You don't have to lift heavy to build muscle, says new research in April's Journal of Applied Physiology. After 10 weeks, young guys doing three sets of knee extensions with light weights three times a week increased muscle volume as much as guys doing the same with heavy weights, the study found.

The key: Both groups performed each set until exhaustion -- meaning they physically couldn't lift the weight again. (That's exactly how you'll feel after only 15 minutes with the high-intensity muscle building workouts in the Men’s Health Big Book of 15-Minute Workouts.)

"Certain muscles, such as the deltoids, quadriceps, and calves, respond especially well to high-rep sets due to their muscle fiber makeup," says Chad Waterbury, a neurophysiologist and Santa Monica-based trainer. Case in point: Cyclists -- who do thousands of light reps on any given ride -- usually have giant calves.

Waterbury says training with light loads and high reps works well in three different scenarios: When you have joint pain from heavy training, when you need to move weight quickly to boost force, and when you need to do more reps to increase muscle endurance. (For an endurance routine you can do anytime, anywhere, download this Total-Body Outdoor Workout.)

Add a light-load, high-rep session a day or two after heavy lifting to give your joints and central nervous system a break. "Stick to performing light-training sets at the end of your heavier workouts or on another day as an extra workout," Waterbury says. "Performing high-rep sets to exhaustion at the beginning of a workout will negatively affect your performance for the rest of that workout."

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For maximum muscle growth, the key is to accelerate the lifting phase, Waterbury says. "With light loads you should lift as fast as possible with perfect form, squeeze the peak contraction, and lower under control. This added acceleration during contraction taps into the largest motor units that have the most potential for growth."

To figure out what a "light" load is for you, find the most weight you can lift for no more than three reps in a given exercise. Divide that weight by three. (Want to build huge muscles fast? Do this 15-Minute Total Body Blast. It'll skyrocket your metabolism to torch fat while packing on major muscle.)

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