Mobility problems are pervasive in older adults. Mobility limitations affect personal independence, need for human help, and quality of life. Limited mobility predicts future health, function, and survival. Like other geriatric syndromes, mobility disorders are caused by diseases and impairments across many organ systems, so evaluation and management require multiple perspectives and disciplines. Health-care providers should be able to assess and treat mobility problems. They should be able to measure and interpret clinical indicators of mobility such as gait speed, the short physical performance battery, and the performance-oriented mobility assessment. They should know the physiological and biomechanical mechanisms underlying normal and abnormal mobility, the differential diagnosis of the causes of mobility disorders, and the approaches to management of mobility problems.

Defining Mobility

Mobility is the ability to move one’s own body through space. Mobility requires force production and feedback control systems to navigate the mass of the body through a three-dimensional environment. Walking is the fundamental mobility task for human life. Mobility also includes a wide range of other important activities that require moving one’s body, from turning over in bed to climbing stairs. Mobility tasks have an inherent hierarchical order based on the biomechanical and physiological demands made on the body. This orderedness is apparent in the developmental tasks of infancy and childhood, when mobility independence is first achieved. The simplest and first mobility task is turning over in bed, followed by sitting upright, transfers from lying to sitting and from sitting to standing, locomotion with an increased base of support (like crawling or using a walker), to independent two legged walking, then more challenging tasks like ascending and descending stairs, running, climbing ladders, and sports.

Mobility disability is best defined within a conceptual framework such as that of disablement (Table 115-1). Disability is caused by pathologic processes that lead to organ system impairments and functional limitations. Disability causes handicap by limiting life roles such as work or caregiving. Disability in mobility occurs at the level of the whole person and is manifested by the inability to carry out normal mobility activities like bathing or shopping. Mobility disability is caused by functional limitations in walking, transferring, or climbing stairs, which are, in turn, caused by problems with strength, endurance, coordination, balance, and range of motion. These functional limitations can be caused by numerous pathological processes. Disablement can be modified by psychological, social, and environmental factors. Mobility disability can precipitate a cycle of negative consequences because it often leads to decreased activity, which in turn worsens functional limitations and causes organ system deconditioning, including muscle weakness, loss of joint range of motion, and poor cardiovascular endurance.

Classification Methods for Mobility

Mobility classification is often driven by a tacit assumption of orderedness. No single current instrument assesses the full range of mobility from the lowest levels of rolling over in bed to the highest levels of endurance and coordination required for athletics or dance. Mobility assessment tools for older adults generally address one or more of the following three mobility levels: nonambulatory, ambulatory, and vigorous, corresponding broadly to Tinetti’s levels of frail, transitional, and vigorous mobility status. Mobility levels are surprisingly stable. While day-to-day variability does occur, people in general tend to remain in one level or decline very gradually unless a major event has occurred. Within the nonambulatory level, there are important mobility skills that affect independence in personal care activities, care needs, and demand for human help; these activities include bed mobility, self transfer skills, and wheelchair mobility. Within the vigorous mobility level, the degree of fitness, as represented by the ability to perform demanding or challenging mobility activities, may be a useful indicator of the extent of “successful aging” and “physiological reserve.” As an indicator of the extent of physiological reserve, vigorous mobility status may be a marker of ability to tolerate physiologic stressors such as acute illness, surgery, or periods of reduced activity.

Mobility can be assessed by self-report, professional observation, or direct measurement. Instruments to assess mobility from all three perspectives have been developed (Table 115-2). Each has advantages and disadvantages. These tools have been used to estimate the population incidence and prevalence of mobility disorders, predict the consequences of mobility problems, screen patient populations, determine care needs, and reimburse services in rehabilitation settings. More detailed instruments have been developed specifically for sorting out causes and mechanisms. Detailed instruments will be discussed separately in the section on causes of mobility limitations. Mobility measures have varying strengths and limitations, depending on characteristics such as reliability and validity, respondent burden, feasibility and convenience of use, and assessor skill required. Self-report measures are the easiest type of measure to obtain when gathering data from large populations. Self-report measures have high face validity in that they reflect the opinion of the person themselves. Since self-report measures usually ask about a period of time, such as recent weeks or months, these are capable of reflecting fluctuating ability over time. Self-report measures can be limited by problems with reliability, accuracy, and nonresponse. Because these are usually ordinal scales, they lack ability to discriminate small but important differences. Professional report measures reflect the opinion of an experienced assessor, can integrate over time, and may be more feasible when an individual is considered an unreliable informant or is unable to cooperate with testing. Professional report is limited by the need for assessor experience and training and can be vulnerable to problems with interassessor reliability unless extensive efforts to standardize assessment are made. Since professional reports are usually based on ordinal scales, ability to discriminate small but important differences might be limited. Mobility testing by measuring actual performance is somewhat more independent of opinion. Performance testing can produce very quantitative results, which discriminate small but important or subclinical differences. Performance measures are limited in that they require direct observation, subject cooperation, and standardization of instructions and procedures. These measure capability rather than actual daily mobility activities. The need for subject cooperation can lead to problems with nonresponse. Performance measures do not account well for short-term fluctuations over hours to weeks. Despite these limitations, performance testing may have direct applications in clinical settings because it can provide useful information and is brief and quantitative.

