For most readers -- i.e., people who exercise at light to moderate intensities -- performing cardio and strength training on one day is fine. But the asterisks begin to fly as intensity levels increase.
The majority of exercisers, according to the American College of Sports Medicine, aren't putting high enough demands on their bodies to significantly compromise the value of either activity or risk injury.
So do your 20 minutes on the treadmill watching CNN and follow it up with a half-hour set of strength-training moves. Or do a strength-training circuit of six exercises with no rest between sets and keep your heart tapping throughout.
"You wouldn't want to have a heavy lifting day after running 10 miles" on the same day, says William Kraemer, a professor of kinesiology, physiology and neurobiology at the University of Connecticut. That's because your body is restoring energy, rebuilding damaged tissue and recovering hormones exhausted during the first exercise bout.
After hard activity, "your body wants to say, 'We're in repair mode,' " Kraemer says. If you jump to another demanding task while your system is busy synthesizing protein to repair stressed muscles, the body goes back into metabolic mode, which interrupts the recovery. "You want to be fully metabolically and neurologically capable for the big stuff. Whatever [activity] you do first will have the highest quality," he says.
Kraemer suggests following a tougher strength session with a light run, or doing hard interval cardio training before light lifting. Though the strength-cardio sequence matters little, Walter Thompson, a kinesiology, health and nutrition professor at Georgia State University, recommends an ordered approach within a strength-training session.
This essentially involves doing the moves with bigger payoff earlier in your workout. So, for example, work large muscles (quads) before smaller ones (abductors); perform multi-joint exercises (bench press) before single-jointers (biceps curls).