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This launches a new bi-weekly column on the science of sport. Drawing on the latest research, Alex Hutchinson answers your fitness and workout questions.

The question

What should I do first at the gym: cardio or weights?

The answer

Let's start with one incontrovertible fact: You can't fulfill your ultimate potential as both a weightlifter and a marathoner at the same time. Too many hours sweating on the elliptical will hinder your ability to put on muscle (a fact that was demonstrated in a classic study almost 30 years ago), and pumping too much iron will slow your endurance gains.

The problem is that most of us won't choose: We want some combination of reasonable cardiovascular fitness and non-vanishing muscles - a desire shared by many elite athletes. The women's basketball teams who gathered in Saskatoon last weekend to vie for the national university championship, for instance, need strength and explosiveness, but also have to last for a full 40 minutes.

"We typically have our athletes lift weights, jump and sprint one day, then do their aerobic work the next day," says Derek Hansen, head strength and conditioning coach for the 2007 national champs from Simon Fraser University. If they are combining a heavy weights session with a cardio session, he says, then the weights come first, since building power is their first priority.

This approach - starting with whichever activity is most important to you - is widely used by elite athletes.

Until recently, scientists thought it was simply a matter of logistics: If you're tired from the treadmill, then you can't lift as much weight, so over time you put on less muscle.

But new techniques now allow researchers to measure directly which proteins are produced in muscles after a session on the treadmill or a set of bench presses.

It turns out that the sequence of cellular events that leads to bigger muscles is partly controlled by the same "master switch" - an enzyme called AMP kinase - that controls adaptations for better endurance. But you can't have it both ways: The switch is set to either "bigger muscles" or "better endurance," and the body can't switch instantly between the two.

That means whatever mode you start in will be dominant for your whole workout.

So if your goal is beach muscles, dive straight into your weights routine (being careful to warm up with some easy sets). If you're preparing for a five-kilometre race, do your full cardio workout before tacking on weights at the end.

And if you're looking for the best of both worlds, Mr. Hansen suggests mixing it up, both within a single session and from day to day: "The variability will be good, as it challenges your body and metabolism."

Alex Hutchinson is a former member of Canada's long-distance running team, and has a PhD in physics.

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