Psychological aspects of fear, mobility confidence, and activity avoidance can have great effect on mobility disability, separately or in combination with observable mobility limitations. Several instruments to assess psychological factors related to mobility disability have been developed (Table 115-3). Items from these scales might have use in the clinical setting.

Prevalence

The epidemiology of mobility can be considered from the perspective of basic or higher-level mobility. Examples of basic mobility problems are getting around inside the house and transfers from bed or chair. Examples of higher-level mobility problems are getting around outside the house, ability to walk one-quarter or one-half mile, and ability to climb stairs. Basic mobility problems are uncommon in community-dwelling older persons but are very frequent in institutionalized older people. Among community-dwelling persons aged 65 years and older, approximately 5% are dependent in getting in and out of a chair or bed and 7.5% are dependent in getting around inside. Among institutionalized persons older than 65 years, approximately 80% are dependent in getting in and out of a chair or bed and getting around inside. Mobility disability increases dramatically with age; dependence in getting around inside increases from 5% of persons aged 65 to 74 years to 30% of persons aged 85+ years. Women tend to have higher rates of mobility disability than men and nonwhites than whites.

Approximately 13% of Americans older than 60 years report higher-level mobility problems in that they have difficulty going outside the home alone. There is a marked increase with age. There is also geographic variation; self-reported difficulty going outside the home alone is more common among older adults in the Southern United States. Higher-level mobility problems, defined as difficulty walking a quarter mile or climbing stairs, increase with age, are more common in women than in men, and appear to be decreasing during recent decades.

Risk of Adverse Consequences Associated with Mobility Status

Mobility problems have serious consequences. Mobility status predicts mortality. Older people with difficulty walking 2 km (a little more than a mile) or climbing one flight of stairs are twice as likely to die during the next 8 years compared to those with no difficulty. Mortality risk is even higher among those who both have mobility difficulty and are physically inactive. Poor mobility performance, even in the absence of self-reported mobility limitations, is an independent predictor of death and nursing home placement. In community-dwelling older adults, baseline physical performance score was associated with a twofold increase of death. Among persons who report no mobility problems, gait speed less than 1.0 m/s has been associated with an increased risk of death compared to a gait speed of more than 1.0 m/s. In another community-based population, 9-year mortality for older persons with gait speeds less than 0.6 m/s was 78% compared to 48% in persons with gait speed between 0.6 and 1.0 m/s and was 26% in persons with gait speeds more than 1.0 m/s. A change in gait speed of approximately 0.1 m/s (equivalent to approximately 0.2 miles per hour) or a change of one point in the SPPB or 20 m in a 6-minute walk test appears to be clinically important. Older persons who have a 0.1 m/s decline in gait speed over 1 year have a doubled risk of dying during the subsequent 5 years. Older persons who have a 0.1 m/s improvement in gait speed over 1 year have a 40% decreased risk of dying in the following 8 years.

Poor mobility performance is an independent predictor of future self-care difficulty and mobility disability. Among community-dwelling persons older than 70 years with no baseline self-care disability and no higher-level self-reported difficulty in ability to walk one-half mile and climb stairs, baseline physical performance score was a powerful predictor of incident disability in both activities of daily living and in higher-level mobility disability. Mobility self-report and performance have been shown to predict disability and mortality in older populations from numerous countries and cultures, including Mexican American, British, Italian, French, Dutch, Spanish, Scandinavian, Australian, Japanese, and Chinese.

Poor mobility performance is also an independent predictor of hospitalization and nursing home placement. In a population-based study of older adults who reported no disability at baseline, poor baseline physical performance score was associated with a twofold increased risk of hospitalization and more days in the hospital over the following four years. The increased hospitalization risk was independent of baseline health status. The increased risk was mostly associated with hospitalization for geriatric conditions, such as dementia, pressure ulcer, hip fractures, other fracture, pneumonia, and dehydration.

