The proportion of people older than 80 yr is growing rapidly. To guarantee the sustainability of health and social care systems while enhancing quality of life, it is important to find ways to promote the functional capacity of older people. Mobility is a key issue in maintaining independence in old age. Mobility refers to a person’s ability to move himself or herself independently and safely from one place to another. In the present article, we focus on epidemiological studies that have examined walking as an indicator of mobility. Mobility in older people can be assessed either through self-report or through performance-based measures. Performance-based measures rely on a rater’s assessment of a subject’s performance of a specific mobility task, measured in a controlled environment, and provide information on the upper limit of mobility. Maximal gait speed is an example of a widely used performance-based indicator of mobility. Self-report measures are subject completed, relying on self-perception of mobility. They typically assess the subject’s perceived difficulties, restrictions, or need for assistance associated with the activity. An example of a widely used self-report measure of mobility would be a hierarchical series of questions posed to the participants, “Do you have difficulty walking a distance of 1 mile/a quarter of a mile/across a room,” with the response options “no difficulty,” “some difficulty,” “a great deal of difficulty,” or “unable without help from another person.”
Mobility limitations — referring to performance deficits assessed using an objective mobility test or perceived difficulties in mobility — increase with advancing age and are often the first signs of further functional decline. Mobility limitations hinder the ability to manage tasks of daily life and may lead to the need for help and an increased risk for disability and institutionalization (6,18). To prevent disability, it is important to identify people who are not yet disabled but who are at risk for disability progression in the near future (2). For this purpose, knowledge of the progression of the mobility limitation is important.
In the present article, the purpose is to provide a broad view of the progression of mobility decline in older people. We present recent findings from our study center on the individual and environmental factors on the pathway to mobility limitations among older people.
PROGRESSION OF MOBILITY DECLINE IN THEORY
A widely recognized theoretical model explaining mobility decline in epidemiological studies is the disablement process model by Nagi (17), later expanded by Verbrugge and Jette (36), which shows the disablement process pathway from pathology to disability. Pathology, referring to physiological abnormalities, such as chronic diseases or injury, and also to physiological changes with advancing age, affects specific body systems and may result in impairments such as decreased muscle strength and balance or sensory impairments. These impairments usually lead to functional limitations, which in turn finally may cause disability. In the disablement model, mobility usually is considered to belong in the category of functional limitations. Disability refers to a situation where individual capabilities are not sufficient to meet the requirements posed by the living environment. The disablement process model outlines intraindividual and extraindividual factors that can either reduce or increase functional limitations. Whereas the main pathway emphasizes the physiological process, intraindividual factors focus on lifestyle and behavioral changes, psychosocial attributes and coping, and activity accommodations. Extraindividual factors include factors such as the built and physical and social environment. In addition, different predisposing risk factors, such as certain demographic, social, lifestyle, and behavioral characteristics of an individual, may have a direct or indirect effect on the development of functional limitations.
Environmental gerontology studies the relationship between aging people and their physical and social environment and how these relationships shape health, functioning, and quality of life in old age. The widely accepted ecological model of aging, also known as the “competence-press model” (9), describes the person-environment relationship. Among older people, environmental factors, such as hills or long distances to services, may operate as barriers to mobility or as an opportunity to maintain mobility. The effect of environmental factors on mobility depends on the interaction between the environment and the individual and is called person-environment fit. If individual competence and the demands of the environment are in balance, a person is able to function optimally. When this balance is lost and person-environment misfit occurs, difficulties emerge (9).
There are similarities between the ecological model of aging and the disablement process model. The person-environment perspective of the disablement process model emphasizes that the environment and the individual are of equal importance in the disability process. The environment is seen on the “demand” side and the person on the “(cap)ability” side of the model, as in the ecological model of aging. Both theories emphasize that disability occurs when there is a misfit between the environment and the individual. In both models, a challenging environment is seen as either a threat or as an opportunity to maintain functional capacity.
Although these two theories share a similar content, there are a few differences between them. The disablement process model shows the pathway from pathology to disability, seeking to outline the underlying physiological changes and contributing factors, whereas the ecological model of aging shows the interplay between the individual and the environment from a general perspective and sees the relationship as a dynamic process, explaining the mechanisms behind the interaction. The ecological model of aging has a strong psychological emphasis, whereas the disablement process model emphasizes physiological changes. Combining the salient aspects of these two models into an analytical approach may provide a good foundation to better understand the progression of mobility decline.
PREDICTORS OF MOBILITY DECLINE
Pain and Obesity as Predictors of Mobility Decline
In the long-term, most chronic conditions along with aging will have a detrimental influence on mobility through various mechanisms influencing the musculoskeletal, neurological, or cardiorespiratory systems. In this section, we discuss two important pathologies predicting mobility loss, namely, pain and obesity.
