Dynamic properties of partially denervated muscle in children with brachial plexus birth palsy.

Contraction time, time to peak rate of tension development, half-relaxation time and maximum twitch tension of partially denervated flexor carpi ulnaris muscle were measured in children with brachial plexus birth palsy. The extent of weakness of the affected muscle was assessed by expressing its maximum twitch tension as a percentage of the tension of the contralateral normal muscle. Contraction time, time to peak rate and half-relaxation time were prolonged in children with severe weakness, while in children with moderate weakness only half-relaxation time was prolonged. The contralateral normal flexor carpi ulnaris muscle showed age differences in its contractile properties, while in the affected muscle such differences were not found. This result suggests that denervation at birth impairs normal development of muscle contractile properties.