There are now several articles that describe the status of research done on the medical benefits of 
yoga. One of the most comprehensive is by S.B. Khalsa ["Yoga as a therapeutic intervention: a bibliometric analysis of published research studies," Indian Journal of Physiology and Pharmacology (2004
Jul;48(3):269-85) Abstract: PubMed. Fulltext: Journal Archives. ] who points out that ". . . for the time period 1973 to 1989, the
number of Indian and U.S. published RCT's [randomized controlled
trials] was 11 and 2, respectively. From 1990 to 2004 the number of
Indian-based studies was 21, a two-fold increase, whereas the number of
U.S. studies was 16, an eight-fold increase." Focusing on a select number of studies and looking forward from a public health perspective, Laci Scott in "Yoga off the mat: How far and to whom do the benefits extend?" (M.P.H. thesis, The University of Texas School of Public Health, 2008 Abstract], suggests that "Future research should seek to increase sample size, to diversify recruitment to allow for the randomization of treatment and control groups, and to allow for long-term follow-up."
Another study that deals less with the medical details and more with historical aspects is Elizabeth De
Michelis's very perceptive,
richly documented overview "A Preliminary survey of modern yoga studies," Asian Medicine
(2007; 3, issue 1:1-19). De Michelis points out that "The 'medicalisation' of yoga, and its
dialogue with science, started in the 1920's in India . . . ." and carries her discussion forward to the present.
What might future yoga medical research ideally encompass? There are many variables - funding being the most obvious. But the trajectory for more comprehensive, focused yoga medical studies is already evident. YMN Daily suggests a likely scenario for future yoga research done in the U.S.: (1) longer-term clinical trials involving larger randomized treatment and control groups; (2) greater "customization" of research approaches to fit the condition (already seen in studies relating to diabetes and osteoporosis); and likewise (3) more "customization" of treatment. More subtle gradations are now appearing that focus on specific approaches (Iyengar, Ashtanga) or asanas.
