Here's a good reason to multi-task at the weight room: When weight trainers use the rest periods between sets to stretch the working muscle group, says a new study, they experience a greater strength gain over weight trainers who don't stretch. "After 10 weeks, the stretching group had a strength gain in their leg curl test that exceeded the non-stretching group by almost four pounds," says study leader Wayne Westcott, Ph.D., senior fitness director at the South Shore YMCA in Quincy, Massachusetts, where the tests were conducted. "We did a second study and the combined data revealed a 16.5 lb. strength gain for the strength training only group while the stretching group had a 19.6 lb. increase, an almost 20 percent greater strength gain."

Westcott conducted a third trial to determine whether it was better to stretch muscles right after each set or to do one longer stretching session at the end of the circuit. While there was no difference in strength gain between the two methods, Westcott recommends the lift-stretch-lift method. "You need to rest anyway so why not do some stretching? It's the most efficient use of the total time frame."

But how does flexibility help you flex more muscle? "We know that shortening, or contracting, a muscle builds strength and it seems that muscle lengthening does, too, to some degree," says Westcott. "When we stretch muscles in close proximity to muscle strengthening it appears to increase the muscle's receptivity to the strength-building stimulus."

There are many stretches you can do without getting off the machine: On the seated leg curl machine, for example, leaning forward and holding your toes or ankles will stretch your hamstrings. Others, like triceps stretches, can be done standing between sets. After finishing your reps on the triceps machine, disengage and stretch the triceps by reaching across your chest for the opposite shoulder blade with one hand and gently tugging on your elbow with the other. "With all these stretches, allow the muscle to relax and lengthen for 20 seconds," recommends Westcott. "You should feel some tautness, but don't push it to the point where you feel pain." The bottom line is, he says, don't just sit there, stretch something.