hypothetical models in which symptoms (angina, dyspnea, low back pain) 138
precede impairments (e.g., muscle strength), suggesting that symptoms could be incorporated into the model between the entry points and impairments. However, because symptoms can be caused by chronic diseases, injuries, and disuse, they might better be incorporated at the same level as impairments.
Modify Definitions (and Measures) to Incorporate Positive Levels Despite the fact that they are often referring to the same concepts, the language of exercise scientists differs from that of disability researchers. In particular, the fact that all points in the disablement process model are defined negatively (e.g., impairments, limitations) limits its usefulness for physical activity research.
Impairments or Physiologic Fitness? In the current model,34 impairments (organ-level factors) are defined as abnormalities in specific body systems such as musculoskeletal, cardiovascular, and neurologic. Exercise scientists refer to the same organlevel systems in terms of physiologic fitness. Common components of fitness in a variety of organ systems include cardiovascular and muscular endurance, flexibility (range of motion), balance/coordination, muscle strength, reaction time, visual acuity, power, and body composition.2,9 Each can be defined on a broad continuum, ranging from very low to very high fitness levels; thus, they are concerned with the full range, not just impairments. One potential generic label for this category is physiologic function, which could accommodate concepts of impairments (problems) and positive physiologic fitness levels. An advantage of this modification is that it could formalize efforts by researchers from both disciplines to explore the effects of having high fitness levels or “reserve” on the disablement process. Noting that hospitalization and immobilization are associated with precipitous functional declines, Fried et al.33 suggest that physiologic reserve can help patients maintain functioning in the face of these events. Disability research could examine the effect of physiologic reserve on withstanding an injury, hospitalization, or a serious health problem without progressing to a functional limitation. Physical activity researchers could determine if increasing fitness levels (i.e., approaching the “reserve” end of the fitness continuum) could slow the progression to functional limitations.
Functional Limitations or Physical Functioning? A parallel problem exists with the functional limitations step in the pathway. The highest score one can typically
American Journal of Preventive Medicine, Volume 25, Number 3Sii
get on self-reported functional limitations measures is the absence of limitations,2 such as not having any difficulty walking. This could be why many exercise interventions conducted in community-based samples of older adults have been unable to detect effects of exercise on physical functioning, as these subgroups tend to have minimal limitations. Some researchers have attempted to extend the range of measurement of self-reported physical functioning into the positive side. Three such approaches include: (1) for those reporting no difficulty, asking how easy a function is35; (2) adding items about participation in various physical activities36; and (3) adding discretionary or advanced functions (walk 1 mile, run one-half mile).2,37 Performance-based measures of functional tasks were developed, in part, to assess a broader functioning range, such as obtaining more variation among those at the ceiling of self-reported physical limitations.38 For example, on a performance test of walking, the highest score can indicate a fairly high performance level (e.g., above-average walking speed). However, as Guralnik and Ferrucci2 articulate so clearly in this supplement, performance measures were not designed with the disablement model in mind and thus cannot easily be used to assess processes of disablement. Developers of performance-based measures were attempting to overcome several problems with self-report measures in older adults and tried to tap a broad range of functional tasks. Hence, as shown in Table 3 in Guralnik and Ferrucci,2 many available performance measures assess more than one point in the disablement process. This is also true of self-report function and disability measures.23 Regarding the need to extend the functional limitations category into more positive ranges, there are options for both self-report and performance measures to tap higher functioning levels. Thus, relabeling this category would help, perhaps as “physical functioning.” Although other functioning dimensions are part of the original model, such as cognitive and sensory functioning, to adapt this model for physical activity research, the physical dimension is probably the most crucial.
Performance of Physical Functions—A Distinct Point in the Pathway? Much research suggests that performance measures of physical functioning are not simply an alternative to self-reported measures but might instead reflect a distinct step in the disablement process. Fried and Guralnik33 and Fried et al.39 have found that performancebased physical function tests identify a larger proportion of older adult samples than would be identified by self-report, particularly for ambulation (walking, climbing stairs). In a prospective study of older women with no mobility difficulty, they found
that lower mobility performance levels at baseline predicted declines in self-reported mobility difficulty.40 Another example is an exercise intervention by Cress et al.41 in which improved physical functioning was observed based on several performance measures, but not on self-reported measures. Last, self-reported walking difficulty and measured maximal walking speed were independently associated with self-rated health in older women.42 These and other findings that self-report and performance-based physical functioning measures behave differently are strong indications that they represent distinct concepts. It may be time to consider distinguishing performance of physical functions as a separate step in the pathway between impairments (physiologic fitness) and functional limitations; the latter category would then become “perceptions” of physical functioning or limitations rather than performance. This new performance step would not include performance of complex activities or disability. By incorporating performance as a potential distinct point in the sequential model, we can explore, for example, the extent to which physiologic fitness (impairments) is associated with subsequent performance of basic physical functions (e.g., whether fitness reserve slows decline in walking speed or the amount of weight one can lift), and in turn, how variations in performance of physical functions subsequently affect self-reported limitations (e.g., whether performance reserve such as fast walking speed slows the onset of self-reported walking difficulty).
Conclusions The articles in this supplement raise many issues that should facilitate the design and testing of physical activity interventions to affect the disablement process more specifically. Many issues have been raised regarding physical activity and disability3,5 that could be addressed by such collaborative research, including designing functional limitation–sensitive exercise guidelines, identifying types of exercise that prevent disability progression without exacerbating pain and musculoskeletal problems, and determining whether exercise equipment needs to be tailored for persons with disabilities. Exercise researchers can examine how interventions affect older adults who are at different points in the disablement process and whether interventions can “reverse” the process in addition to simply improving outcomes. Simultaneously, disability researchers can expand knowledge of specific transitions, including backward transitions (reversing the process). To continue to explore pathways to disability, and ways that physical activity interventions might affect them, interventions need to include good improved measures that assess distinctly each point in the process, as discussed in several articles in this supplement. An example of the benefit of including measures at all Am J Prev Med 2003;25(3Sii)
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levels was illustrated in the study by Cress et al.41; if they had included only self-report physical functioning measures, they may never have known that the intervention could improve performance; that is, true improvements may never have been detected. There are countless other ways in which the model could be modified to account for the many behavioral and psychosocial determinants of fitness, functioning, and disability; however, the few suggested here can provide a starting point for discussing how to expand this model to bridge the gaps between the disciplines and advance science in this area.
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