A 2005 study conducted by the American Council on Exercise (ACE) revealed that a moderate regimen of Hatha Yoga (three 55-minute classes per week for 8 weeks) had significant positive benefits in the areas of flexibility, balance and muscular strength and endurance. It also showed no significant benefits toward pulmonary function or aerobic capacity. A follow-up study was done that compared the physiological effects of Hatha yoga with those of Power yoga. The results of these study can be found here.