Physical Function in Older People

Noran N. Hairi1,2, Tee Guat Hiong3, Awang Bulgiba1,2 and Izzuna Mudla4 1Department of Social and Preventive Medicine, Faculty of Medicine, University of Malaya, Kuala Lumpur, 2JCUM, Centre for Clinical Epidemiology and Evidence-Based Medicine, Faculty of Medicine, University of Malaya, Kuala Lumpur, 3Institute for Public Health, National Institutes of Health, Ministry of Health, 4Ministry of Health, Malaysia


Introduction
Aging is a natural process. Improved maternal and infant health, better survival in infancy, childhood and early adult life, has led to increase life expectancy of older people. As of 2008, 7% (506 million) of the world's population was aged 65 years and older, an increased of 10.4 million since 2007 (Kinsella K and Wan He 2009). The current pace of population aging varies widely. While developed countries have relatively high proportions of people aged 65 years and over, the most rapid increases in older people are in the developing world. As of 2008, 62% (313 million) of the world's population aged 65 and over lived in developing countries (Kinsella K and Wan He 2009). Many developing countries will be experiencing a sudden rise in the proportion of older people within a single generation, with far less well developed infrastructure. In contrast, most developed countries have had decades to adjust to the changing age structure and this change has been supported by relative economic prosperity.

Theories of population health change
The implications of longer life mean increased risk of poor physical function as expounded by the theories of population health change. Four theories have been proposed in discussing the consequences of increased life expectancy in older people.
The expansion of Morbidity/Disability Theory (Gruenberg EM 1977), suggests that the gain in life expectancy in older people is mainly due to technological advances and secondary prevention strategies that have extended the life of older people with disability and underlying illness. This results in living with non-fatal diseases such as vision loss, arthritis, chronic pain and other diseases of old age, therefore living longer means living with more years of disability.
The opposing theory is called the Compression of Morbidity/Disability Theory (Fries 1980;Fries 2005). He suggested that primary prevention strategies modify risk factors for disability in the literature. Basic ADLs are self-care tasks such as bathing, dressing, grooming and eating . The IADL's are tasks that are physically and cognitively more complicated and difficult but are necessary for independent living in the community such as getting groceries, preparing meals, performing everyday household chores. ADL and IADL are measures of disability that reflect how an individual's limitation interacts with the demands of the environment.
The evaluation of mobility refers to the individual's locomotor system. Mobility disability is a critical component of activities of daily living . Mobility disability is defined as difficulty or dependency in functioning due to decreased walking ability, manoeuvrability and speed.
The building blocks of restrictions in performing ADLs are termed functional limitations (Guralnik and Luigi 2003). Functional limitations are measures independent of environmental influences, and may explain the changes in functional aspects of health. Functional limitation refers to restriction in physical performance of tasks required for independent living, such as walking, balancing and standing.
Physical function is a general term that reflects one's ability to perform mobility tasks, ADLs and IADLs. Throughout this chapter "poor physical function" is used as a general term to refer to physical disability, mobility disability and functional limitation.

The disablement process
To discuss poor physical function in older people, it is important to have an understanding of the progression that ends with loss of physical function, or the disablement process. The disablement process describes how chronic and acute conditions affect functioning in specific body systems, basic human performance, and people's functioning in necessary, usual, expected, and personally desired roles in society (Verbrugge and Jette 1994). It also describes how personal and environmental factors speed up or slow down this process. There are two major models describing disability and related concepts. This chapter will describe both models. -the Nagi Model (Nagi 1976) and the International Classification of Impairments, Disabilities and Handicaps (ICIDH) (World Health Organization 1980) and its current version, the International Classification of Functioning, Disability and Health (ICF) (World Health Organization 2001) developed by the World Health Organization (WHO).

