Electromyographic studies on flexor hypertonia of the extremities of newborn infants.

In order to investigate flexor hypertonia of the extremities in newborns, a neurological examination and superficial electromyogram (EMG) recordings were carried out on 50 full-term infants both before and after the first 48 hours of life (the 'birth-shock period). EMG amplituds on the flexor side of the upper arm in the traction response were low during the first 48 hours despite flexion of the elbow (hypertonia); after the first 48 hours EMG amplitudes increased in a large percentage of the infants although elbow flexion decreased (hypotonia). This finding suggests that the frflex response to passive movement (traction) is weaker during the first two days of life than after this time. Flexor tone of the upper arm in recoil of the forearm increased during the first 48 hours in spite of low EMG amplitudes; after the first 48 hours the amplitudes increased although flexor tone of the upper arm decreased. The popliteal angle was more than 90 degrees in 27 infants and less than 90 degrees in 23 infants during the first 48 hours; after this period the angle was more than 90 degrees in 11 infants and less than 90 degrees in 39 infants. EMG amplitudes on the flexor side of the thigh in most cases increased after the first 48 hours for both groups of infants, i.e. those with an angle more than 90 degrees and those with an angle less than 90 degrees. It is suggested that flexor hypertonia of the extremities in newborn infants is not a result of muscle contraction (i.e. real muscle strength) but rather is residual from the infant's position in utero.