A painless, non-invasive technique for measuring the effects of age on the relaxation of calf muscle in 22 healthy children is reported from the Departments of Paediatric Neurology and Physiology, Royal Hospital for Sick Children, Edinburgh. The study was undertaken as a prelude to investigations of contractile properties of muscles in children with cerebral palsy and other motor handicaps. Soleus muscle twitches were generated by a single Achilles tendon tap which caused a monosynaptic reflex muscle-twitch contraction, recorded by EMG. Half-relaxation times halved from about 90 ms at age 3 years to 40 ms at age 10. Compared to a 19-year-old healthy male, relaxation was prolonged in a 3-year-old boy. The younger the child, the slower the muscle-relaxation time. Muscle maturation rate-limits motor tasks, and modifies the effects of early brain or spinal cord damage. 
COMMENT. Having attempted quantitative measurements of motor function and muscle tone and relaxation in children with cerebral palsy for the purpose of evaluation of muscle relaxant drugs, I am aware of the paucity of reliable measures of muscle function in children . Measurements of muscle tone by speed and height of hammer recoil were abandoned in favor of quantitative tests of muscle function involving range, rapidity, strength, and coordination of voluntary movements. The Edinburgh method of measurement for muscle relaxation should be of value in assessment of various treatments of cerebral palsy.