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Resistance Exercise Training

2006; Lippincott Williams & Wilkins; Volume: 113; Issue: 22 Linguagem: Inglês

10.1161/circulationaha.105.584060

1524-4539

Randy W. Braith, Kerry J. Stewart,

Muscle metabolism and nutrition

HomeCirculationVol. 113, No. 22Resistance Exercise Training Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessReview ArticlePDF/EPUBResistance Exercise TrainingIts Role in the Prevention of Cardiovascular Disease Randy W. Braith, PhD and Kerry J. Stewart, EdD Randy W. BraithRandy W. Braith From the Center for Exercise Science, College of Health and Human Performance and the College of Medicine, University of Florida, Gainesville (R.W.B.); and the Division of Cardiology, Johns Hopkins School of Medicine, Baltimore, Md (K.J.S.). and Kerry J. StewartKerry J. Stewart From the Center for Exercise Science, College of Health and Human Performance and the College of Medicine, University of Florida, Gainesville (R.W.B.); and the Division of Cardiology, Johns Hopkins School of Medicine, Baltimore, Md (K.J.S.). Originally published6 Jun 2006https://doi.org/10.1161/CIRCULATIONAHA.105.584060Circulation. 2006;113:2642–2650The metabolic effects of reduced muscle mass, engendered by normal aging or decreased physical activity, lead to a high prevalence of obesity, insulin resistance, type 2 diabetes, dyslipidemia, and hypertension.1–4 These risk factors are associated with abnormalities in cardiovascular structure and function such as arterial stiffness and impaired endothelial function. Skeletal muscle is the primary metabolic "sink" for glucose and triglyceride disposal and is an important determinant of resting metabolic rate. Accordingly, it has been hypothesized that resistance exercise training (RT) and subsequent increases in muscle mass may reduce multiple cardiovascular (CV) disease risk factors.5–8 The inclusion of RT as part of an exercise program for promoting health and preventing disease has been endorsed by the American Heart Association,9 American College of Sports Medicine,10 and the American Diabetes Association11 as an integral part of an overall health and fitness program. Cross-sectional studies have shown that muscular strength is inversely associated with all-cause mortality12 and the prevalence of metabolic syndrome,13,14 independent of cardiorespiratory fitness levels. To date, however, the evidence that RT reduces CV risk factors remains equivocal.This review will critically evaluate whether RT modifies CV risk factors and improves characteristics of CV structure and function. The topics will be limited to the effects of RT on major and independent risk factors for CV disease including diabetes mellitus, hypertension, dyslipidemia, and advancing age.4 The quantitative relation between these risk factors and CV events has been elucidated by the Framingham Heart Study4 and other studies. The topics will also include 2 predisposing risk factors—obesity and physical inactivity—that are designated as major risk factors by the American Heart Association.1,2,4 To the extent possible, this review will examine the separate and independent effects of RT in studies that did not include a concomitant aerobic exercise component. However, in those instances where the data from RT studies are equivocal, studies that combined RT and aerobic exercise will be acknowledged to help the clinician formulate recommendations for their patients. Additionally, the review will focus mainly on primary prevention, for example, risk reduction in persons without established CV disease. Many low- to moderate-risk patients with established CV disease should be encouraged to incorporate RT into their physical conditioning program, especially those who rely on their upper extremities for work or recreational pursuits. However, the safety and effectiveness of RT in other populations of CV patients (eg, women, older patients with low aerobic fitness, patients with severe left ventricular dysfunction) have not been well studied. Accordingly, these patient subsets may require more careful evaluation and initial monitoring, and RT guidelines and recommendations must be modified accordingly. Moreover, there is only a limited body of literature assessing the independent benefits of RT on CV risk factors in patients with established CV disease. Studies conducted in cooperation with comprehensive cardiac rehabilitation programs typically include the confounding influences of aerobic activity, initiation of vasoactive and lipid-lowering drugs, and nutritional education with subsequent dietary modifications.Rationale for Resistance TrainingThere is overwhelming research evidence that RT prevents decline in skeletal muscle mass and function when the mechanical stimuli provided by tasks of daily living are not sufficient to offset these declines with aging.15–17 Adults who do not perform regular RT lose approximately 0.46 kg of muscle per annum from the fifth decade on.18 Furthermore, adults who do not perform RT experience a 50% reduction in type 2 muscle fibers, the fibers responsible for high levels of strength, by age 80 years.19 The profound beneficial effects of RT on the musculoskeletal system can contribute to the maintenance of functional abilities