When you tense your upper body muscles to scrub the outdoor grill or work at the computer, you might expect your neck and shoulders to ache afterwords. But why would your leg muscles start to hurt if they didn’t even get a workout? A new study by Hong-You Ge, M.D., Ph.D., of Denmark explains why a two-minute contraction of the shoulder muscles can increase leg pain in people with fibromyalgia.*

Sustained muscle contractions are known to trigger the release of pain-relievers into the spinal cord to reduce feelings of achiness in those muscles that are getting the workout. It’s a natural process that soothes pain, at least in healthy subjects. But this system that provides analgesia after exercise appears to be working the wrong way in fibromyalgia patients.

Study participants were asked to contract the large shoulder muscles in their upper back until their muscles gave away to fatigue. Twenty-two fibro patients were compared to the same number of healthy control subjects. Everyone in the study was middle-aged, so age was not a factor.

The upper shoulder muscles’ sensitivity to pressure pain was measured before and immediately after the contraction, as well as 20 minutes later. Pain sensitivity was also checked in a lower leg muscle that was relaxed the entire time. The research team predicted that the spinal cord was misinterpreting the messages from the contracting muscles and making pain worse throughout the body in people with fibromyalgia.

The brief muscle contraction in the healthy group produced less pain in the shoulder area right after the exercise and 20 minutes later. This is what should happen when taxing one’s muscles, so that waste products like lactic acid don’t leave people hurting. If this did not happen, people would learn to avoid exercise whenever possible and that wouldn’t be healthy. Pain sensitivity in the lower leg was unaffected. The results were quite the opposite for those with fibro.

Pain levels started out much higher in the group of fibromyalgia patients (as expected). In addition, the spinal cord did not kick in to relieve post-exercise soreness in the shoulders. Worse yet, pain sensitivity increased significantly in the leg muscles that were relaxed throughout the study.

What does this mean? The system in the spinal cord that people rely upon to ease post-exercise discomfort seems to be responding to workouts by increasing the pain in fibro. So, if you are a fibro patient on your feet all day long, your activity can cause sore leg muscles and also make other muscles hurt, such as those in your arms.

Exercise is an essential part of staying fit and healthy, but it is hard to do if it increases your pain, as this study demonstrates. The trick is to not overwork any of your muscles. This explains why fibromyalgia patients state that keeping their pain under control requires frequent rest breaks and changing positions to avoid straining any muscle group.

Increasing physical function has to be done extraordinarily slow in fibromyalgia patients just to keep the pain levels stable. As your muscles get bigger and stronger, your body will be able to sustain more activity before the spinal cord decides to amplify your pain. Improving fitness has to be done very carefully and gradually, or increased muscle aches will constantly trip you up (unlike those healthy folks who feel great after a heavy workout).

* Ge HY, et al. Descending pain modulation and its interaction with peripheral sensitization following sustained isometric muscle contraction in fibromyalgia. Eur J Pain 16(2):196-203, 2012. (Available online ahead of print June 28, 2011.)