Dynamic electromyography and muscle biopsy changes in a 4-year follow-up: study of patients with a history of polio.

Eighteen patients who had had polio 29-56 years prior to the first investigation were studied on two occasions, 4 years apart. Isokinetic and isometric strength measurements and Macro EMG were performed in 28 legs. Muscle biopsy specimens were obtained on both occasions from 11 legs. On average the muscle strength was 56% of control values at the first examination, and decreased by another 8% during the observation period. The muscle fiber area was increased compared to that of controls and did not change significantly. Macro EMG, comprising muscle fiber area and number of muscle fibers, and/or single fiber EMG showed clear signs of reinnervation in all legs. The motor units at the first examination were increased 11-fold, on average, compared with age-matched control values. During the observation period, reinnervation continued and the size of motor units increased by another 56% as a result of ongoing denervation, that is, loss of neurons. This compensation was particularly pronounced in patients with stable conditions. The parameters studied did not reveal any definite pattern predicting future development of new muscle weakness in individual subjects.