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Muscle activity and postural behaviour in newborn infants. A polymyographic study
Insights
Newborn muscle activity, measured by electromyography (EMG), shows distinct patterns during different behavioral states. Chin muscle activity is particularly specific in state 1, with tonic activity increasing stepwise and decreasing abruptly.
Area of Science:
- Neonatal physiology
- Neuroscience
- Developmental motor control
Background:
- Understanding spontaneous muscle activity in newborns is crucial for assessing neurological development.
- Previous research has explored neonatal motor patterns, but detailed analysis of tonic activity across behavioral states is limited.
Purpose of the Study:
- To analyze spontaneous electromyography (EMG) activity of various muscles in healthy newborns during different behavioral states.
- To characterize tonic muscle activity patterns, particularly in state 1 (quiet sleep), and their relationship to postural behavior and state transitions.
Main Methods:
- Surface electromyography (EMG) was used to record muscle activity in 9 healthy newborns on days 5-6 of life for 6-8 hours.
- Infants were positioned in alternating side positions.
- Averaged EMG activities were analyzed in relation to behavioral states (state 1 and state 2) and postural behavior, using an improved recording technique and logarithmic presentation.
Main Results:
- Chin muscles exhibited the most specific EMG pattern in state 1, with tonic activity present only 15% of the time and increasing stepwise after phasic activity.
- Other muscles, including neck and forearm, showed less tonic activity in state 1, with durations varying by muscle group.
- Tonic activity exceeding 30 seconds was absent in state 2, except during transitions. Muscles in the chin, neck, and upper extremities were more active than lower extremities in both states.
Conclusions:
- Sustained tonic activity in newborns, particularly in state 1, may be linked to an increased gain in the gamma-loop, potentially influencing respiratory regularity.
- Observed muscle activity differences reflect maturational and functional variations between muscle groups.
- Systematic changes in body position did not consistently affect motility or tonic activity.
Abstract:
In a group of 9 healthy newborns, spontaneous activity of different muscles was studied for 6 or 8 hours by means of surface electromyography on the 5th or 6th day of life. According to the local nursing routine the infants were placed in alternating left and right side positions. Averaged EMG activities were analysed with respect to behavioural states and observed postural behaviour. An improved recording technique and the logarithmic presentation of averaged EMG activities allowed the study of characteristics of tonic activity in state 1. The chin muscles showed the most specific EMG pattern in state 1. Chin muscle quiescence was observed during only 15% of that state. Lack of tonic activity was observed more in the beginning than at the end, but never in the middle of state 1. By visual analysis of 41 patterns of averaged EMG activities from the chin muscle it was found that increases of tonic activity only happen stepwise following concomitant phasic activities. Relaxation of tonic activity is either asymptotic or abrupt or by steps. The latter two are especially characteristic for the transition to state 2. Long periods of sustained tonic activity and low motility were characteristic for a long lasting state 1. The interval between the beginning of state 1 and the first increase of tonic activity varied more than the interval between the end of state 1 and the final drop of tonic activity back to the noise level. Other muscles showed less tonic activity in state 1: the neck area (sterncleidomastoid muscle 48% of state 1 time, trapezius m. 32%) was followed by the muscles of the forearm (extensor carpi m. 18%, flexor carpi m. 16%). These muscles also lead the rank order of the modulation of tonic activity. Tonic activity sustained for more than 30 sec. was not observed in state 2, except during transitions. In state 2 as in state 1 the muscles from chin, neck and upper extremities were more active than the muscles from the lower extremities. Durations of activities ranged from 28% (chin) to 13% (ant. tibialis) of state 2 time. No dominance of flexor muscles, which might account for the observed flexed posture, was found. Systematically changed side positions during the recording had no consistent effect on either motility or presence of tonic activity when corresponding muscles from the upper- and the lower-lying body side were compared during state 1. Likewise in state 2 no consistent effect on the total duration of phasic activity was observed. The results are discussed in relation to other known state dependent motor phenomena. It is suggested that sustained tonic activity results from an increased gain of the gamma-loop which may also contribute to the regularity of respiration. Differences between muscles probably reflect maturational and functional differences.