Human myotatic reflex development of the lower extremities.

Lower extremity (LE) myotatic reflexes were tested by percussion (taps) to the patellar and Achilles tendons. Surface electromyographic recordings were obtained from 5 LE muscles during tendon taps. Results indicated that LE myotatic reflexes underwent considerable change during early human development. The changes were non-linear and highly variable. Reflex irradiation (the presence of reflex responses in muscles other than the one being directly stimulated by a tendon tap) was present in the newborn but to a lesser extent than was in evidence later on during the first year of life. The percentage of time reflex irradiation was detected in heteronymous muscle groups appeared to achieve maximal levels during the first year and then progressively decline. The decline in reflex irradiation was most dramatic between the first and second years of life. Irradiated responses were still recorded from 2 year-old children but with less frequency than in children less than 1 year of age. With the exception of responses in muscles that were direct antagonists to the stimulated muscle, irradiation was not observed in children 3-5 years of age.