Obese people may be more sensitive to pain than people who aren’t obese, a new study suggests.

All of the older adults who completed the study had osteoarthritis of the knee, a disease that causes inflammation and extreme pain in the knees.

Participants were given a mild electrical stimulation on their left ankle to measure their pain reflex. The stimulus was given before and after the participants took part in a 45-minute coping skills training session that included a progressive muscle relaxation exercise.

The obese patients showed a greater physical response to the electrical stimulation than did the non-obese people, both before and after the training session. This indicates they had a lower tolerance for the painful stimulation despite reporting, in questionnaires, that they felt no more pain than non-obese people.

“The relaxation procedure helped both groups cope with pain,” said Charles Emery, the study’s lead author and a professor of psychology at Ohio State University. “Additionally, our tests showed both groups had higher physical pain thresholds after the relaxation session. But the obese participants still had a lower threshold for tolerating the pain.”

“This is important because if an obese person begins an exercise program, he may not cognitively experience pain when in fact it is hurting the body on some level,” Emery said. “That could lead to severe pain down the road.”

Emery and his colleagues presented their findings on March 4 in Denver at the annual meeting of the American Psychosomatic Society.

The researchers wanted to see if coping skills training, including progressive relaxation techniques would help people with osteoarthritis to better cope with the pain that the disease can cause. Also called degenerative joint disease, osteoarthritis affects more than 20 million people in the United States.

But they were particularly interested in seeing how the obese group responded to pain; according to Emery, a small number of studies have looked at pain sensitivity in obese people, but many of these studies report conflicting results.

“Some studies say that obese people are more tolerant of pain, while other studies say they are less tolerant,” Emery said.

About a third of the study’s 62 participants were obese. Researchers determined who was obese based on participants’ body mass index (BMI) scores, which relates height to weight. Obese patients in this study had a BMI greater than 30 but less than 35. (Scores higher than 35 are considered morbidly obese.)

The participants underwent two rounds of electrical stimulation - once before, and once after a 45-minute training session where they learned different ways of coping with pain, including instruction in progressive muscle relaxation therapy.

The electrical stimulation came from an iPod-sized device that delivered a slight Electrical shock to a patient’s sural nerve, a nerve that extends along the ankle and into the calf. This kind of electrical stimulation causes sensations of tingling and mild pain in the lower leg.

The researchers determined the body’s response to sural nerve stimulation by measuring the reflex of the lower leg muscles that surround the sural nerve. When the brain senses pain, it sends a message to the body to contract and move the muscles in order to get away from the source of the pain.

“This kind of evaluation is in some ways a more objective way of measuring the body’s response to pain, as opposed to simply asking someone if they feel pain,” Emery said.

But the researchers did ask participants how much pain they felt. Participants completed questionnaires about anxiety and pain perception after each round of electrical stimulations. All participants, obese or not, reported that they felt less pain after the relaxation session than they did before.

Yet results of the sural nerve stimulus test showed that the obese participants did not tolerate the painful stimulus as well as the non-obese individuals.

“Our findings show the importance of looking at objective as well as subjective measurements of how the body responds to pain stimuli,” Emery said.

Emery conducted the study with colleagues from Ohio State, Ohio and Duke universities.

Editor’s note: Emery will attend the American Psychosomatic Society meeting from March 3 through March 6. The best way to reach him during this time is to call the phone number, above.