Moderate or Vigorous Intensity Exercise: Which Is Better for Improving Aerobic Fitness?

Abstract

Landmark early studies of physical activity during work or leisure, such as the Harvard alumni study, found that weekly energy expenditure was protective of heart disease in a dose-response manner.1 As a consequence, public pronouncements on health have recommended that the general population engage in regular physical activity. Most notably, the Surgeon General recommended, in 1996, that “people of all ages [should] include a minimum of 30 minutes of physical activity of moderate intensity (such as brisk walking) on most, if not all, days of the week.”2 While many epidemiological studies used physical activity as the independent variable in assessing the risk of disease, other studies have used aerobic capacity, i.e., the individual's maximal oxygen consumption (VO2max) determined during a graded exercise test. These studies have found that higher levels of VO2max are associated with reduced risk of disease or mortality.3, 4 Of course individuals who engage in regular physical activity would be expected to have higher aerobic capacities, however, engaging in physical activity at a low intensity might not increase aerobic capacity. Therefore, it would be important to know whether accumulating physical activity or increasing aerobic fitness is more beneficial so that exercise recommendations can properly target one or the other. The question of fitness vs. activity has been controversial for several years. In 1989, Blair et al.3 reported a large drop in all-cause mortality between the least fit quintile and the next higher fitness group, prompting the conclusion that only a modest amount of physical activity was needed to reap the majority of the benefit, even though mortality continued to decrease across the higher fitness quintiles. Other studies have since confirmed that mortality declines across a wide range of fitness levels.4, 5 Of particular importance is the recent meta-analysis of Williams,5 who independently examined both activity and fitness in relation to cardiovascular disease. As illustrated in Figure 1, he found a 64% decline in disease risk from the least to most fit, but only a 30% decline from the least to most active. While there may be a hereditary component to aerobic fitness, Blair et al. reported that an increase in aerobic fitness decreases one's risk of all-cause mortality. For each 1-minute increase in their treadmill test times (approx. 1.4 mL•min−18•kg−1 increase in VO2max), men reduced their risk of mortality by 8%.6 The reduction in risk of cardiovascular disease is twice as great across the range of aerobic capacity than the range of physical activity. Adapted from Med Sci Sports Exerc. 2001;33:754–761.5 Given the importance of aerobic capacity, recommendations for increasing physical activity should consider how best to increase VO2max. Exercise prescriptions can be formulated using the FITT principle: Frequency, Intensity, Time (duration of exercise), and Type (mode of exercise). The American College of Sports Medicine (ACSM) recommends a minimum frequency of 3–5 times per week, an intensity of 50%–85% of VO2 reserve (VO2R, the difference between resting and maximum oxygen uptake) for most individuals, but as low as 40% VO2R for very low fit clients, a duration of 20–60 min/session, and exercises that employ a large muscle mass in a continuous, rhythmic fashion, such as walking, jogging, bicycling, etc.7 Of these variables, the one of most concern for improving VO2max is intensity. To prescribe an intensity at a particular percentage of VO2R, one calculates a target heart rate at the same percentage of heart rate (HR) reserve (for example, 50% of HR reserve for a client with a resting HR of 70 bpm and maximum HR of 180 bpm would be: 0.5(180–70) + 70=125 bpm). The ACSM considers 50% of VO2R to be a threshold intensity for most clients and 40% to be the threshold for those with very low fitness.8 Exercising at an intensity below these values would not be expected to increase VO2max. A recent analysis of training studies by Swain and Franklin9 considered these values, and recommended lowering them to 45% VO2R for clients with VO2max≥40 mL•min−1•kg−1 and 30% VO2R for those below that level. While these are the lower limits for improving VO2max, what intensity is optimal for this purpose? Above-threshold intensities may produce larger improvements in VO2max. Of course, exercising at a higher intensity for a given period increases the total amount of work performed. To be certain that the intensity itself, rather than the volume of exercise, is responsible for greater improvements in VO2max, studies must control the volume of exercise by increasing the exercise duration of subjects who exercise at lower intensities, so that the same number of calories is expended as in the higher intensity