Regular physical activity improves general health but the frequency and intensity of physical activity required to produce benefits is not clear. To address this issue, Duncan and associates (Archives of Internal Medicine, 2005; 165: 2362-2369) conducted a randomized controlled clinical trial to evaluate the effects of different prescriptions of exercise intensity and frequency on cardiovascular disease risk factors.
Four-hundred and ninety-two sedentary adults were randomized to one of five study conditions: moderate-intensity/low frequency; hard-intensity/low frequency; moderate-intensity/high frequency; hard-intensity/high frequency; and a comparison group that received physician advice and written materials about exercise. The duration (30 minutes) and mode (walking) of exercise were held constant across the conditions. The interventions involved 11 group meetings over 6 months and 6 quarterly follow-up meetings over an additional 18 months. The main study outcomes were the 6- and 24-month changes in cardio-respiratory fitness (measured as maximum oxygen consumption), high-density lipoprotein cholesterol and the ratio of total cholesterol to high-density lipoprotein cholesterol.
At 6 months, the hard-intensity/low frequency, moderate-intensity/high frequency, hard-intensity/high frequency conditions produced significant increases in maximum oxygen consumption compared to the physician advice comparison group. However, only the hard-intensity/high frequency conditions produced improvements in both high-density lipoprotein cholesterol and the total cholesterol high-density lipoprotein cholesterol ratio (ps < .05). At 24-months, the increases in maximum oxygen consumption remained significantly higher, compared to baseline, in the hard-intensity/low frequency, moderate-intensity/high frequency, hard-intensity/high frequency conditions. The hard-intensity/high frequency condition also maintained the significant improvement in oxygen consumption compared to the physician advice group, but had no significant 24-month effects on cholesterol and high-density lipoprotein cholesterol.
A prescription of walking exercise at either hard intensity or high frequency produced significant long-term benefits in cardio-respiratory fitness. The combination of hard intensity/high frequency exercise might provide additional benefits including an improved lipid profile.
This study provides strong evidence that significant improvements in cardio-respiratory fitness can be achieved and maintained over 24 months via an exercise prescription involving walking 30 minutes per day at either moderate (45%-55% of training heart rate zone) or hard (65%-75% of training heart rate zone) intensity 3 to 4 days per week. Moreover, the study indicates that greater benefits are conferred when the individual performs either more frequent exercise or the combination of more frequent and more intense exercise.
Perhaps the most important finding of this study was that benefits were derived from walking exercise only. That is, participants were not required to engage in other forms of exercise (resistance training) that might require committing a great deal of time and effort to the endeavor. Simply walking for 30 minutes a day (either as a 30 minute continuous bout or for at least 10 minutes per bout) 3-4 days per week (the low frequency condition) or 5-7 days per week (the high frequency condition) were sufficient to confer significant benefits. Of course, the greatest benefit came with more frequent, more intense walking. However, patients, as they become fitter and more confident, can gradually progress in both the frequency and intensity of their walking exercise. This methodologically rigorous randomized controlled clinical trial indicates that counseling sedentary adults to become more physically active via walking exercise provides significant long-term benefits with respect to cardiovascular risk factors. The challenge, of course, remains assisting patients to adopt and sustain the walking exercise program over the long-term.