INTERMITTENT EXERCISE EFFECTS ON PHYSIOLOGIC TOLERANCE DURING UNCOMPENSABLE HEAT STRESS

Abstract

1616 This study determined if exercise-rest cycles (vs continuous exercise) alter physiologic tolerance and exercise capability during uncompensable heat stress (UCHS). 12 men wearing protective clothing (clo = 1.5; im = 0.33) completed four trials: moderate intensity (420 W) continuous exercise(MC), moderate intensity exercise-rest cycles (MR), high intensity (610 W) continuous exercise (HC), and high intensity exercise-rest cycles (HR). Exercise-rest cycle times were 30 and 10 min. The evaporative heat loss required (Ereq) to maintain steady-state core temperature was greater than the maximal capacity (Emax) of the climate for evaporative heat loss. The average percent of Ereq/Emax was 200±18, 181±36, 417±24 and 283±28 for MC, MR, HC, and HR trials, respectively. Exhaustion from heat strain occurred in 37 of 48 trials (18 of 24 for continuous trials and 19 of 24 for exercise-rest trials) and the remaining 11 trials were discontinued due to core temperature criteria and were not included in this analysis. Exposure time was longer (P<0.05) in MR(64.2±30.4 min) than MC (44.7±14.0 min), HC (37.1±11.8 min) and HR (39.0±12.2 min). Total amount of exercise was greater(P<0.05) during MR than MC (1.5±0.6 vs 1.1±0.4 MJ), but similar (P>0.05) during HR and HC (1.4±0.4 and 1.2±0.3 MJ). Core temperatures at exhaustion were 38.6, 38.9, 38.9 and 39.0 °C for MC, MR, HC and HR, respectively and were not altered (P>0.05) by exercise intensity or exercise-rest cycles. In summary, during UCHS: a) exercise-rest cycles can extend exposure time and work capability, but only at lower exercise intensities; and b) physiologic tolerance to heat strain is not altered by exercise intensity or exercise-rest cycles.