The Nagi disablement model
The pathway proposed by Nagi in 1965 to describe progression from disease to disability is shown in Figure.1. Nagi's disability model is based on four related components that described the sequential steps in the theoretical pathway from disease to disability (Nagi 1976). In the Nagi pathway, pathology (e.g. sarcopenia) first leads to impairment (e.g. lower extremity weakness) (Steven M Albert and Vicki A Freedman 2010). When lower extremity weakness crosses a certain threshold, functional limitation (e.g. slow gait speed) becomes evident (Steven M Albert and Vicki A Freedman 2010). When this happens, a person has a disability (e.g. difficulty or needing help with walking across a small room).
According to this pathway, pathology refers to biochemical and physiological abnormalities that are medically labeled as disease, injury or congenital/developmental conditions (Ferrucci, et al. 2007;Nagi 1976;Verbrugge and Jette 1994). Impairment is the consequence www.intechopen.com and degree of pathology (Nagi 1976;Verbrugge and Jette 1994). Functional limitations are limitations in performance at the level of the whole organism or person (Ferrucci, et al. 2007). By contrast, disability is defined as limitation in performance of socially defined roles and tasks within a socio-cultural and physical environment (Ferrucci, et al. 2007). Disability can also refer to the expression of functional limitation in a social context. An important advantage of utilizing different definitions for functional limitation and disability, as proposed by Nagi, is that they can be considered as sequential steps on the pathway from disease to disability. The validity of this theoretical pathway is supported by a large body of literature (Ferrucci, et al. 2007; (1965) Nagi's model was extended to include personal and environmental factors that influence the evolution of the disablement process (Verbrugge and Jette 1994). Verbrugge and Jette differentiate the "main pathways" of the disablement process (i.e. Nagi's original concepts) with factors hypothesized or known to influence the ongoing process of disablement ( Figure  2). This model emphasizes that predisposing risk factors, intra-individual and extraindividual factors may modify the relationship of the four components in the main pathway (Ferrucci, et al. 2007;Guralnik and Luigi 2003; Steven M Albert and Vicki A Freedman 2010; Verbrugge and Jette 1994). Risk factors are predisposing phenomena that are present prior to the onset of a disabling event that can affect the presence and/or severity of the disablement process. Intra-individual factors are those that operate within a person such as lifestyle and behavioural changes, psychosocial attributes and coping skills. Extra-individual factors are those that perform outside or external to the person. Nagi's definition of disability and the elaboration by Verbrugge and Jette also operationalizes disability as a broad range of role behaviours that are relevant to daily activities. This includes basic ADL, IADL, paid and unpaid role activities, such as occupation, social activities and leisure activities.

