Activating mechanisms. Myofascial pain syndromes occur in muscles that have been overused as well as underused (Rosen 1993). The mechanism underlying their activation in these varied situations is not one of use, but rather one of exceeding a "critical load." Other factors such as faulty positioning, malalignment of forces, and underlying joint dysfunction provide important external influences. In these situations, it is presumed that muscle dysfunction occurs in response to an abnormal load imposed on the muscle at any point in time, resulting in premature muscle fatigue and then muscle failure once the critical load has been exceeded. Myofascial trigger points would then develop at a point where the muscle was no longer capable of handling the load, perhaps as a result of local injury or disruption of the myofibrillar structure.
Perpetuating mechanisms. Several perpetuating factors have been suggested to explain both the chronicity of myofascial pain and the apparent failure of appropriate treatment in many patients. Myofascial perpetuating factors are those mechanical, systemic, and psychological conditions that facilitate either injury of the affected muscle group or provocation of the neuromuscular feedback group theorized to result in chronic trigger point manifestations.
Of the many mechanical factors that serve to perpetuate myofascial pain, deconditioning caused by lack of exercise, inadequate muscle use, or impaired posture often develops as a result of the acute myofascial pain condition. Faulty posture is becoming an increasingly common perpetuating mechanical factor with the proliferation of computerized work stations.
Systemic perpetuating factors, including nutritional deficiencies, electrolyte disorders, metabolic or endocrine dysfunction, sleep disturbances, or other concurrent illnesses have all been intuitively known to cause musculoskeletal stress, although no experimental evidence exists to support their role as a perpetuating factor for myofascial pain (Fricton 1990) (see Table 3).
Table 3. Perpetuating Factors in Myofascial Pain Syndrome
• Poor posture, sustained contraction, repetitive motion, immobility, bruxism, dental malocclusion, nonergonomic furniture
• Leg length inequity, small hemipelvis, scoliosis, spinal spondylosis or spondylolisthesis
• Inflammatory, degenerative
• Bursitis, tendonitis
• Radiculopathy, spasticity, RSD neuropathic and nociceptive pain
• Insomnia, restless legs, nocturnal myoclonus
• B complex vitamins
• Uremic or hepatic disease
• Thyroid, diabetes
Other concurrent disease
• Primary fibromyalgia, anemia, chronic fatigue syndrome, infections
• Connective tissue, vasculitis