Calves often enlarged. Disease progresses slowly but will affect all voluntary muscles. Survival rare beyond late twenties. Almost identical to Duchenne but often much less severe. Can be significant heart involvements.
Joint deformities are common.
Frequent cardiac complications are common. Autosomal recessive, X-linked recessive. Disease may span many decades. Delayed relaxation of muscles after contraction. Congenital myotonic form is more severe.