World Health Organization's models of disablement
In 1980, the World Health Organization (WHO) proposed a theoretical framework to describe the sequence from disease/disorder to impairment, disability and handicap named the International Classification of Impairments, Disabilities and Handicaps (ICIDH) (World Health Organization 1980) (Figure 3). At the foundation of the pathway is pathology, which is defined as any abnormality of macroscopic, microscopic or biochemical structure or function affecting an organ or organ system (Ferrucci, et al. 2007;Verbrugge and Jette 1994). The second step is impairment, defined as any abnormality of structure or function at the whole organism level, independent of any specific environment, symptom, or sign (Ferrucci, et al. 2007;Verbrugge and Jette 1994). At the third step is disability, which derives from the  Process (1994) interaction between the organism and the environment and is defined as any change or restriction in an individual's goal-directed behaviour (Ferrucci, et al. 2007;Verbrugge and Jette 1994). Finally, handicap is defined as any alteration in a person's status in society, including alterations in roles. Each level of the pathway should be considered as independent and may or may not be determined by the previous level and/or cause the successive level (Ferrucci, et al. 2007;Verbrugge and Jette 1994). This approach raised criticisms for several reasons: it was thought to be too medically-orientated, ignoring social and psychological dimensions; the negative connotation of the term 'handicap'; and the omission of environmental factors. Some of these limitations were overcome by the model proposed by Nagi.
In 2001, the WHO presented a revision of the classification under a new name called the International Classification of Functioning, Disability and Health (ICF) (World Health Organization 2001) ( Figure 4). The revised model moves away from the idea that disability is a consequence of disease or aging and focuses on components of health as human functioning. The ICF has two parts, each with two components (  (Jette AM and Keysor J 2003). The first element of the ICF, the Body Functions and Structures is similar to Nagi's concept of pathology and impairment while the second component of the ICF, the Activities and Participation closely corresponds to Nagi's concept of functional limitations and disability (Jette AM and Keysor J 2003)(as shown in Table 2). The greatest limitations of the ICF is the aggregation of "activities and participation" into one domain (Guralnik and Ferrucci 2009). Using the ICF, the concepts of activity limitation and participation restriction are difficult to separate, unlike Nagi's concept of functional limitations and disability. The ICF currently does not offer crisp distinction between activity and participation, although there is an increasing movement towards defining "activities" and "participation". The Institute of Medicine (IOM) discussed this concern in its report entitled Future of Disability in America (Marilyn J. Field and Alan M. Jette 2007). Some sections of the report cited verbatim are as shown below: "A first and well recognized aspect of the ICF that needs further development involves the interpretation and categorization of the concepts of activity and participation (page 42)" "Although this committee does not endorse any particular approach to resolving the problem, it believes that the lack of operational differentiation between the concepts of activity and participation is a significant deficit in the ICF. (page 44)" Since the ICF's distinction between activity and participation is still in the developmental stage, many studies have used the Nagi Disablement Model as a conceptual framework in their research to understand the dynamic relationships among factors associated with physical function. Furthermore, the ICF is not inherently a dynamic model, similar to the ICD-10, the ICF is a classification system that offers standardized internationally accepted language. It is also worth noting that the Nagi Disablement Model has been successfully used as a theoretical pathway that was empirically tested in many datasets (Guralnik and Ferrucci 2009). For example, evidence demonstrates the predictive value of disease for impairment (arthritis causing reduced strength) (Guralnik and Luigi 2003), of impairment for functional limitations (reduced strength leading to reduced gait speed) (Guralnik and Ferrucci 2009) and of functional limitations for disability (lower extremity limitations leading to activity of daily living and mobility disability) (Penninx, et al. 2000).

Physical function measurement tools
Poor physical function can be assessed by using instruments based on self-report and by objective measurements or performance based tests. In the domain of physical and mobility www.intechopen.com disability, self report and proxy report of difficulty or inability to perform ADLs and IADLs and mobility questionnaires have been the standard assessment tools (Guralnik and Luigi 2003;Kovar and Lawton 1994). There are more than 100 published basic ADL or IADL scales, with considerable variations in the number of questions, item content, and scoring method. Examples of some of the instruments used to measure disability include Katz ADL, The Barthel Index, Instrumental Activities of Daily Living Scale (IADL), and The Health Assessment Questionnaire (HAQ) Disability Scale. The comparison of the quality of the physical disability and mobility disability tools is as shown in Table 3. Objective measurements of disability are also available but are rarely used (Cress ME 1996;Kuriansky JB 1976 Table 4. All in all, poor physical function has been assessed with a wide variety of instruments. There is no single best way to perform an assessment and there is no single instrument that is ideal. The lack of standardization that results from the use of multiple instruments makes it difficult to compare findings across studies (Jette 1994;Kovar and Lawton 1994;Wiener, et al. 1990).

Prevalence of physical disability, mobility disability and functional limitation in developed countries
Several studies in developed countries have sought to gauge the prevalence of physical disability, mobility disability and functional limitation among older people.
The basic set of either the six-item ADL or the five-item ADL has been found to be most useful for valid comparison across studies. Using the six-item ADL, the prevalence of disability from the National Long-Term Care Survey in the United States ranges from 12.4% to 13.2 % from 1982 to 2005 (Lafortune 2007). Disability surveys that capture five-item ADL show lower prevalence of disability:-6% in Canada, 10% in France and 11% in Sweden (Lafortune 2007). Using the five-item ADL, significant variations in disability have been reported between populations in the United States, China and Singapore (Chen, et al. 1995;Ng, et al. 2006;Wiener, et al. 1990;Zhe, et al. 1999 Table 3. Comparison of the physical disability and mobility disability assessment tools www.intechopen.com Table 4. Comparison of the functional limitation assessment tools.

