Head and body positions affect thermoregulatory tonus in deltoid muscles.

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