Done in a series, yoga positions tone muscles and increase flexibility, while proper breathing brings calm and concentration. 

"A lot of people are no longer interested in exercise that beats them to death," says John Schumacher, director of Bethesda, Md.-based Unity Woods Yoga Center, where class enrollment has nearly doubled in four years. "We get construction workers, lawyers, psychologists, bank tellers, a whole range." A quarter of class participants are men, Schumacher says -- a percentage that's increasing. 

Athletes in peak form are turning to yoga for an extra edge. For example, marathoners-in-training from the New York Road Runners Club in Manhattan gather for weekly yoga classes designed to build strength and agility. The athletes follow a series outlined in Power Yoga (Simon & Schuster, $15), currently one of the top-selling yoga books. Author Beryl Bender Birch says the practice is a "high-heat, high-energy workout" that especially appeals to people who want to build sinewy, not bulky, strength and avoid or recover from injury. 

Most yoga taught in America is hatha yoga, of which Iyengar is the most popular form. But gaining ground is Ashtanga yoga, from which power yoga is derived. This more strenuous yoga builds strength from positions that require prolonged muscle contraction. Among the growing number of practitioners is Madonna. The one-time Material Girl, who started doing Ashtanga yoga after the birth of her daughter, says it has given her better muscle tone, more energy and greater strength.