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Mobility disability is very common among older people. Results from a United States National Prevalence Survey of Disability revealed that among older people aged 65 years and over, 30% had difficulty with mobility (Nordstrom, et al. 2007). In the United Kingdom, the Hertfordshire Cohort Study found that 32% of men and 46% of women aged 59 years and over reported that their health limited them in performing mobility activities (Syddall, et al. 2009). Data from the Netherlands National Health Survey showed that approximately 18% and 37% of older Dutch people aged 65 to 74 years and 75 years and over respectively reported mobility disability (Picavet and Hoeymans 2002).
Assessing functional limitation adds valuable information about the steps in the disability pathway. Gait speed, often termed walking speed has been regarded as the best single measure to evaluate functional limitation (Guralnik and Luigi 2003). It has also shown to be a strong and consistent predictor of adverse outcomes in older people. In a pooled analysis of individual data from nine major cohorts, gait speed has been shown to be a predictor of mortality in older people (Studenski, et al. 2011 (Studenski, et al. 2011).

Prevalence of physical disability, mobility disability and functional limitation in developing countries
The burden of poor physical function has been studied extensively in developed countries but there is little data available for older people in developing countries. Comparison of physical disability, mobility disability and functional limitation distribution between countries is difficult due to methodological differences in definition and measurements used. Surveys from around the world used different approaches in measuring disability. Different instruments within the same country often report different rates. Across countries the variation is even more cumbersome. Nevertheless, the studies discussed below used comparable methods to a certain extent.
Prevalence studies on the five-item ADL disability among older people had been carried out in several low income developing countries. The Cambodian study in 2004 showed a prevalence of 23.7% among older people aged 60 years and over (Zimmer 2008); the 1998 Housing and Population Census of the Ethiopian Government reported a prevalence of 28.6% among Ethiopians adults aged 55 years and above (Teferra 2005). In addition, two studies were conducted in Nepal among older people aged 60 years and above with prevalence of 8.8% in Kathmandu city, 2005 and a much higher prevalence of 55.8% in rural Chitwan Valley, 1998 (Shrestha 2004).
Studies on the five-item ADL disability among older people in lower middle income developing countries reported varied prevalence: 10% and 9% among older people aged 65 years and over in Sri Lanka (Nugegoda and Balasuriya 1995) and urban Chinese in Shanghai www.intechopen.com (Chen, et al. 1995)respectively. Whereas, among older people aged 60 years and over, the prevalence were: 12% in Indians (Shantibala, et al. 2007), 10.9% and 14.7% in Filipinos in the years 1996 and 2001 respectively (Ofstedal et al), 6.5% in Beijing Chinese (Zhe, et al. 1999), 18.7% in Indonesian men and 12.1% in Indonesian women (Evi Nurvidya Arifin 2009).
Several studies in lower middle income countries have used six-item ADL scales. In the WHO Collaborative Study on Social and Health Aspects of Aging in 1990, the prevalence of six item ADL disability among older people age 60 and over was: 22.4% in Egyptian men, 28.5% in Egyptian women, 32.0% in Tunisian men and 46.8% in Tunisian women (Yount and Agree 2005). The Ibadan Study of Aging in Nigeria in 2004 reported the prevalence of six-item ADL disability at only 3% among Nigerians aged 65 years and over(Oye Gureje, et al. 2006). The prevalence of disability among Nigerians is low because of the difference in criterion definition used. In the Ibadan Study of Aging, disability is based on difficulty experience with four levels of responses. This resulted in a more restrictive definition of disability, as compared to studies that defined disability based on any level of difficulty.
Many studies on the prevalence of physical disability were conducted in upper middle i n c o m e c o u n t r i e s . F r o m t h e S u r v e y o n H e a l t h a n d W e l l -B e i n g o f E l d e r s . P a l l o n i et al reported that the prevalence of six-item ADL disability among older people were: 19% in Argentina, 14% in Barbados, 24% in Brazil, 26% in Chile, 21% in Cuba, 19% in Mexico and 17% in Uruguay (Palloni and McEniry 2007). The prevalence of six-item ADL disability among older people aged 60 years and over in Puerto Rico in a 2003 study was 20% (Palloni, et al. 2005).
The epidemiology of poor physical function among older people in developing countries is incompletely understood with many unanswered questions.

