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Bad back? These are the best sex positions to ease the pain.

University of Waterloo professor Stuart McGill with a research subject. (Courtesy of University of Waterloo)

For a long time, spooning was considered a sexual position everyone could get behind, so to speak. But for people who suffer from back pain, is it really the best option?

For the first time, a new study out of the University of Waterloo in Canada has some real answers to your questions about how to avoid exacerbating (or creating) back pain during sex.

"Spooning had previously been considered a one-position-fits-all for both men and women with back pain," said Natalie Sidorkewicz, lead author of the study, which will be published in the journal Spine. "That ignores the fact that there are different kinds of back pain triggered by different kinds of movements."

And that's where this study comes in. The Waterloo research team recruited 10 healthy male subjects and 10 healthy female partners (the couples were required to have at  least one year of sexual experience with each each other) and they did not suffer from lower back pain.

The couples were outfitted with reflective markers that acted as sensors, similar to what video game animators and visual effects artists use, to model the spine angles used during five different sexual positions. They also monitored how hard muscles worked during sex, and even which muscles were effected by orgasm (more on that later).

The first phase of the study looked closely at male movement in the "spooning," two variations of the "missionary position," and two variations of the rear-entry quadruped (also known as "doggy-style") position. (In this case the study used heterosexual partners, but the recommendations can be applied to anyone--male or female-- who is controlling or not controlling the movement in these positions.)

In general, Sidorkewicz advises that in any position, controlling the movement with the hip and knee rather than the spine will be more spine sparing, and for the person not controlling the movement, maintaining a neutral spine is key to reducing lower back strain.

Their more specific recommendations focus on two kinds of back pain.

People who are "flexion-intolerant" might experience worsening pain when they touch their toes or sit for long periods of time. For those people, spooning is actually the worst position for the male (or the person controlling the movement).

After that, flexion-intolerant back pain sufferers should avoid the missionary position where the arms are propped on the elbows, then the rear entry position in which the female (or male) partner not controlling the movement is propped on their hands. The two best positions are the missionary variation where the person controlling the movement is propped on their hands, and the rear-entry position where the female (or male) partner is propped on their elbows.

Extension-intolerant back pain sufferer might experience pain by lying on the back or stomach. For those people the recommendations are essentially reversed. Spooning might be the most pain free position for those people, and the rear-entry position where the partner not controlling the movement is propped on their elbows is the worst position.

Researchers noted that for two positions, missionary and rear entry, not all variations are created equally. In the study, slight changes in the posture of the female partner changed the kind of muscle movement the male partner used in the rear entry position. And a slight change to the posture of the male partner in the missionary also changed the kind of muscle movement that was used.

"That really lends itself to the interaction between the couple and how the partner could be a part of that intervention," Sidorkewicz said.

It is the first study of its kind to look at the biomechanics of the spine during sex, which now gives doctors real evidence to make recommendations to the many patients who suffer from lower back pain. At any given time, 31 million people in the U.S experience lower back pain, according to the American Chiropractic Association.

"If you look at some of the existing literature and text books that make recommendations on sexual positions and technique, remember they were just people's opinion," University of Waterloo professor Stuart McGill, a co-author of the study, said in a University of Waterloo video. "There was no science that had been performed before to create the foundation for the recommendations."

Upcoming research based on the data collected in this study will be published in the coming months. They include a separate study focused on the spine movements of the female partners, the effect of orgasm on spine movements, and a look at the impact of the sex position recommendations on back pain sufferers.

Sidorkewicz suspects that the guidelines produced from this study could also reduce some of the awkwardness in the doctor's office when it comes to talking about pain in the bedroom.

"The survey reports were all pointing for a study like this to be done," Sidorkewicz said. "A lot of the low back pain patents that were being interviewed were saying that they were having a lot of pain with the movement and as a result that was making them reduce the frequency of their sexual activity."

"We're hoping this will facilitate a dialogue between practitioners and their patients."

For more about which sexual positions to avoid for flexion-intolerant and extension-intolerant back-pain sufferers, see this chart. (Viewer discretion advised).