Calf exercise-induced vasodilation is blunted in healthy older adults with increased walking performance fatigue
Introduction
Understanding the mechanisms that contribute to loss of physical function with age has significant implications for improving health and quality of life for older adults. Walking speed has proven to be an informative measure of physical function. Not only does it represent the ability of older adults to perform activities related to independent living (Verghese et al., 2011), but walking speed also predicts future disability and all-cause mortality (Newman et al., 2006, Studenski et al., 2011). Walking speed at a normal or fast pace decreases with advancing age (Ko et al., 2010). Outside of severe clinical conditions (e.g., stroke) where walking speed can fall abruptly, the vast majority of older adults experience a progressive decline in walking speed (Guralnik et al., 2001). This indicates that the underlying physiological deficits that impair walking ability manifest slowly. Older adults may compensate for these deficits by altering walking performance to maintain function (Rantakokko et al., 2013), which may present as an inability to sustain walking speed (i.e., fatigue) during an endurance walk test (Simonsick et al., 2014).
Increased fatigability during walking may occur if aged arteries have a reduced ability to deliver oxygen and nutrients to active muscle for energy utilization. Leg muscle oxidative capacity (e.g., capillary density, oxidative enzyme activity) is strongly associated with walking speed in normally active, non-endurance trained older adults (Coen et al., 2013, Nicklas et al., 2008). Muscle blood flow is related to the oxidative potential of muscle (Laughlin and Armstrong, 1985), which declines with age (Yu et al., 2007); thus, lower muscle blood flow during exercise may reflect reduced muscle oxidative capacity in older adults. However, aging is also associated with a decline in vascular function that manifests as a reduced ability of peripheral arteries to dilate (Celermajer et al., 1994) and increase blood flow (Westby et al., 2011). Indeed, lower muscle blood flow responses to leg exercise have been observed in healthy older adults as compared to younger adults (Donato et al., 2006, Proctor et al., 2003a). Interestingly, drug therapy in older adults to increase vascular function (arterial vasodilatory capacity) improves blood flow responses to leg exercise that result in greater muscle oxygen consumption (Chen et al., 1999) and faster walking speed (Cowley et al., 1990). Therefore, it is possible that age-related changes in muscle blood flow may be a physiological deficit that plays a role in walking performance fatigue in older adults.
The aim of this study was to determine if calf muscle blood flow and vasodilation are associated with walk test fatigability in healthy, normally active older adults. We hypothesized that older adults unable to sustain walking speed during a fast-paced 400 m walk test would have i) a lower ability to maximally dilate calf muscle vasculature, and/or ii) smaller muscle blood flow and vascular conductance responses to plantar-flexion exercise as compared to adults able to maintain walking speed.
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Participants
Forty-five older adults (25 women and 20 men) between the ages of 60 and 78 yrs provided written informed consent to participate in this study. These participants were screened to identify factors well known to alter vascular function independent of aging. Exclusion criteria included a personal history of physician-diagnosed cardiovascular disease, diabetes, and pulmonary disease. In addition, adults were not eligible to participate if taking medications for high blood pressure, cholesterol, or
Walking performance fatigue
Twelve adults (26%) slowed walking speed during the walk test while all other adults either sustained or increased walking speed. Adults that did not slow walking speed were categorized into one maintained walking speed group for comparison purposes. Characteristics for each group are shown in Table 1. Adults that slowed walking speed were of similar age, body fat, resting blood pressure, fasting blood glucose, and maximal grip strength as compared to the maintained walking speed group. In
Discussion
The major finding of this study is that leg blood flow and vascular conductance during calf muscle exercise is associated with walking performance fatigue in healthy community-dwelling older adults. Specifically, adults that slowed walking speed during a 400 m walk test had lower SFA blood flow and vascular conductance during plantar flexion exercise than adults that maintained walking speed. This observation was made after normalizing hemodynamics to calf lean mass indicating that adults with
Conclusion
Older adults that slow walking speed during an endurance walk performance test display a lower calf blood flow and vascular conductance response to exercise. The lower calf hemodynamic response to exercise suggests reduced blood flow support for bioenergetics during walking. Examination of the hemodynamic response revealed that the initial increase from rest to the first stage of calf exercise was linearly related to walking fatigability with a lower vascular response associated with greater
Conflict of interest
The authors have no perceived or real conflict of interest to disclose.
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