Musculoskeletal pain is common among older people and is associated with impaired balance and mobility limitations (12). The pathway from musculoskeletal pain to mobility limitation is not clear. It has been suggested that severe pain in the lower body decreases physical activity that may then lead to a decline in muscle strength and the development of mobility limitations (33). However, Karttunen et al. (7) found that even after adjusting for potential factors on the pathway, such as self-rated health, depressive symptoms, different chronic conditions, and muscle strength, musculoskeletal pain almost doubled the risk for mobility limitations among older people. Leveille et al. (11) also found that musculoskeletal pain had a direct effect on mobility limitations, independent of the main disablement pathway, via impairments and functional limitation. Presumably, different pathways are present, depending on the underlying cause of pain. For example, hip fracture, a common severe fall-related injury among older people, causes persistent pain in many cases. After hip fracture, along with protracted pain, the muscle strength and power of the fractured leg remain poor. This leads to muscle power asymmetry of the lower extremities and subsequent mobility decline, although the fracture itself has been treated (22).
Obesity is a rising health problem in both western and nonindustrialized countries. However, the influence of obesity on mobility decline in old age has only recently received more systematic scientific attention. People with excess weight carry a mechanical load, which increases their energy expenditure, placing increased demands on aerobic capacity and muscle strength compared with normal-weight individuals doing similar physical tasks. In a 22-yr follow-up, we observed that people who were overweight in midlife but did not have any impairment approximately had double the risk for future mobility limitation compared with normal-weight people (35). However, when overweight was accompanied by two or more impairments (poor performance in handgrip strength, squatting test, or self-reported running difficulties) in midlife, the risk for old-age mobility limitation was more than sixfold compared with those with normal body weight in midlife. The increased risk for mobility limitation among obese and overweight people was explained partly by their increased inflammation (C-reactive protein) and low muscle strength, as indicated by handgrip strength in the lowest quartile (34).
It is possible that some people genetically are more vulnerable to mobility loss and weight gain than others. Twin and family studies provide an opportunity to examine how large a proportion of individual differences are explained by genetic differences. Among monozygotic twins, who share all their genes, phenotypic differences are caused by differences in the environment, that is, behavior, living habits, work, or living conditions. Among dizygotic twins, the genetic resemblance is 50% and, consequently, phenotypic differences between the members of the pair may result from either genetic or environmental differences. We studied a sample of older female twins during a 30-yr period to determine the proportion of genetic factors underlying individual differences in changes in body mass index (BMI) and whether the same genetic factors underlie mobility in old age. We observed that the inverse association between BMI and mobility was explained by shared genes, where genes predisposing people to obesity in middle and old age increased the risk for mobility limitation in later life (19,20).
Muscle Strength and Sensory Impairments in Mobility Decline
The impairments most commonly studied in relation to walking difficulties are those that directly influence walking, namely, muscle strength and balance. In our early studies, we reported that the maximal isometric muscle strength of handgrip, elbow flexion, knee extension, and trunk flexion and extension correlates with mobility assessed as self-reported difficulties in walking outdoors (26). The association of mobility and maximal isometric strength also was observed for muscle groups other than those in the lower extremities, as the strength test results showed a within-person correlation. We later reported that the quadriceps strength requirement for a standard stair-mounting task (stepping on a 40-cm-high box) is similar for men and women of different ages (28). This demonstrates that the reason a larger proportion of 80-yr-old people than 75-yr-old people, and of women than men, was unable to do the task was because of their lower muscle strength. We also reported on quadriceps strength thresholds for walking (27) and identified the minimum required knee extension strength for walking (1.22 m·s−1) and the reserve capacity threshold for the same walking speed. These studies demonstrated that muscle strength is important for the mobility of older people but that the association between muscle strength and mobility is not linear. The strength of the older person needs to be above a certain minimum for performance of the task to be at all possible. Above this minimum, increasing muscle strength improves task performance until the reserve capacity threshold of muscle strength is reached. Above this value, increased muscle strength does not improve task performance as strength no longer limits task performance. The most commonly used measure of muscle strength in large studies is handgrip strength (31). In a sample of older people, Sallinen et al. (32) determined the thresholds of handgrip strength for the likelihood of mobility limitation, which he defined as difficulty walking 0.5 km or climbing stairs. In cross-sectional data, the overall handgrip strength cut points for the likelihood of mobility limitation were 37 kg (sensitivity, 62%; specificity, 76%) for men and 21 kg (sensitivity, 67%; specificity, 73%) for women. There were differences in the thresholds according to BMI among men. Men with higher BMI had higher cut points; but for women, the cut point did not depend on BMI.
