I asked Roger, a kinesiology professor who teaches the subject at a university in Canada, about how much exercise is enough. Roger said:
“You’ve discovered one of the fundamental exercise conundrums: how to define exercise intensity and what does it mean. The problem is there are no really clear parameters. What may be moderate intensity for one may be very severe for another.
“It’s clear that the more intense the activity, the greater the potential benefit. But the greater intensity requirement for many may end up being too stressful and lead to diminishment of activity down the road; they may decide, ‘I don’t want to put myself through that discomfort/pain again, so I’m not going to do the exercise.’
“Is it better to do twenty-minute sessions three days a week of high intensity or thirty minutes of moderate intensity five days a week? I lean toward the guidelines of “moderation and common sense.” Let each person decide which is best.
“The higher intensity might produce a slightly greater benefit, but it may turn an individual off. The moderate will produce benefit, and as fitness improves today’s high intensity might become tomorrow’s moderation. Plus, the total energy output on the moderate intensity (150 minutes per week versus sixty minutes plus additional higher metabolism recovery energy expenditure) might also be beneficial in body fat regulation. You could suggest a plan of starting with the moderate intensity protocol and then experiment with a higher intensity bout on occasion.
“Regarding the stress proteins, it would appear that moderate intensity requires sixty minutes to produce significant changes versus thirty minutes for cardiovascular improvement. You could recommend that moderate-intensity exercise be increased to sixty minutes to provide that protection. The individual could stick to the thirty-minute routine but throw in a couple of short, higher intensity bouts (fifteen to thirty seconds) to also produce stress protein benefits.
“There are so many variables: how much time is available? How does higher intensity feel —during the activity, after the activity, the next day? Does it impact on repeating the activity? What is the goal? I like to do everything possible to get people moving and repeating the movement. If this means not so intense, so be it.”