Developmental trends of jumping reaction time by means of EMG in mentally retarded children.

Developmental trends of jumping reaction time (RT) in mentally retarded children were studied cross-sectionally. Fifty-three boys, ranging in CA from 7 to 18 years, took part in the jumping RT task. RT measurements were obtained from EMG of an agonist muscle (rectus femoris). There was a significant decrease in RT with increasing CA. Correlation between RT and MA was highly significant, but discontinuity in the distribution was found between MA of 6 and 7 years. The subjects with a MA below 6 were characterized by large variability as well as slowness in RT. RT was not correlated with CA holding MA constant among the subjects with a MA below 6. The subjects with a MA above 7 showed relatively small variability within and between subjects. RT in subjects with a MA above 7 decreased with CA holding MA constant but RIV showed no change. RT in children with a very low MA is prolonged by inefficiency which reflects difficulty in maintaining attention or arousal level. The efficiency in information processing may change between the MAs of 6 and 7 years.