[Staircase phenomenon in children with progressive muscular dystrophy and dermatomyositis].

The electrical and mechanical activity of the isometric twitch of flexor carpi ulnaris muscle during two per second indirect supramaximal stimulation for 90 sec was examined in 14 children with muscular distrophy and 8 children with dermatomyositis. The muscle electrical responses show no significant changes in the amplitude of its first negative phase. The first derivative of the dinamogram shows some of the following abnormalities in 8 of the examined children with muscular distrophy and in 2 of these with dermatomyositis: 1. Prolonged and increased negative staircase; 2. Insufficient or absent positive staircase potentiation. These abnormalities of the staircase phenomenon disclose disorders of the contractile function of the examined muscle.