HEAD AND BODY POSITIONS AFFECT THERMOREGULATORY TONUS IN DELTOID MUSCLES

Electromyograms (EMGs) were recorded from both deltoid muscles (DMs) w hen subjects were sitting, standing, and bending the head laterally at different cold sensations to clarify the role of head and body positi ons in the thermoregulatory tonus. At room temperature (22 degrees C), neither EMG related to thermoregulatory muscle tonus (TMT) nor were c hanges in EMG intensity observed during changes in posture. At a cold temperature (5 degrees C), when the subjects felt slightly cold or col d, the averaged surface EMG intensity of TMT was similar to 2-3 mu V d uring sitting with the head held upright. The EMG of TMT was 7.23 +/- 2.99 mu V in subjects sitting with the head upright when feeling cold and extremely cold and increased to 14.75 +/- 5.15 mu V (P < 0.01) whe n the subjects stood at ease and to 19.96 +/- 11.01 mu V (P < 0.01) wh en the subjects stood on tiptoes. Lateral head bending in subjects sta nding at ease and feeling extremely cold increased the EMG 40-60% (P < 0.01) in the contralateral DM without any significant effect on the i psilateral DM. The results suggest that in humans TMT is influenced by tonic postural reflexes.