Exercise

Evidence for impaired skeletal muscle contraction-induced rapid vasodilation in aging humans

We tested the hypothesis that aging is associated with an impaired contraction-induced rapid vasodilation in healthy adults. We reasoned that employing single contractions of a small muscle mass would allow us to isolate the local rapid vasodilatory responses independent of systemic hemodynamic and sympathetic neural influences on forearm hemodynamics. We measured forearm blood flow (Doppler ultrasound) and arterial blood pressure (Finapres) on a beat-by-beat basis and calculated the changes in forearm vascular conductance (ΔFVC) in response to forearm contractions in 18 young (24 ± 1 yr) and 13 older (62 ± 2 yr) healthy subjects. Single, 1-s dynamic forearm contractions were performed with the experimental arm slightly above heart level at 5, 10, 20, and 40% of the subjects' maximal voluntary contraction (MVC) in random order. In general, muscle contractions evoked a rapid increase in FVC that reached a peak within approximately four to five cardiac cycles postcontraction in both age groups. At 5% MVC, there were no significant age-related differences in contraction-induced forearm vasodilation. However, the peak vasodilatory responses were impaired ∼40–45% in older adults at 10, 20, and 40% MVC, as were the total vasodilatory responses (area under curve ∼40–50%; all P < 0.05). Additionally, the immediate vasodilation (first cardiac cycle postcontraction) for the 20% and 40% MVC trials was also impaired ∼50% with age ( P < 0.05). There were no significant age-group differences in MVC or forearm fat-free mass, and these variables were not correlated with local vasodilation within a given exercise intensity. Under the experimental conditions employed, the blunted responses with age reflect impaired local contraction-induced rapid vasodilation.

Alpha-adrenergic control of skeletal muscle circulation at rest and during exercise in aging humans

Aging is associated with many changes in autonomic nervous system function that often lead to impairments in the normal ability to respond to physiological stressors commonly encountered in daily life. In addition, many of these chronic age-related changes in autonomic-circulatory function can potentially predispose the older adult to elevated risk for acute and chronic cardiovascular complications. One of the most pronounced and repeatable findings with respect to changes in the autonomic nervous system with human aging is the progressive increase in basal muscle sympathetic nerve activity (MSNA) directed to skeletal muscle vascular beds. Although the mechanism(s) underlying this sustained age-associated increase in MSNA are not completely understood, several changes in sympathetic a-adrenergic function occur with age. In this review, the authors discuss how aging affects (1) a-adrenergic control of skeletal muscle vascular tone under resting conditions and the differences that exist in this control of the upper and lower limbs (forearm vs leg circulation); (2) vasoconstrictor responsiveness to endogenous norepinephrine release, as well as the specific responsiveness of postjunctional a(1)- and a(2)-adrenergic receptors; and (3) sympathetic a-adrenergic control of muscle blood flow and vascular tone during exercise in humans. Further, they discuss how these changes in sympathetic a-adrenergic control of skeletal muscle blood vessels have important physiological and clinical implications for the aging human.

Abdominal adiposity is a stronger predictor of insulin resistance than fitness among 50-95 year olds

OBJECTIVE

Physical inactivity and increased adiposity contribute to insulin resistance; less is known, however, about the relative contributions of these factors in older adults. The aim of this study was to determine whether cardiovascular fitness, whole-body adiposity, or abdominal adiposity is the strongest predictor of insulin resistance into old age.

RESEARCH DESIGN AND METHODS

Subjects included 407 men and women aged 50-95 years (means +/- SD 69 +/- 11 years). Insulin resistance was estimated using the insulin sensitivity index (ISI) of Matsuda and DeFronzo [ISI = 10,000/square root of (fasting glucose x fasting insulin) x (mean glucose x mean insulin during an oral glucose tolerance test); lower ISI = greater insulin resistance]. Fitness was determined with a treadmill maximal oxygen consumption (Vo(2max)) test. Whole-body adiposity measures included BMI and percent fat by dual-energy X-ray absorptiometry or hydrodensitometry; abdominal adiposity was estimated by waist circumference.

