By Amy Norton
The less positive finding: Both therapies fell far short of helping everyone.
Still, many failed to get meaningful relief, the researchers noted.
Experts said the findings reflect the reality of chronic low back pain: No single treatment has proven widely effective.
"We do not have any magic bullet," said lead researcher Dr. Robert Saper.
According to guidelines published earlier this year by the American College of Physicians (ACP), non-drug options should be the first-line treatment against low back pain.
That's largely because pain medications and other drugs are not very effective, and carry side effects.
Yoga was included on the ACP's list of options, said Saper, an associate professor at Boston University School of Medicine.
But even in recommending yoga -- and tactics like heat wraps, acupuncture and massage -- the ACP stressed that the evidence was limited. So far, studies have shown "small" to "moderate" benefits with each therapy.
Elissa Stein, 53, a freelance writer from New York City, said she'd dealt with bouts of debilitating back pain ever since she was injured in a car accident at age 16.
"It's terrible when you feel you're not capable of doing anything except taking a muscle relaxant," she said.
She decided to try yoga about 14 years ago.
At first, Stein said, yoga offered a "good stretch." But then she started to realize other benefits -- namely, the focus on conscious breathing and meditation.
"Now I'm helping myself, instead of relying on medication," Stein said.
The new study focused on patients who might not often have access to complementary therapies. It included 320 patients who were low-income and mostly minority, all of whom had persistent back pain for at least 12 weeks. Their mean age was 46.
The patients were randomly assigned to either 12 weekly yoga classes, 15 physical therapy sessions, or an "education" group that received a book on managing back pain.
After the first 12 weeks, yoga practitioners were assigned to either keep taking classes or practice at home, with the help of a DVD, manual and yoga props.
At the 12-week point, both the yoga and physical therapy groups were faring better than the education group.
Almost half of the yoga group (48 percent) had seen a "clinically meaningful" improvement in their pain and disability -- enough to make a difference in their daily lives, Saper explained.
That was also true for 37 percent of physical therapy patients, versus 23 percent of education patients. (While the yoga numbers look better, the difference between yoga and physical therapy was not statistically significant.)
Typically, both yoga practitioners and physical therapy patients saw the benefits last over one year, the researchers said.
"This was an excellent study," said Dr. Stefan Kertesz, who wrote an accompanying editorial. To him, yoga should be "one more tool" that doctors suggest to back pain patients.
"But we also shouldn't oversell it -- the way drugs and procedures were historically oversold," said Kertesz, an associate professor at the University of Alabama at Birmingham School of Medicine. "The reality is, yoga was not a panacea for most of these patients."
In the real world, Kertesz said, the treatment decision will likely come down to a patient's preferences -- and what's practical.
Saper agreed. "One yoga class can be $18 or $20," he pointed out, and insurance is unlikely to cover it.
At the same time, Saper noted, physical therapy is expensive, and even insured patients can face steep co-pays.
Those kinds of barriers are a big problem, according to Kertesz. "How do we make these things more accessible to people?" he said.
People who are interested in yoga should be aware there are many different styles, Saper pointed out. The class "down the street," he said, may not be appropriate for someone with serious pain issues.
The classes in the trial were standardized and included gentler poses -- often aided with chairs and other props -- breathing practices and meditation.
The study was published online June 19 in the Annals of Internal Medicine.