and prevent osteoporosis, sarcopenia, and accompanying falls, fractures, and disabilities.15,17,18 A comprehensive comparison of the chronic effects of RT versus aerobic exercise training in multiple organ systems is presented in Table 1. TABLE 1. Comparison of the Effects of Aerobic Training to Resistance Training on Health and Fitness VariablesVariableAerobic ExerciseResistance Exercise↑ indicates increased; ↓, decreased; and ↔, negligible effect.Bone mineral density↑↑↑↑Body composition Fat mass↓↓↓ Muscle mass↔↑↑Strength↔↑↑↑Glucose metabolism Insulin response to glucose challenge↓↓↓↓ Basal insulin levels↓↓ Insulin sensitivity↑↑↑↑Serum lipids High-density lipoprotein↑↔↑↔ Low-density lipoprotein↓↔↓↔Resting heart rate↓↓↔Blood pressure at rest Systolic↓↓↓ Diastolic↓↓↓Physical endurance↑↑↑↑↑Basal metabolism↑↑↑Long-term adaptation to RT lowers cortisol response to acute stress,20 increases total energy expenditure and physical activity in healthy18,21 and frail older adults,22 and relieves anxiety, depression, and insomnia in clinical depression.23 RT has beneficial effects on bone density,18,24 osteoarthritic symptoms,22,25 hypertension,26,27 lipid profiles,28 and exercise tolerance in coronary artery disease.29 Conversely, the loss of skeletal muscle mass and contractile function that accompanies aging, for example, sarcopenia, is linked to peripheral insulin resistance, dyslipidemia, and increased adiposity.30 Sarcopenia is also accelerated in patients with chronic heart failure, a condition characterized by peripheral skeletal muscle abnormalities and muscle wasting. Several studies have shown beneficial effects of RT on muscle mass and strength in patients with chronic heart failure.31,32 However, the relative mitigating effects of RT on primary and secondary CV risk factors remains controversial.Resistance Training and DiabetesDiabetes mellitus, glucose intolerance, and insulin resistance are central features of coronary artery disease risk, being strongly related to hypertension and dyslipidemia, proinflammatory markers, thrombogenic factors, and endothelial dysfunction. These abnormalities increase with age and represent the early stages of CV disease that precede the clinical manifestations of CV disease.33 Maintaining good glycemic control hinges on enhancing insulin availability or secretion and overcoming insulin resistance. Unfortunately, central obesity and physical inactivity hinder medical management and may hasten development of chronic complications, particularly in elderly people with long-standing diabetes. Even when glycemic control is near optimal with medication, reducing insulin resistance by any other means must be explored in view of these adverse consequences.Muscle contraction increases glucose uptake in skeletal muscle,34 thereby forming the basis for recommending RT for individuals with abnormal glucose metabolism. Aerobic exercise uses large muscle groups for extended periods of time, but whole-body RT programs may provide equally high or higher recruitment of muscle mass over a comparable period of time. The American College of Sports Medicine has recommended the use of progressive RT as part of a well-rounded exercise program for individuals with type 2 diabetes.35 Similarly, in the absence of contraindications, the American Diabetes Association also recommends RT for those with type 2 diabetes.11 These recommendations are supported by evidence that RT is an integral component in the therapeutic management of glycemic control in both young and older type 2 diabetics,28,36–38 particularly if the RT is performed in a supervised versus home-based program.39Glucose Tolerance and Insulin SensitivityA frequent postulate is that RT and subsequent increases in skeletal muscle mass may improve glucose and insulin responses to a glucose load.6,40 However, there are little data available showing that RT prevents type 2 diabetes. RT does not usually alter glucose tolerance or glycemic control regardless of age,5–7,16,41,42 unless baseline glucose tolerance is abnormal.36,38,40–44 Nevertheless, RT reduces acute insulin responses during an oral glucose tolerance test in healthy young, middle-aged, and older men in most studies.5,6,45 RT also reduces acute insulin responses during glucose tolerance testing in diabetic men36,38 and women36,38,44 and improves insulin sensitivity during hyperglycemic and hyperinsulinemic-euglycemic clamps in diabetic and/or insulin-resistant middle-aged6,46,47 and older men42 and diabetic middle-aged48 and older women.42Glycemic ControlRT decreases glycosylated hemoglobin (HbA1c) levels in diabetic men28,36,37,43 and women,28,36,37 regardless of age. This effect is observed even in the absence of a lasting effect of RT on fasting glucose or insulin levels.37 Improved glycemic control and decreased HbA1c levels are important for reducing the microvascular and macrovascular complications of diabetes. For example, the UK Prospective Diabetes Study49 reported that each percentage point reduction in HbA1c was associated with a 35% reduction in microvascular complications, whereas the European Prospective Investigation of Cancer and Nutrition (EPIC)–Norfolk prospective population study showed that an increase of 1 percentage point in HbA1c was associated with a 