But let’s face it, walking holds little appeal — or physiological benefit — for anyone who already exercises. “I nominate the squat,” said Stuart Phillips, Ph.D., a professor of kinesiology at McMaster University and an expert on the effects of resistance training on the human body. The squat “activates the body’s biggest muscles, those in the buttocks, back and legs.” It’s simple. “Just fold your arms across your chest,” he said, “bend your knees and lower your trunk until your thighs are about parallel with the floor. Do that 25 times. It’s a very potent exercise.” Use a barbell once the body-weight squats grow easy.

The squat, and weight training in general, are particularly good at combating sarcopenia, he said, or the inevitable and debilitating loss of muscle mass that accompanies advancing age. “Each of us is experiencing sarcopenia right this minute,” he said. “We just don’t realize it.” Endurance exercise, he added, unlike resistance training, does little to slow the condition.

Image ENERGY WELL SPENT In METs (metabolic equivalent to task), a measure of energy exerted for a given activity.

Resistance training is good for weight control, as well. In studies conducted by other researchers, a regimen of simple weight training by sedentary men and women led to a significant decrease in waist circumference and abdominal fat. It also has been found to lower the risk of developing diabetes and cardiovascular disease. Counterintuitively, weight training may even improve cardiovascular fitness, Phillips said, as measured by changes in a person’s VO2max, or the maximum amount of oxygen that the heart and lungs can deliver to the muscles. Most physiologists believe that only endurance-exercise training can raise someone’s VO2max. But in small experiments, he said, weight training, by itself, effectively increased cardiovascular fitness.

“I used to run marathons,” he said. Now he mostly weight-trains, “and I’m in better shape.”

But there’s something undignified and boring about a squats-only routine. And the science supporting weight training as an all-purpose exercise approach, while provocative, remains inconclusive. Is there a single activity that has proved to be, at once, more strenuous than walking while building power like the squat?

“I think, actually, that you can make a strong case for H.I.T.,” Gibala said. High-intensity interval training, or H.I.T. as it’s familiarly known among physiologists, is essentially all-interval exercise. As studied in Gibala’s lab, it involves grunting through a series of short, strenuous intervals on specialized stationary bicycles, known as Wingate ergometers. In his first experiments, riders completed 30 seconds of cycling at the highest intensity the riders could stand. After resting for four minutes, the volunteers repeated the interval several times, for a total of two to three minutes of extremely intense exercise. After two weeks, the H.I.T. riders, with less than 20 minutes of hard effort behind them, had increased their aerobic capacity as much as riders who had pedaled leisurely for more than 10 hours. Other researchers also have found that H.I.T. reduces blood-sugar levels and diabetes risk, and Gibala anticipates that it will aid in weight control, although he hasn’t studied that topic fully yet.

The approach seems promising, since most of us have minimal time to exercise each week. Gibala last month published a new study of H.I.T., requiring only a stationary bicycle and some degree of grit. In this modified version, you sprint for 60 seconds at a pace that feels unpleasant but sustainable, followed by 60 seconds of pedaling easily, then another 60-second sprint and recovery, 10 times in all. “There’s no particular reason why” H.I.T. shouldn’t be adaptable to almost any sport, Gibala said, as long as you adequately push yourself.