Last updated: Fri, Jun 30, 2017
Electromyography (EMG) is observation of the electrical activity associated with muscle activity. Each motor neuron extends from the spinal cord to a muscle, and when it conveys a "contract" message to the muscle fiber, it sets off an action potential in the muscle fiber very similar to the action potential in a nerve fiber. EMG has been used in pain research and in the pain clinic to investigate relationships between muscle activity and pain. Surface EMG is used for this purpose. An electrode placed on the skin senses the potential (voltage) of an area of muscle in adjacent sites. A difference in voltage indicates that a command to contract has been sent to and received by the underlying muscle. Since the sensor is at the skin surface, it can't sense underlying muscles.
Flor and Meyer, authors of the chapter on physiological measures in the Handbook of Pain Assessment, caution that [a] causal role of muscular dysfunctions has so far not been demonstrated for pain disorders.
1 As we've seen, however, there are researchers who would attribute muscular causes to conditions including certain headaches and back pain. Nevertheless, investigators have looked at the relations between pain and several varieties of EMG effect.
Resting baseline levels of muscle tension have been investigated. However, the evidence for permanently elevated baseline levels in patients with chronic musculoskeletal pain problems has been scarce.
2 Low baseline EMG levels were found in one study of FMS patients.3
Evidence has been found, on the other hand, that chronic pain patients have a stronger muscular reaction to various stressors. A study of grocery store checkout clerks found that, in their work environment, those with higher EMG levels reported greater pain.4 A number of studies have found that stress reactivity is higher in those muscles that are painful—for example, back muscles in CBP patients, jaw muscles in TMD (temporo-mandibular disorder) patients. Patients who have a strong EMG reaction to stress also have high fear of movement and reinjury.5
The time to return to normal EMG state after a stressor is complete has been studied, but not adequately to support firm conclusions.6
It has been noted that chronic back pain patients often have unusual posture and gait. However, it isn't known whether this is a cause or a result of the painful condition. Also, it is not know whether too much tension or too little tension might be the problem. One group of researchers established that chronic low back pain patients show higher EMG ratings in a standing posture, but patients with disc disorders also show higher ratings in a sitting position.7 (I think I could have told them this;)) Most studies have reported low tension during movement in low back patients, and the researchers suggested training to counteract this.8
Researchers have suggested that musculoskeletal problems may be caused or maintained by an inability to correctly perceive bodily states (see also Inadequate Perception of Bodily States). The researchers concluded that the chronic pain patients were deficient in their abilities to read their own bodies.9 The patients were shown a bar on a video screen and asked to tense a muscle in proportion to the height of the bar. The EMG rating from the subject's muscle was then compared to the bar height. The patients perceived higher muscle tension, had higher pain aversiveness while tensing, and had higher pain. The researchers concluded that [t]hese data confirm and clarify previous reports of deficient tension perception and show concurrent overestimation of bodily symptoms in chronic musculoskeletal pain patients.
10 (I should think that this could alternatively be interpreted as “pain patients aren't comfortable tensing their muscles.”)