Suppression of shivering by breath holding, relaxation, mental arithmetic, and warm water ingestion.

Four methods of suppressing cold-induced shivering were evaluated in 26 young male volunteers exposed to 0.0 +/- 1.0 degrees C air for 135 min. Voluntary relaxation of musculature (R), breath holding (BH), warm (50 degrees C) water ingestion (W), and performance of a mental arithmetic task (MA) were applied in a counterbalanced order following 2 h of cold exposure. Surface electromyograms of seven muscles were recorded and converted to root mean square voltage (RMS) as a measure of shivering intensity. Mean skin and rectal temperatures decreased significantly, 4.9 degrees C and 0.3 degrees C, respectively (p < or = 0.01). Mean reduction of EMG activity was 35% during R, 24% during BH, 18% during MA, and 5% during W. R was significantly more effective than BH, MA, and W, and BH and MA were significantly more effective than W in reducing shivering. These results indicate that, at small decreases in rectal temperature, shivering can be voluntarily suppressed to some extent during relaxation, breath holding, and mental arithmetic.

Core temperature response to immersed bicycle ergometer exercise at water temperatures of 21 degrees, 25 degrees, and 29 degrees C.

A bicycle ergometer modified for aquatic exercise was used to determine the effects of immersion on core temperature during submaximal exercise at different water temperatures. An exercise intensity (60% of maximal oxygen consumption) and duration (30 minutes) considered appropriate for cardiovascular conditioning were used. These data will be useful in cardiovascular and leg-strengthening hydrotherapy programs. Rectal temperature, skin temperature, and a rating of thermal comfort were studied in five normal men (14.8% +/- 5.6% fat) during headout immersion at water temperatures of 21.1 degrees, 25.3 degrees, and 29.4 degrees C and exercise in air of 21.1 degrees C. Subjects were immersed for 30 minutes during static and exercise (63% +/- 0.6% maximal oxygen consumption) conditions. Data were collected every 5 minutes and analyzed by repeated measured analysis of variance. At water temperatures, rectal temperature fell from control during static immersion (p less than or equal to 0.05) and was lower than control at the end of the 30-minute recovery period (p less than or equal to 0.05). During exercise there was no change in rectal temperature at water temperatures of 21.1 degrees and 25.3 degrees C; however, rectal temperature rose at water temperatures of 29.4 degrees (p less than or equal to 0.05) and air 21.1 degrees C (p less than or equal to 0.05). At the end of recovery rectal temperature was lower than control at water temperatures 21.1 degrees C (p less than or equal to 0.05) and greater than control at water temperatures 29.4 degrees C (p less than or equal to 0.05). There was no change from control in rectal temperatures at water temperatures 25.3 degrees C and air at 21.1 degrees C. These results indicate that immersion in 25.3 degrees and 21.1 degrees C water effectively attenuates the rise in rectal temperature during exercise at 63% of maximal oxygen consumption, whereas immersion in 29.4 degrees C water does not. In addition, both skin and rectal temperatures affect perception of thermal state but do not give the subjects an accurate estimation of thermal balance.

Synchronized slow-amplitude modulations in the electromyograms of shivering muscles.

The electromyograms (EMG) of shivering human subjects exposed to 0 degrees C air in an environmental chamber were analyzed to detect slow-amplitude modulations (SAMs, less than 1 Hz) in the EMGs of widely separated muscles and to study the relationship of these SAMs to respiration rate and skin temperature. Distinct amplitude modulations were observed in the raw EMGs during shivering. The peaks in EMG activity occurred simultaneously in the majority of the monitored muscles in all subjects. Pearson correlations between the average rectified EMGs of 93% of the muscles were significant (P less than 0.05). Visual analysis of the EMG and respiration signals indicated that the peaks in muscular activity occurred 6-12 times/min, whereas respiration ranged from 10 to 23 cycles/min. For all subjects respiration was at a higher frequency than amplitude modulation in the EMG. Comparison of EMG records with expiratory flow rate traces in shivering subjects indicated no one-to-one correlation between the occurrence of respiration and EMG amplitude modulations. Respiratory flow rate and average rectified EMG showed significant correlation in only 33% of the cases. In addition, skin temperature changes could not be correlated with the SAMS.