Mobility may be part of an underlying constellation of core factors that link multiple outcomes associated with aging. Poor mobility as measured by timed chair stands is one of four factors proposed to be common risk factors for geriatric syndromes, including incontinence, falls, and functional decline. Conversely, good mobility, along with good cognition and nutritional status, is an independent predictor of recovery of independence after a period of disability. Abnormalities of gait and slow gait speed have been found to precede the onset of cognitive decline and dementia. Slow and abnormal gait is a risk factor especially for vascular dementia. Among older adults, simultaneous abnormalities of mobility, cognition, and mood are more common than would be expected by chance, perhaps implying potential common underlying causes.

Severe mobility disability, sometimes called immobility, has widespread and devastating consequences. It accelerates impairments in multiple organ systems, including bone, muscle, heart, circulation, lung, skin, blood, bowel, kidney, nutrition, and metabolism. Loss of organ system function can be rapid and severe; muscle strength can decline by 1% to 5% per day of enforced bed rest. Skin breakdown and pressure ulcer start to occur after only hours of persistent and unrelieved pressure. Major consequences of clinical significance include decreased plasma volume, orthostatic hypotension, accelerated loss of bone density, muscle weakness, decreased pulmonary ventilation, and constipation leading to fecal impaction. When even temporary bed rest is combined with the increased vulnerability of aging and acute illness, there is a marked increased risk of death, disability, and institutionalization.

The causes of mobility problems are complex. Unique and complementary etiologic perspectives can all contribute to a better understanding of mobility. Three perspectives are described here: epidemiological, biomechanical, and biomedical. Each has advantages and disadvantages for the clinician and the researcher.

Epidemiological Perspective: Risk Factors

Epidemiological risk factors for the onset of higher-level mobility disability include demographics, health behaviors, psychological factors, and diseases. Demographic factors associated with increased risk include advancing age, lower income, and lower educational level. Behavior-related risk factors include current smoking, alcohol abstention, low physical activity, high body mass index, and high waist circumference. Physical activity, a health behavior that is a key to mobility, is influenced by multiple psychological, social, and environmental factors. Common reasons given by older adults for limiting or avoiding physical activity include lack of an exercise companion, lack of interest, fatigue, fear of falling, weather, and safety. Self-reported conditions identified as major barriers to physical activity by older adults include arthritis and past injury. Psychological factors that influence mobility include negative attitudes toward aging, fear of falling, and poor emotional vitality. Self-reported conditions associated with increased risk of new higher-level mobility disability include baseline and incident heart attack and stroke, baseline hypertension, diabetes, angina, dyspnea, exertional leg pain and incident cancer, and hip fracture.

Biomechanical Perspective: Direct Assessment of the Body in Motion

Age affects the biomechanics of walking. Normal gait can be defined in terms of the gait cycle with two main phases: stance and swing (Figure 115-1). A normal cycle begins with a push off from the forefoot, then a swing through and heel strike, timed tightly to be followed by the push off of the other leg. Normal gait has highly characteristic patterns of foot, ankle, knee, hip, pelvis, trunk, and arm motion. Gait biomechanics can also be viewed from the perspective of the pattern of steps (footprints) (Figure 115-2). Step length is the forward distance between two foot falls. Stride length is the distance covered by one foot until it falls again. Stride length is, therefore, twice the step length, assuming the step lengths are the same on both sides. Step width is the lateral distance between two foot falls. With age, gait speed slows, step length decreases, and the proportion of the gait cycle when both feet are in contact with the ground (double support time) increases. During gait, older people compared to young adults tend to have more thoracic kyphosis, more anterior pelvic tilt, increased hip flexion, and greater external rotation of the foot. Older people tend to generate less power about the ankle and use hip flexion to compensate more than young adults. Normal gait has a very regular spatial and temporal pattern. An irregular gait can be either regularly irregular, like a limp, or irregularly irregular, with no pattern at all (Figure 115-3). Irregularly irregular gait, often called gait variability, predicts falls and mobility disability.

Normal walking maximizes energy efficiency. When walking changes owing to biomechanical alterations caused by disease or aging, walking becomes more energy demanding. Normal walking also requires excellent control of balance and timing. When problems develop with balance and timing, the priority for safe walking may be to increase stability and support at the expense of losses of energy efficiency. Thus, many changes with aging increase the energy cost of walking and decrease gait efficiency.