The Interaction between Mobility Status and Exercise Specificity in Older Adults

Clemens Markus Brahms; Tibor Hortobágyi; Reto Werner Kressig; Urs Granacher

Disclosures

Exerc Sport Sci Rev. 2021;49(1):15-22. 

In This Article

Abstract and Introduction

Abstract

Many adults older than 60 yr experience mobility limitations. Although physical exercise improves older adults' mobility, differences in baseline mobility produce large variations in individual responses to interventions, and these responses could further vary by the type and dose of exercise. Here, we propose an exercise prescription model for older adults based on their current mobility status.

Introduction

Mobility is associated with health and quality of life in old age. Although it has been broadly defined as the ability to move independently or with the use of assistive devices within one's environment, mobility is inherently linked to walking.[1,2] Mobility depends on an individual's physical, cognitive, and psychosocial resources and is influenced by environmental, biographical, and cultural factors.[3,4] Natural aging and sedentariness favor the evolution of mobility limitations, and consequently many older adults have difficulties walking and performing other activities of daily living.[5] Prevalence rates of mobility limitations vary with definition. For example, 59% and 43% of adults aged 60–74 yr in the United States and Europe, respectively, and as much as 75% of individuals 80 yr or older have mobility limitations, that is, difficulties walking for longer distances or climbing steps without resting.[3,6–8] Moreover, 40% of adults aged 60–74 yr and 65% of people aged 75 yr have at least one chronic disease such as heart disease, diabetes, or osteoarthritis, which increases the risk for future mobility limitations.[5] For the purpose of this narrative review, we will define mobility as the ability to walk independently within one's environment. We will further discuss mobility in the context of walking speed, as it correlates with or predicts other dimensions of mobility, such as static and dynamic balance, chair rise time, timed up and go test performance, and stair ascent and descent.[9–12] Furthermore, training-induced increases in walking speed are often accompanied by concomitant increases in other measures of mobility, and several large-cohort studies have demonstrated its predictive nature with respect to future health status, mobility limitations, and mortality.[13–17] Moreover, we define mobility limitations as objective difficulties to move oneself around by walking or as the need of equipment or personal assistance for walking.[2]

Physical activity, defined as movement generated by contracting skeletal muscle that leads to an increase in energy expenditure, can reduce common risk factors for mobility limitations, such as obesity, hypertension, and diabetes.[18,19] Similarly, exercise, a subset of physical activity characterized by planned, structured, and repetitive movement, can preserve or improve physical determinants of mobility, such as muscle strength, balance, and flexibility.[20] In light of this evidence, the World Health Organization (WHO) has recognized physical activity and exercise as fundamental strategies to improve older adults' mobility and health. To facilitate the translation of scientific results into practice, guidelines have been established for specific age groups. Accordingly, international health organizations such as the WHO and the American College of Sports Medicine recommend that all adults 65 yr or older perform either 75 min of vigorous-intensity physical activity (4.8–6.8 metabolic equivalent of task [MET], 76%–96% of maximum heart rate [HRmax]), 150 min of moderate-intensity physical activity (3.2–4.8 MET, 64%–76% HRmax) per week, or an equivalent combination thereof, as well as strength exercises on two or more days per week.[21,22] The guidelines further loosely address older adults with "poor mobility" and recommend that these individuals additionally perform balance exercises on three or more days per week.[21] Despite their effectiveness, universal exercise guidelines suggest that older individuals represent a homogeneous group of inactive yet "healthy" individuals. However, it is often overlooked that older adults' mobility varies greatly and overall diversity generally increases with age.[23]

In a landmark article based on his J.B. Wolffe Memorial Lecture, Haskell[24] proposed a model that assumes the magnitude of exercise-related health benefits changes as a function of baseline activity status. The model predicts that for any given increase in activity, larger effects will be observed in inactive individuals. In accordance with the law of diminishing returns, the effects will be blunted in moderately active individuals and barely detectable in highly active individuals. However, despite its relevance, the model is vague with respect to predictors as well as outcome measures, and it also does not discuss dose-response relations in the context of training specificity. We, therefore, adapted the model to address the hypothesis that differences in baseline mobility status cause substantial variation in the responsiveness to training, and these responses further vary by exercise type and dose (Figure 1). That is, the effects of resistance and aerobic training on mobility are inversely related to the severity of mobility limitations: the greater the mobility limitation, the greater the training-induced improvements in mobility.[25] In these low-baseline individuals, the interventions produced a dominant overall conditioning effect, which masks specific training adaptations. For example, even a specific gait retraining intervention originally designed for stroke patients may result in improved cardiovascular function in sedentary adults if their baseline function is low enough. Hortobágyi et al.[26] expressed this idea by stating that interventions intended to reduce specific dysfunctions will produce a general effect in healthy but sedentary individuals.

Figure 1.

Conceptual model of the principle of diminishing returns in a mobility context. The logarithmic growth of training adaptations is a function of improved mobility status. Δ represents the magnitude of exercise-induced mobility benefits as walking speed improves from 0.8 to 1.0 m·s−1 and from 1.0 to 1.2 m·s−1, respectively. Individuals with high mobility status have to invest more time to induce adaptations compared with individuals with low mobility status. Based on information from (24).

A reinterpretation of Haskell's model within a mobility framework further indicates that exercise programs with the main goal to promote physical activity will fail to generate an appropriate adaptive response in older adults with high baseline mobility status. In contrast, older adults with low mobility status benefit from these unspecific interventions and may even improve their mobility at least during the early stages of training.[27] To test these assumptions, accurate and meaningful information about an individual's current mobility status must be obtained. Since walking speed represents the integrative output of multiple body systems and functions, such as postural control, muscle strength, and aerobic capacity, its assessment could serve as a first step in the process of designing and implementing targeted exercise intervention for older adults.[28] As indicated in Figure 2, walking speed can predict future mobility limitations and overall health in aging adults and even outperforms more complex, multicomponent mobility scales, such as the Short Physical Performance Battery.[30] The literature has established cutoff values to cluster subpopulations of older adults according to their mobility status. For example, individuals with habitual walking speeds of >1.0 m·s−1 are generally at a low risk of experiencing future mobility limitations, whereas older adults who habitually walk at speeds of 0.6 m·s−1 represent a high-risk population.[13] Furthermore, a threshold of 0.8 m·s−1 has been successful in identifying older adults at increased risk for mobility limitations, frailty, hospitalization, and death.[31–33] Based on the white paper of Fritz and Lusardi,[28] we propose that older adults with walking speeds of <0.8 m·s−1 be classified as having low mobility status, whereas those with habitual walking speeds ranging from 0.8 to 1.4 m·s−1 and walking speeds of >1.4 m·s−1 be classified as having normal and high mobility status, respectively.

Figure 2.

Mobility throughout the lifespan. Visual representation demonstrating the effects of exercise on walking speed and its implications for mobility status. Based on information from (29).

This Perspective for Progress paper provides a synopsis of the efficacy of balance, strength, and power training to improve mobility in subpopulations of older adults that differ with respect to baseline mobility. We also propose a conceptual model that illustrates the adaptive potential to improve mobility as a function of an individual's baseline mobility status and argue that optimal exercise dose-response relations vary according to exercise type and baseline mobility. First, we discuss the general effects of exercise on mobility and its underlying physiological mechanisms.

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