Risk factors for poor physical function
Several factors have been identified as risk factors for disability and functional limitation. These include non-modifiable risk factors (e.g. gender, ethnicity and genetics) and modifiable risk factors, which include both individual factors (such as sedentary lifestyle, unhealthy behaviours) as well as characteristics of the environment (e.g. household hazards, disadvantaged neighbourhood conditions, common forms of transportation).
The association between female gender and poor physical function is consistently reported in many studies. Some studies have shown that the higher prevalence of poor physical function in female is unexplainable by known differences in sociodemographic and health related factors(Auxiliadora Graciani, et al. 2004;Dunlop, et al. 1997).
There are few reports of ethnic differences in frequency of poor physical function. How differences in sociodemographic and health-related factors explain the ethnic differences in poor physical function is still unclear. Older black and Hispanic Americans have a higher prevalence of poor physical function than their white counterparts (Kelly- Moore and Ferraro 2004). Other studies from the United States have found that African-American have higher disability rates compared to the Whites even after adjustment for education (Liao, et al. 1999) and chronic disease (Kingston and Smith 1997), although one study reported that social and health factors explained these differences(Kelly-Moore and Ferraro 2004). Ng et al showed that Indians and Malay s i n S i n g a p o r e h a v e h i g h e r r i s k o f d i s a b i l i t y t h a n Singaporean Chinese (Ng, et al. 2006).
Older people in less advantaged socioeconomic positions report more physical disability, mobility disability and functional limitation. Lower level of education tends to be associated with a higher prevalence of poor physical function at all ages (Lafortune 2007). A lower level of education is often associated with lower income, lower standards of living, higher risk of work-related injuries, and adoption of less healthy behaviours. The "education" effect has been shown to be a proxy for broader "socioeconomic status".
Changes in the prevalence of chronic diseases play a dominant role in explaining the prevalence of poor physical function among older people. However, not all diseases are associated with poor physical function and some are more strongly associated than others. Diseases with large effects on poor physical function include stroke and other neurological diseases, diabetes, heart diseases, depression, arthritis and other musculoskeletal diseases (Avlund 2004). It has also been reported that the presence of more than one chronic disease in an individual, often called co-morbidity is associated with poor physical function (Guralnik, et al. 1993;Schmitz, et al. 2007). Guralnik et al showed that the presence of a single chronic disease is a significant predictor of poor physical function, with the risk increasing incrementally up to the presence of four or more chronic diseases (Guralnik, et al. 1993).
There is also some evidence of association between smoking, heavy alcohol consumption and lack of physical activity with poor physical function (Tas, et al. 2007b).
The majority of studies on risk factors for poor physical function have focused on chronic diseases and lifestyle behaviours. There are a number of health-related factors that have rarely been investigated. These include the co-existence of depression and visual impairment; chronic pain; and the role of muscle strength, muscle mass (sarcopenia) and muscle quality.