RESULTS

Waist circumference was the strongest independent correlate of ISI (r = -0.52, P < 0.0001), explaining 28% of the variance when controlling for sex, BMI, percent fat, and Vo(2max). BMI (r = -0.45), percent fat (r = -0.40), and Vo(2max) (r = 0.22) independently predicted ISI (all P < 0.0001); however, after controlling for waist circumference, only Vo(2max) remained significant (r = 0.13, P = 0.009).

CONCLUSIONS

Adiposity and fitness continue to be significant predictors of insulin sensitivity into old age, with abdominal obesity being the most important single factor. These findings support the measurement of waist circumference to assess health risk among older adults.

Aerobic power and insulin action improve in response to endurance exercise training in healthy 77-87 yr olds

Previous studies have demonstrated that frail octogenarians have an attenuated capacity for cardiovascular adaptations to endurance exercise training. In the present study, we determined the magnitude of cardiovascular and metabolic adaptations to high-intensity endurance exercise training in healthy, nonfrail elderly subjects. Ten subjects [8 men, 2 women, 80.3 yr (SD2.5)] completed 10-12 mo (108 exercise sessions) of a supervised endurance exercise training program consisting of 2.5 sessions/wk (SD 0.2), 58 min/session (SD 6), at an intensity of 83% (SD 5) of peak heart rate. Primary outcomes were maximal attainable aerobic power [peak aerobic capacity (Vo(2peak))]; serum lipids, oral glucose tolerance, and insulin action during a hyperglycemic clamp; body composition by dual-energy X-ray absorptiometry, and energy expenditure using doubly labeled water and indirect calorimetry. The training program resulted in an increase in Vo(2peak) of 15% (SD 7) [22.9 (SD 3.3) to 26.2 ml.kg(-1).min(-1) (SD 4.0); P < 0.0001]. Favorable lipid changes included reductions in total cholesterol (-8%; P = 0.002) and LDL cholesterol (-10%; P = 0.003), with no significant change in HDL cholesterol or triglycerides. Insulin action improved, as evidenced by a 29% increase in glucose disposal rate relative to insulin concentration during the hyperglycemic clamp. Fat mass decreased by 1.8 kg (SD 1.4) (P = 0.003); lean mass did not change. Total energy expenditure increased by 400 kcal/day because of an increase in physical activity. No change occurred in resting metabolism. In summary, healthy nonfrail octogenarians can adapt to high-intensity endurance exercise training with improvements in aerobic power, insulin action, and serum lipid and lipoprotein risk factors for coronary heart disease; however, the adaptations in aerobic power and insulin action are attenuated compared with middle-aged individuals.

Chronic sympathetic activation: consequence and cause of age-associated obesity?

Primary aging in adult humans is associated with a progressive, tonic activation of the peripheral sympathetic nervous system (SNS). The purpose of this SNS activation and its physiological impact are, however, unknown. We hypothesize that the chronic stimulation of the SNS with aging is driven in part by a progressive accumulation of body fat. This "error" is sensed by the central nervous system via increases in adiposity-sensitive humoral signals (e.g., leptin, insulin) that cross the blood-brain barrier, activate subcortical areas involved in the regulation of energy balance (e.g., ventromedial hypothalamus), and stimulate SNS outflow to peripheral tissues. The SNS activation is intended to increase beta-adrenergic thermogenesis in order to expend excess energy as heat rather than by storage of fat. Recent evidence, however, indicates that these adjustments are not effective in augmenting energy expenditure with aging. Indeed, older sedentary adults demonstrate reduced, not increased, beta-adrenergic stimulation of metabolic rate because of reduced tissue responsiveness, presumably mediated by SNS-induced impairment of beta-adrenergic signaling. As a result, age-associated SNS activation, initiated as a consequence of accumulating adiposity with the intent of preventing further fat storage, ironically, may in time evolve into a potential mechanism contributing to the development of obesity with aging.