That issue is surprisingly contentious in the sports world. Many competitive athletes, their coaches and athletic trainers have come to believe that aerobic exercise, if practiced in close proximity to strength training, reduces the ability of muscles to strengthen and grow. Conversely, many contend that weight training performed on the same day as aerobic exercise blunts the endurance training response.

This phenomenon, known variously as “muscle interference” or “exercise antagonism,” is a frequent topic on fitness-related chat boards. But to date, most of the discussions have been based on anecdotal evidence or simple conjecture. There has been little science supporting or challenging the existence of interference.

So, independently, groups of researchers at McMaster University in Ontario and the Karolinska Institute and other institutions in Sweden recently recruited volunteers to test the idea that you get more physiological benefit from performing only one type of exercise on any given day.

The two groups of scientists rounded up very different subjects. In Sweden, the volunteers were healthy and active young men, primarily college students who regularly worked out but didn’t necessarily compete.

The Canadian volunteers were sedentary, middle-aged men who hadn’t exercised much, if at all, in the past year. (No women took part in either study, an omission that is common and frustrating in exercise science.)

The exercise protocols were also different, in interesting ways. In Sweden, the men began by pedaling a stationary bicycle for 45 minutes, using only one leg, an action that supplied the aerobic component of the experiment. Six hours later, they completed a series of strenuous leg extension exercises using both legs.

Essentially, in each participant one leg had undergone combined exercise, featuring both endurance and resistance training on the same day, while the other leg had done endurance training alone.

The scientists took muscle biopsies before and after each session.

For their part, the Canadian researchers had their older volunteers finish three separate trials. In one, the men rode a stationary bicycle for 40 minutes at a moderate pace. On another day, the same volunteers sweated through eight relatively strenuous sets of leg extension exercises. In the final session, the men completed four sets of leg extensions and then rode the bicycle for 20 minutes, finishing half as much of each type of exercise, but in rapid succession.

The scientists biopsied the men’s leg muscles before and after each session.

“Our hypothesis had been that we would see a greater response to each exercise individually,” says Stuart Phillips, a professor of kinesiology at McMaster who oversaw the Canadian study. Specifically, he says, the scientists had expected that endurance training on its own would significantly affect portions of the muscle cell related to energy production, while resistance training would increase protein synthesis within muscles, the first step toward enlarging the muscles.

Combined training, the Canadian scientists had hypothesized, would dampen at least one of the molecular changes; physiologically, one of the responses would predominate and interfere with the other.

That didn’t happen.

Instead, after combined training, the men’s muscles displayed the same amount of change within both cellular pathways as after either type of exercise on its own, even though the men had actually completed only half as much of each.

“We saw no indications of interference,” says Dr. Phillips, whose study was published last month in The Journal of Applied Physiology.

The Swedish investigators arrived at a similar result. Their study, published in March in Medicine & Science in Sports & Exercise, showed little difference in the genetic and biomechanical responses within muscles whether the men performed both aerobic and resistance training or aerobic training alone.

In other words, “aerobic exercise can precede resistance exercise on the same day without compromising” muscle building, the scientists conclude.

And if you prefer your weight training first, the Canadian study scheduled the resistance work before the bike riding, without compromising the results for either type of exercise.

Of course, both studies looked only at immediate results. But Dr. Phillips believes that over the long term, the effects should be the same. “There’s no reason to assume that interference only kicks in later in training,” he said. If it existed, he continues, it presumably would show up in the earliest molecular changes inside muscles, and it did not.

These findings are important for serious competitive athletes who are designing serious, complicated training regimens. But they also have implications for those of us who’ve been, until now, ignorant of the possible existence of exercise antagonism. We can, it seems, remain blissfully unconcerned.

“It appears that you can set up a workout regimen that happens to be convenient for you,” in terms of how and when you shuffle the endurance and resistance elements, says Dr. Phillips, “and you’re not going to get less training response.”

Best of all, Dr. Phillips’s study suggests that you can potentially do less of each form of exercise when you combine them and still gain considerable benefits. “In our study, the men were doing only 50 percent as much” cycling and weight training in the combined session as during the specialized workouts, he points out. “But their muscles couldn’t tell the difference.”

Photo: Paul Conrath/Getty