The benefits of exercise: Does workout intensity matter?

Which is better for your health – a two-hour leisurely stroll, jogging for 40 minutes or a brief, intense, Cross-Fit-style “workout of the day?” Ask 10 fitness professionals and you will likely get a range of nuanced opinions, but in fact, we really do not know the answer.

The American Heart Association recently sponsored a debate on the topic at a meeting held in San Francisco. I was asked to present the argument in favour of high-intensity interval training, owing to laboratory research we’ve done. My opponent was Dr. William Kraus, a professor of Medicine at Duke University and widely regarded expert in preventive cardiology.

Current Canadian Physical Activity Guidelines state that adults should accumulate at least 150 minutes of “moderate- to vigorous-intensity” aerobic physical activity a week to achieve health benefits. This recommendation is consistent with those from other organizations, although some acknowledge the potential influence of exercise intensity. For example, the American College of Sports Medicine advocates a weekly dose of at least 150 minutes of moderate-intensity activity, or 75 minutes of vigorous-intensity activity, to promote health.

Of course, health is complex phenomenon with physical, mental, social and psychological dimensions. With regards to physical health at least, this is where the debate is at:

The Pro Side: Intensity Matters

Several large observational studies have suggested that relative exercise intensity is more important than duration to improve life expectancy and reduce risk factors for chronic disease. The Copenhagen City Heart Study followed 5,000 individuals over 20 years and included survey data on their self-reported daily cycling habits. It found that the “fast” compared to “slow” cyclists lived longer, were leaner, and had lower blood pressure, cholesterol levels and frequency of diabetes. In contrast, life expectancy and risk factor burden were unrelated to the total amount of daily cycling. The main conclusion was that exercise intensity, and not duration, was most important to promote health.

Other smaller studies have examined adaptations to different exercise intensities, while attempting to control for total volume; that is, by matching energy expenditure or the number of calories burned per session. These studies have typically compared subjects who performed interval training (alternating periods of high- and low-intensity exercise) or an equivalent amount of moderate-intensity continuous training. Interval training has shown to be superior for improving cardiorespiratory fitness and clinical markers of health status, in both healthy individuals and people with cardiometabolic disorders.

The Con Side: Volume is Key

Several large-scale, randomized clinical trials have addressed the issue by having groups of subjects perform different amounts and different intensities of exercise, and making comparisons to a control group who does not exercise. While simple in concept, these studies are logistically complex, expensive and typically involve only a few different conditions. One of the most influential has been STRRIDE (Studies of a Targeted Risk Reduction Intervention through Defined Exercise), a project conducted by Dr. Kraus and colleagues.

The STRRIDE study examined the effect of low-amount/moderate-intensity, low-amount/vigorous-intensity or high-amount/vigorous-intensity exercise on metabolic risk factors for cardiovascular disease and diabetes. One of the key findings was a dose-response effect across the various amounts of exercise on clinical markers such as insulin sensitivity and cholesterol levels. This suggests there is a threshold volume or minimal amount of exercise necessary to promote health, with higher intensities conferring no greater benefits.