Physiological maturation of muscles in childhood.

Little is known about the motor muscle physiology of growing children and it has been assumed that increasing motor dexterity and speed correspond solely to brain maturation, although there is evidence that muscles mature from studies of other mammalian species. Such a finding in children would throw light on the possible mechanics of motor delays in speech, hand function, athletic, and musical skills as well as helping towards a better understanding of pathological motor disorders such as the cerebral palsies. Our non-invasive observations of soleus-muscle relaxation times in healthy children suggest that a child's muscles are initially slow to relax but relaxation doubles in speed up to adult rates by early adolescence. Half-relaxation times halved from about 90 ms at age 3 to about 40 ms at age 10. On the basis of these studies, we suggest that muscle maturation rate-limits motor tasks; which should be borne in mind when considering motor development in muscles and when attempting to determine the effects of early brain or spinal cord damage. Such methods of measuring the dynamics of muscle open up new areas of research and offer the possibility of developing operationally-defined in-vivo measurements for use in motor assessment and for following up the effects on muscle or treatments such as physiotherapy, orthotics, surgery, and drugs.