The correlation between electrical after-activity and slowed relaxation in myotonia

The correlation between electrical and mechanical after-activity was studied in resealed fiber segments from patients with recessive generalized myotonia (Becker) and in intact fibers from normal muscles which were bathed in 9-anthracene carboxylic acid. The tests were performed in vitro on small bundles of 100 fibers or less. Electromyographic activity and contractile force were measured simultaneously. The relaxation of rested-state twitches and tetani was slowed and accompanied by after-activity in both types of preparations. Often random activity was recorded. In all cases, the contractile force was highly correlated with the electromyographic signs of myotonia. These observations support the hypothesis that electrical after-activity is fully responsible for the slowed relaxation in recessive generalized myotonia.