Many studies have focused on the independent effects of various impairments on mobility decline. However, coimpairments, which are multiple impairments simultaneously, may have an even greater impact on mobility decline than the sum of the single impairments involved because people become unable to compensate for one impairment with good capacity in another body system (29). For example, in a 3-yr prospective study, the likelihood of severe walking limitation (walking speed below 0.4 m·s−1 or unable to walk 400 m) was more than five times greater in the group with balance and strength impairments than in the group with no impairments. Among those who had balance impairment but normal strength, the risk for severe walking disability was threefold. Among those with good balance, strength impairment did not increase the risk for severe walking limitation (30).
Only limited information exists on the influence of other impairments, for example, sensory impairments, on the mobility of older people. We observed that impairments in sensory functions, such as in vision and hearing, affect mobility decline in older people. Viljanen et al. (37) found in a 3-yr follow-up study that people with hearing impairment had twice the risk for new mobility limitation than people with intact hearing. Auditory information may be more important for safe outdoor mobility than traditionally considered. For example, hearing loss may hinder the ability to divide attention between traffic, discussion, maintaining postural balance, and walking, thus potentially increasing the risk for falls and other accidents. Sensory impairments may accelerate the process of mobility decline by restricting participation in out-of-home activities.
Kulmala et al. (8) found that people with coexisting vision and hearing impairments had more than fourfold risk and people with coexisting impairments in vision, hearing, and balance had almost 30-fold risk for falls compared with people with no vision impairment.
Falling and Its Consequences for Mobility
Falling and fall-related injuries are common among older people, often leading to a sudden and catastrophic disability. Approximately 20% to 40% of community-dwelling individuals older than 65 yr fall every year, and about one half of those who fall repeatedly do so (21). Known individual risk factors for falls include higher age and health-related issues such as gait problems, muscle weakness, dizziness, and other disease-related conditions. In addition, environmental factors play a major role in falls (21). Irrespective of related serious injuries, whether falls have a negative impact on mobility among older people has been little investigated in prospective studies. We suggest that falls also may lead to progressive mobility decline in the absence of consequent injury. In our study, older women with indoor falls were more than three times more likely to report new difficulties in walking 2 km by the end of the 3-yr follow-up compared with those with no falls (Table 1). Outdoor falls did not increase the risk for future mobility limitation (16). A significant proportion of the increased risk for mobility decline among those who sustained at least one indoor fall was caused by their higher baseline obesity, lower walking activity, and higher prevalence of chronic conditions. Among women who sustained indoor falls, were obese, and reported low walking activity, the risk for incident mobility limitation was 17-fold compared with women with none of the risk factors (16).
Preclinical Mobility Limitation
There is great variability in the progression of mobility limitations among older people. For some, it may be sudden and catastrophic; whereas for others, it may be slowly progressive (5). Sudden onset of mobility limitation usually is a result of a traumatic event, such as an injurious fall; whereas a slow progressive decline in mobility is a consequence of worsening health conditions, such as arthritis (5). If the decline in mobility slowly progresses, then in the early stages, before the onset of task difficulty, older people may be able to compensate for underlying impairments or physiological decrements by modifying their performance and thus maintain their everyday function without (any) strong perception of difficulty (1). For example, a person may reduce his or her walking pace or use a mobility aid to manage a certain walking distance, without perceiving any difficulty in doing so. This early stage of mobility decline has been termed preclinical mobility limitation and refers to a stage between good mobility and manifest mobility limitation (3,4,14) (Fig. 1).
Preclinical mobility limitation may be assessed by asking people who do not report task difficulty whether they have changed their way of doing the task (3,4,14). Our study showed that self-reported preclinical mobility limitation was associated with a decline in measured physical performance and that it was highly predictive of further mobility decline (14). Older adults who reported baseline preclinical mobility limitation had up to sixfold higher risk of progressing to major manifest mobility limitation during a 2-yr follow-up compared with participants with no limitation at baseline (Table 2) (14). Furthermore, our 12-month prospective fall surveillance suggested that preclinical mobility limitation combined with a fall history predicted future falls (15). These results, together with previous evidence by Fried et al. (3,4), indicate that self-reported preclinical mobility limitation is a useful measure for early identification of people at high risk for mobility decline, thereby offering an opportunity for early intervention.