28% increase in mortality risk, independent of other CV risk factors.50 RT-induced improvements in glycemic control, however, appear to be intensity-dependent, with beneficial effects occurring when subjects train at 70% to 90% of the 1-repetition maximum strength (1-RM). One-repetition maximum is the maximal weight lifted in 1 attempt during strength testing. In studies where the RT intervention was less than 2 months and/or the exercise intensity was less than 50% of the 1-RM, improvements in HbA1c were modest or undetected.47,51,52Resistance Training and HypertensionAdopting a healthy lifestyle is critical for the prevention of high blood pressure (BP) and is an indispensable part of the treatment of hypertension.53 The American Heart Association9 and the American College of Sports Medicine10 have each endorsed moderate-intensity RT as a complement to aerobic exercise programs in the prevention, treatment, and control of hypertension.Systemic Blood PressureThe rationale for RT as an adjunct to aerobic exercise for controlling BP stems from multiple studies. Two meta-analyses of RT and hypertension are noteworthy.26,27 Inclusion criteria, consistent across both reviews, were (1) inclusion of a randomized nonexercise control group; (2) RT as the only intervention; (3) training for a minimum of 4 weeks; and (4) participants who were sedentary normotensive and/or hypertensive adults with no other concomitant disease. Kelley and Kelley27 examined the effects of RT on resting BP in studies published between January 1966 and December 1998. A total of 11 studies met the inclusion criteria and represented initial and final BP assessments in 182 RT subjects and 138 control subjects. Decreases (P≤0.05) of approximately 3 mm Hg were found for both systolic and diastolic BP across all BP categories as the result of RT. These changes represented a 2% decrease for resting systolic BP and 4% for resting diastolic BP. No differences were found for changes in resting BP between studies that used conventional RT compared with a circuit RT protocol. A conventional RT protocol generally consists of lifting heavier weights with longer rest periods, whereas a circuit RT protocol consists of lifting lighter weights with shorter rest periods between exercises. By moving quickly between exercises and by using lighter weight with higher repetitions, circuit training introduces an aerobic component to the workout.54In the more recent meta-analysis, Cornelissen and Fagard26 pooled data from studies published between 1996 and 2003 that included 9 randomized controlled trials involving 341 participants. The overall effect of RT was a decrease of 3.2 mm Hg (P=0.10) in systolic BP and a decrease of 3.5 mm Hg (P≤0.05) in diastolic BP. Results from these meta-analyses are consistent with conclusions generated by narrative reviews.17,55,56 Although these reductions seem modest, a systolic BP reduction of 3 mm Hg in average populations has been estimated to reduce cardiac morbidity by 5% to 9%, stroke by 8% to 14%, and all-cause mortality by 4%.57 The lack of data on the effects of RT on ambulatory BP warrants further investigation because this may be more indicative of future CV disease morbidity and mortality.58Control of BP is even more important in individuals who already have hypertension. Although there is general agreement that endurance training lowers resting BP in patients with mild to severe hypertension,27,57 there is a paucity of data on the effects of RT alone on BP in individuals with hypertension. Only 20% of the outcomes in the 2 meta-analysis reviews were based on a mean initial resting systolic BP >140 mm Hg, whereas only 13% had a mean initial resting diastolic BP >90 mm Hg.26,27 One study that used RT in combination with aerobic exercise in middle-aged hypertensive men for 10 weeks demonstrated reductions of 13 mm Hg for both systolic and diastolic BP.59 Conversely, a similar program for 6 months in older adults with hypertension showed mean decreases in systolic and diastolic BP of 5.3 and 3.7 mm Hg, respectively.60 The change in systolic BP, although significantly lower than study entry, was not different from values in the control group. Although any reduction in BP is desirable, the available studies do not answer the question regarding the independent benefit of RT in persons initially classified as being hypertensive or prehypertensive.Arterial StiffnessWith aging, hypertension, insulin resistance, and diabetes, there is increased arterial stiffness from degeneration of the arterial media, increased collagen and calcium content, and arterial dilation and hypertrophy. These factors lead to increased systolic BP and an increased risk of cardiac events.61 Several studies have shown that aerobic exercise is associated with reduced arterial stiffness in healthy subjects of all ages,62 competitive aerobic athletes,63 patients with coronary artery disease,64 and hemodialysis patients.65 Moreover, when aerobic exercise is combined with RT, there is no evidence of increased arterial stiffness.66 However, less is known about the independent effects or RT on arterial stiffness. Two cross-sectional studies have suggested that young and middle-aged men who participate in regular RT have greater arterial stiffness than