Muscle fatigue is the most disabling symptom in primary fibromyalgia (PF), which in addition is characterized by generalised pain and muscle stiffness. In order to assess whether the fatigue is of central and/or peripheral origin, skeletal muscle function was studied by measuring maximum voluntary hand grip strength, and by measuring various contraction characteristics in the adductor pollicis muscle after electrical stimulation of the ulnar nerve. The PF-patients were also studied after a regional sympathetic blockade of the forearm with guanethidine. A lower hand grip strength was found in the PF-patients compared to the controls, before as well as during the sympathetic blockade. The developed force, measured during electrical stimulation, did not differ between patients and controls. A lower muscle relaxation rate was found in the PF-patients. The relaxation rate increased in the PF-patients during the sympathetic blockade. The results indicate both a central and a peripheral cause of muscle dysfunction. Activity in the muscle sympathetic system may be one link in the chain of events that leads to muscular symptoms in PF.
Changes of the terminal innervation pattern in polymyositis were studied in nine patients using single fiber electromyography (SFEMG) and histochemical investigations of muscle biopsies. In all patients, and in 16 out of 18 muscles studied, reinnervation was found. The preceding denervation could be caused either by segmental necrosis of the muscle fiber that disconnects one part of the fiber from the end-plate region or by direct intramuscular nerve involvement.
Muscle fibre degeneration and regeneration, inflammation in intramuscular connective tissue and hypoxia in resting muscle are not necessarily associated with pain. However, when sustained or dynamic muscle contractions are performed in an ischaemic muscle, severe pain develops. In the chronic muscle pain syndrome called fibromyalgia (or fibrositis) the most likely cause of the pain is a combination of muscle tension and muscle hypoxia. This conclusion is supported by the finding of a pathological distribution of tissue oxygen tension in painful muscles and a subjective feeling of muscle tension and muscle stiffness in the majority of patients. A decrease in high energy phosphates is found in biopsies from painful muscle. The most characteristic morphological finding is the so called ragged red fibre, which is a finding that can been seen in mitochondrial disorders. The morphological and chemical findings are possibly a consequence of a long standing hypoxia. The possibility that activity in muscle sympathetic nerves is important for the development of chronic muscle pain is discussed.
Isometric muscle force was measured in 217 normal children aged 3.5-15 years. The standard error of a single determination made by the same observer was ca. 9% of the muscle force. When two measurements were made by different observers the standard error of the difference was estimated at ca. 17%. Reference values for isometric force are given for boys and girls separately. With regard to 7 of the 10 muscle groups tested the force was significantly greater in boys than in girls as early as at ca. 10 years of age. Age and weight were the most important predictors of muscle force.
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