group. In Swain and Franklin's review,9 18 studies of aerobic training utilized a total of 37 different exercise groups. Ten of the studies controlled the total amount of work between groups, and all found a numerically greater increase in VO2max in the higher intensity group, with three studies reaching statistically significant differences. It was concluded that the low sample size in most studies precluded more significant findings; the preponderance of evidence favored greater increases in VO2max from higher intensities of exercise. Wenger and Bell,10 in an earlier review, also concluded that higher exercise intensities elicit greater improvements in VO2max. Higher intensities of exercise may be especially important in improving the VO2max of individuals who are already relatively fit. In training, an overload can be defined as a stress that is greater than what is normally encountered. Thus active individuals can expect to increase their VO2max only by further increasing exercise intensity. To test for such an interaction between exercise intensity and baseline fitness, a reanalysis of the studies in Swain and Franklin's review is performed here. All 37 exercise groups were stratified based on the mean initial VO2max of the subjects as low (<30 mL•min−1•kg−1), average (30–39 mL•min−1•kg−1) or high (≥40 mL•min−1•kg−1) fitness. Then the exercise intensities used in the studies were stratified using the ACSM categories of light (<40% VO2R), moderate (40–59% VO2R) and vigorous (60%–84% VO2R).7 The average increase in VO2max reported in the resulting nine fitness-by-intensity subgroups is displayed in Figure 2. Two observations are apparent. First, higher intensities of exercise clearly result in greater improvements in VO2max for all subjects than do lower intensities. Second, at any given intensity, improvements in VO2max are greater for less fit than higher fit subjects. Higher fit subjects require vigorous intensity exercise to substantially improve VO2max. The interaction of initial fitness and exercise training intensity on the improvement in aerobic capacity. Higher intensities are more effective than lower intensities, and higher fit individuals experience less improvement than lower fit individuals at any given intensity. VO2=maximal oxygen consumption; VO2R=maximal oxygen consumption reserve. Based on data obtained from Med Sci Sports Exerc. 2002;34:152–157.9 A number of studies have been performed using interval training at maximal aerobic intensity. Following a warm up, the individual typically exercises for 3–5 min at the lowest power that elicits VO2max, then exercises at a light level for 3–5 min, and repeats the work:recovery periods for a total of 5–6 intervals. As seen in the Table, Hickson et al.11 used this type of training with individuals of average fitness and obtained a substantial increase in VO2max, while studies of trained subjects obtained smaller increases that were inversely proportional to their initial VO2max.12-14 In all four of these studies, additional training of a continuous nature (such as 30–60 min of running) was also performed for at least 1 day per week. Having a high aerobic capacity reduces one's risk of cardiovascular disease; the reduction is greater than that obtained merely by being physically active. Therefore, exercise programs should be designed to obtain optimal improvements in VO2max, provided there are no contraindications to more vigorous exercise training. Vigorous intensity exercise (60%–84% VO2R or HR reserve) is more effective at increasing VO2max than is moderate intensity exercise (40%–59% VO2R or HR reserve), even when the duration of exercise is adjusted so that the same number of calories is expended. While most exercise prescriptions employ continuous exercise, such as 20–60 min at a moderate or vigorous intensity, interval training utilizing 3-min bouts at VO2max may be most effective in augmenting aerobic capacity. There is an interaction between exercise intensity and one's initial fitness level in that individuals with high initial fitness require high intensity exercise to further improve their fitness. Although vigorous and maximal intensities are more effective at increasing VO2max than is a moderate exercise intensity, the health professional must prescribe these higher intensities with caution. The likelihood of experiencing cardiovascular or orthopedic complications is much greater at higher intensities. Initially sedentary individuals should begin with moderate intensity exercise, which is effective at improving their VO2max, and move up to higher intensities only after a period of adaptation. Individuals at risk of cardiovascular complications during exercise should not engage in high intensity exercise unless a careful evaluation of their condition, including a stress test and attention to signs and symptoms, indicates that they can do so safely.