PERSON-ENVIRONMENT INTERACTION IN THE PROGRESSION OF MOBILITY DECLINE
People with mobility limitations report more barriers in their environment than people without limitations. Examples of barriers often reported by older people include poor public transportation, discontinuous or uneven sidewalks, curbs, noise, heavy traffic, inadequate lighting, lack of resting places, sloping terrain, long distances to services, and weather conditions. However, most studies have been limited to cross-sectional analyses and have been unable to reveal whether people report environmental barriers because of their mobility limitation or whether environmental barriers accelerate mobility decline. Our recent study among community-dwelling older people showed that those who reported barriers to walking in the outdoor environment had approximately twofold risk for incident difficulty in 2 km and 0.5 km walking (Fig. 2) (24). In addition, fear of moving outdoors markedly increased the risk for incident walking difficulties (Fig. 3). Fear of moving outdoors is an example of a negative effect resulting from a perceived mismatch between environmental press and individual competence (9) and may lead to avoidance of outdoor activities (25). Fear-related avoidance of walking outdoors may, because of the consequences of physical inactivity, accelerate the progression of mobility decline (23).
PREVENTION OF MOBILITY DECLINE
Many different types of physical activity programs, ranging from simple home exercise programs to intensive highly supervised hospital- or center-based programs, have been used to improve mobility in older people. Although physical activity and exercise are promoted widely as effective means to enhance physical functioning among older people, it is less certain how these promising results can be adapted for use in everyday clinical practice. Physical activity counseling, where the participant is encouraged to exercise and provided with advice about possibilities to exercise, is one example of a low-cost educational intervention to promote physical activity (10). As many older adults use health care services regularly, physical activity counseling in primary health care settings may be an effective means of increasing physical activity and further slowing down the age-related deterioration in mobility. A randomized controlled trial of the effects of physical activity counseling on physical activity and mobility was performed in our study center from 2003 to 2006. The study protocol is described in detail elsewhere (10). Briefly, the participants were people aged 75 to 81 yr. The intervention included a single individual physical activity counseling session followed up with telephone contacts every 4 months for 2 yr. Data were collected in the laboratory at baseline and after 2 yr. During the intervention, intermediate changes were assessed by means of semiannual telephone interviews. In addition, postintervention telephone interviews were conducted semiannually for 1.5 yr. Thus, the total follow-up time was 3.5 yr.
During the intervention, the proportion of participants reporting difficulties in advanced (walking 2 km) and basic mobility (walking 0.5 km) increased in the intervention and control groups but significantly less in the intervention group (Fig. 4) (13). At the end of the 2-yr intervention, the treatment effect on walking 2 km was significant (odds ratio (OR), 0.84; 95% confidence interval (CI), 0.70–0.99), and the effect on walking 0.5 km was parallel but nonsignificant (OR, 0.87; 95% CI, 0.69–1.09). The positive effect of the intervention mainly was caused by the prevention of walking difficulty, rather than recovery from walking difficulty. For advanced mobility, the treatment effect remained significant (OR, 0.82; 95% CI, 0.68–0.99) at the postintervention 1.5-yr follow-up, whereas for basic mobility, the effect gradually disappeared (OR, 1.09; 95% CI, 0.87–1.37). At the 2-yr follow-up point, the number needed to treat for walking 2 km was 15. This indicates that to prevent one people from developing difficulty or to recover from baseline difficulty, 15 people had to receive counseling (13).
Mobility is important for maintaining community independence into old age. Aging changes, pathology, individual vulnerabilities, and environmental barriers increase the risk for mobility decline. Mobility decline may happen gradually for many years, or it may occur overnight because of a catastrophic event such as a hip fracture. To optimize the opportunities for good mobility in old age, a spectrum of actions should be considered, including both preventive and rehabilitative interventions. First of all, all individuals should have access to physical exercise. This is not yet self-evident for older people, as, for example, ageism, financial constraints, or physical barriers may prevent participation. Communities should be accessible, and neighborhoods should include features that facilitate mobility, that is, resting places or green areas. It is intuitive that removing barriers to mobility commonly experienced by older people, such as adding resting places and proper lighting to walking routes, would increase their opportunities for walking.
Attention should be paid to preventive interventions seeking to minimize the individual risk factors for mobility decline, such as obesity, sensory impairments, falls, or physical inactivity. Special interventions should target risk groups. For example, older people who are recovering from an injury or a disease should receive rehabilitation. In all, young and middle-aged people could prevent their future risk for mobility decline by aiming to increase their physiological reserve, whereas younger older people may slow down aging declines by being active, learning new mobility skills, and through good treatment of diseases. Among older old people, specific interventions, rehabilitation, and supportive environments assume greater importance.
This work was funded by the University of Jyväskylä.
The Gerontology Research Center is a joint effort between universities of Jyväskylä and Tampere.
The authors declare no conflicts of interest.
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