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Last updated: Mon, Aug 12, 2024
Although muscles clearly can hurt, and although they are the largest organ system of the body, the musculature is an “orphan organ” in medical practice—no medical discipline deals with it specifically.1 While there is much basic scientific knowledge about it, it is sparsely covered in pain textbooks and in the medical curricula.2
Muscle pain has many of the characteristics of the second pain associated with a twisted ankle that I described in an earlier section (The Pain System in Acute Injury). The pain is commonly described as cramping, aching, or tearing. When a muscle is injured there is commonly no first pain, only second pain, and the injured area is difficult to localize. The pain is often accompanied by nausea and other autonomic reactions such as a drop in blood pressure and sweating.
Muscle can become damaged as the result of un-dramatic processes. Repetitive work at a low level of force can lead to pain. Although the muscle as a whole is not overstressed, small units of the muscle do all of the work at each repetition, and are referred to as “Cinderella units.” These are groups of muscle fibers that fire together and have "just the right" strength to do the job. These units are overstressed and can become inflamed. Inflamed muscle releases a chemical called nerve growth factor that can sensitize neurons in the spinal cord without directly generating pain.
Spinal neurons can be dramatically sensitized by nociceptive stimulation from the muscles. An experiment with rats showed that muscle pain from the calf (yes, rats have calves) caused spinal activation to expand to include two additional spinal segments. The result of this, mapped to human anatomy, is that painful or silent damaged muscles in the calf could cause pain to be felt in the pelvis. Sensitization of spinal neurons can cause pain to be referred not only to other muscles but also to areas of skin. Spinal neurons that respond to noxious stimulation of muscle often receive input from other deep tissues, such as joints and tendons. When these neurons are sensitized, pain from any of these areas can be felt as the result of muscle irritation.
Not only can spinal sensitization result in large increases of referred pain in remote areas, it can occur as the result of very little nociceptive input. Siegfried Mense, a prominent muscle researcher, claims that Every input from muscle nociceptors to the spinal cord probably sensitizes central neurons.
3 Signals from the muscles that are below the level of perception can cause spinal sensitization.4 The sensitized spinal neurons are more active while they are not receiving nociceptive input, and respond more strongly when they do receive such input.
Nociceptive stimulation from muscles activates the pain network of the brain differently than stimulation of the skin. Differences in activation have been seen in the cingulate cortex and in the primary somatosensory cortex.
A common medical misconception has been the idea of the spinally-reinforced muscle spasm. The theory has been that once a muscle spasm begins it is reinforced through spinal reflex pathways. Research shows, however, that persistent spasm is often the result of an injury in another muscle that is then referred to the muscle that is in spasm.5
Medical treatment of muscle pain has been strongly influenced by two syndromes in which pain is prominently felt in the muscles, fibromyalgia and myofascial pain. A later section (Fibromyalgia, Whiplash, and Myofascial Pain (+CWP)) discusses those conditions in more detail, but in the context of muscle pain generally they are important for the way they have attracted attention to tender spots and trigger points in muscle. In the case of fibromyalgia, early attention focused on the presence of tender areas of muscle mostly in the upper back. There was controversy over whether this “subjective” symptom was significant. Recent findings point clearly to central sensitization, but the role of damage in the muscles themselves is unclear.
"Myofascial" is a compound word. Myo refers to muscle, and fascia refers to sheets of connective fibers that surround, delimit, and connect muscles. Hence myofascial pain can be taken to mean pain of the muscle or the fascia that encloses it. However, myofascial pain is also the name given to a muscular pain syndrome that is defined by the presence of trigger points. A trigger point, at least in the context of myofascial pain, is a small, hard area in a muscle that is painful to the touch and that triggers pain in a remote area when irritated. Trigger points became a standard focus of care in muscle pain. Since trigger points are difficult to identify accurately, treatments based on trigger points have suffered from irregular success.
The emphasis on trigger points, however, may have limited clinical interest in muscle pain when trigger points can't be found.