age-matched sedentary control subjects.67,68 However, only 3 interventional studies have examined the effect of RT on arterial function.69–71 Miyachi et al70 reported that RT 3 days per week for 4 months decreased carotid arterial compliance by 19% (P<0.05) in young, healthy men who were novice weight trainers. Interestingly, carotid arterial compliance returned to baseline values within 2 months after RT was discontinued. Cortez-Cooper et al69 reported that high intensity RT for 4 days per week for 3 months in young, healthy women who were novice weight trainers (n=23; age 29±1 years, mean±SD) increased carotid augmentation index (a measure of arterial wave reflection and arterial stiffness) from −8±13% to 1±18% (P 4 hours per week maintained this reduced risk when confounding variables were controlled. In contrast, Kohl et al97 studied 1193 women and 5460 men and reported no significant association between muscle strength and total or low-density lipoprotein cholesterol for either gender. However, there was a direct association between both upper and lower body strength and triglyceride levels in men.Most interventional studies have failed to adequately control for normal variations in lipoproteins and lacked proper dietary controls and/or lacked statistical power. When these factors are controlled, most studies show no improvement in lipid profiles after RT in either middle-aged or older adults.40,87,98,99 However, the lack of significant lipoprotein-lipid changes with RT may be due to the fact that total cholesterol values for most study groups has been ≤200 mg/dL at study entry. Individuals with normal lipoprotein-lipid profiles may require greater exercise stimulus and energy expenditure coupled with significant reductions in body weight to further improve lipid profiles. Alternatively, Shoup and Durstine100 postulated that for changes in blood lipids to occur, lipoprotein lipase levels need to increase or remain elevated postexercise, or associated hepatic lipase must be suppressed to reduce conversion of high-density lipoprotein cholesterol (HDL-C) subfraction HDL2-C to HDL3-C. It is possible that the RT stimulus in most studies does not generate these cellular changes.Screening and PrecautionsThe risk-to-benefit ratio of RT is highly favorable for most healthy individuals. The hemodynamic response to aerobic exercise is an increase in heart rate, with a progressive rise in systolic BP with little or no change in diastolic BP, and a widening of the pulse pressure.9 These responses result primarily in a volume load on the heart. Conversely, RT causes a marked rise in both systolic and diastolic BP and consequently mean blood pressures, with less of a rise in heart rate compared with aerobic exercise.101 Thus, RT imposes primarily a pressure load on the heart. Because of the considerably lower response of heart rate during RT, this mode of exercise generally results in a lower rate-pressure product compared with aerobic exercise. Among the many studies of RT in healthy adults, there have been no reported cardiovascular complications. The American College of Sports Medicine95 and the American Heart Association9 indicate that the contraindications to RT are similar to those for endurance exercise. Thus, the same screening criteria used for healthy adults before participation in endurance exercise would apply.Selected individuals should consult their healthcare practitioners before beginning a vigorous exercise program. For those at moderate risk or higher, such as men age 45 years and over, women age 55 years and over, those with major risk factors for arteriosclerosis, and those with diabetes at any age, the American College of Sports Medicine recommends a medical history and physical examination including an exercise stress test before initiating a vigorous exercise program.95 Because of the marked rise in BP with RT, those with uncontrolled hypertension (systolic BP ≥160 mm Hg and/or diastolic BP ≥100 mm Hg) should be controlled to lower levels before starting an exercise program.102 High-intensity RT should also be avoided by individuals who have active proliferative retinopathy or moderate or worse nonproliferative diabetic retinopathy. To minimize excessive BP responses, individuals should be told to avoid extended breath-holding during their workouts.35,80,103Resistance Training Exercise PrescriptionA summary of RT guidelines is presented in Table 2. RT of all major muscle groups can be accomplished through the use of expedient programs. Indeed, adherence rates in RT interventional studies are high and due, in part, to the minimal time requirement for full participation. Most studies report that RT 3 days per week elicits superior strength gains when compared with training regimens of lower frequency.77,104 However, if training intensity remains high (7 to 10 repetitions performed to momentary muscular failure), RT only 2 days per week produces approximately 80% of the strength benefits reported by studies using traditional 3 days-per-week routines (Figure).104 Using scientific evidence and expert opinion, the American Heart Association, with endorsement of the American College of Sports Medicine, has promulgated RT guidelines for individuals with and without CV disease.9,95 The guidelin