Co-existing depressive symptoms and visual impairment as risk factors of poor physical function
The accumulation of deficits across more than one health domain, including physiological, sensory, cognitive and psychological domain, is likely to explain the development of poor physical function better than decline in just one single health domain. Whitson et al showed that individuals with co-existing visual impairment and cognitive impairment are at high risk of disability (Whitson, et al. 2007). Lin et al showed that that the burden of having both vision and hearing impairment is greater than the sum of each single impairment (Lin, et al. 2004). Rantanen at al reported that the odds of severe mobility disability were ten times www.intechopen.com greater among those who had both strength and balance impairments compared to those with just one or other impairment (Taina Rantanen, et al. 2001). Thus, it appears that certain pairs of co-existing conditions have a strong effect on physical function risk.
Depression and visual impairment are common conditions among older people and are also modifiable to a certain degree; depression can be treated and visual impairment can be corrected. However, it is unclear whether there is a synergistic effect of depressive symptoms (psychological health domain) and visual impairment (sensory domain) on the risk of poor physical function among older people.

Chronic pain and poor physical function
There are gender-based differences in mortality and morbidity; with men experiencing higher mortality rates and women generally having higher levels of morbidity (Steven M Albert and Vicki A Freedman 2010). In the pain literature, a robust and common finding is that women reported more pain, have lower pain thresholds and tolerance, and show different attitudes in coping with pain as compared to men (Roger, et al. 2009;Unruh 1996).
Longitudinal and cross sectional population based studies have shown that the impact of pain goes beyond physical distress (Keogh, et al. 2006). The presence of pain is also associated with poor physical function (Duong, et al. 2005).
In contrast to gender differences in pain, the evidence about gender differences in pain outcomes, such as poor physical function, remains inconclusive. Cunningham et al found no difference in musculoskeletal pain related restriction in daily activities between genders (Cunningham and Kelsey 1984). The Health, Aging and Body Composition (ABC) Study also found no gender differences in the relationship between low back pain and physical function (Weiner, et al. 2003). However, studies that used pain-related disabilities items as an outcome found there were gender differences in reporting pain-related disabilities (Keefe, et al. 2000;Réthelyi, et al. 2001;Stubbs, et al. 2010).

Sarcopenia as risk factors for poor physical function
It is well established that the aging process in humans is associated with loss of muscle mass and strength (Doherty 2003;Y Rolland 2008 ). Age-related decline in muscle mass has been documented by lean body mass measurements with dual X-ray absorptiometry (DXA) and muscle cross sectional areas quantified by imaging methods such as X-ray computed tomography (CT) and magnetic resonance imaging (MRI). The age-related loss of muscle mass results from loss of both slow and fast motor units, with an accelerated loss of fast motor units. These changes in muscle morphology results in sharp age-related changes in muscle strength and muscle function (Lang, et al. 2010). Muscle quality is an indicator of muscle function, quantified by strength per unit muscle mass. Another morphological aspect of aging skeletal muscle is the infiltration of muscle tissue components by lipids. The aging process is thought to result in increased frequency of fat cells within muscle tissue (Anne B. Newman, et al. 2003).
Age-related loss of muscle mass, strength and quality is called "sarcopenia". Recent longitudinal studies have demonstrated that age-related loss of muscle strength increases the risk of poor physical function among older people (Giampaoli, et al. 1999;Taina Rantanen, et al. 2001). Perhaps because of the various operational definitions used, the relationship between age-related muscle mass (sarcopenia) and poor physical function has not been consistent (I Janssen 2006;MJ. Delmonico, et al. 2007).

Conclusion
In summary, poor physical function -physical disability, mobility disability and functional limitation is developing as the population ages. Poor physical functions are complex processes with multiple risk factors at work (Steven M Albert and Vicki A Freedman 2010). As such, multifactor interventions are needed to improve and maximize older people's physical functions. Identifying the appropriate target population and window of time for targeting an intervention is critical to its success. Furthermore, the issue of sustainability and adherence to these interventions are also important for long term success (Steven M Albert and Vicki A Freedman 2010). Research up to date is still incomplete in guiding public health practitioners and clinicians as to which interventions will improve and maximize older people's physical function in the long run.