Force variability during isometric biceps contraction in children with secondary dystonia due to cerebral palsy.

Children with secondary dystonia due to cerebral palsy (CP) often exhibit excess variability in their movements. To investigate the relationship between excess variability and single-joint control in these children, we compared their force variations during single-joint isometric tasks to age-matched controls. Subjects performed isometric biceps contractions at six target force levels scaled to their maximum voluntary contraction. Similar studies in healthy adults have shown that movement variability exhibits characteristics of signal-dependent noise (SDN). We look for the first time at whether variability in single-joint force contractions in healthy children and children with CP exhibit similar characteristics of SDN. Results showed that compared to controls, patients exhibited increased variability (standard deviation about the mean), increased force error bias (difference between the mean and target), and increased dependence of force error bias on target force level. Signal-dependence of the noise was significant only in healthy subjects due to the higher overall variability in the patients. A control experiment using filtered visual feedback confirmed that overcorrection for perceived error was not the cause of the observed variability. These results contribute to a better understanding of movement variability in childhood dystonia and have potential use in therapeutic interventions designed to increase force production reliability.