Thermal strain of intermittent and continuous exercise at 10 and 35 °C in man

While the heat strain of continuous exercise in a hot environment has been relatively well documented, little is known concerning the strain of intermittent exercise in the heat, particularly for short-duration (30 min or less) exercise. The purpose of the present study which had ethical committee approval, was to examine the physiological and metabolic responses to continuous (CT) and intermittent (I) exercise of the same mean power output in hot (H, 350C) and cool (C, 10 °C) environmental conditions. Eight well-trained male subjects performed four trials separated by - 1 week. Intermittent trials consisted of 90 s at 40 % VP2 max) a 5 s rest, a maximal sprint of - 6 s (constant work) and a passive rest of - 15-20 s. Continuous trials consisted of cycling at 60 r.p.m. at the same average exercise intensity as that achieved during the intermittent test. At the end of each 30 min trial subjects cycled to exhaustion at 100% Vo2max Thermal, physiological and metabolic variables were recorded at 5 min intervals and at exhaustion throughout all trials. A trial was terminated at volitional exhaustion, or if the subject appeared to be unduly distressed or if the aural temperature rose above 39 °C. Table 1. Mean aural and rectal temperatures, heart rate (HR), oxygen uptake (VO2), blood lactate and plasma ammonia during the 30 min exercise test during the cool continuous (CCT), cool intermittent (CI), hot continuous (HC) and hot intermittent (HI) trials for eight male subjects Aural Rectal HR (OC) (OC) (beats min') CCT 370 + 07 376 + 05 127 + 14 CI 372 +07 378 + 0-6 151 + 10 HCT 37-5 + 0-6 377 + 05 146 + 11 HI 37-8+009 38&0+07 166 +9 ab abd ab VO, Lactate Ammonia (1 min-') (mM) (aM) CCT 2-24+0-17 1P4+0-5 28-5+11P9 CI 2-61 + 0-44 6-5 + 29 669 + 31-9 HCT 2-27+0X18 1-8+007 58-2+17-2 HI 2-74 + 0-35 7*9 + 36 100-3 + 550 bc abd ab Values are means + S.D; a, P< 0 01 H vs. C; b, P< 0 01 I vs. CT; c, P < 0 05 H vs. C; d, interaction P < 0 01. Two-way analysis of variance with repeated measures. Some of the thermal, physiological and metabolic responses to the four exercise trials are shown in Table 1. Time to exhaustion was 441 + 268, 312 + 186, 289 + 65 and 82 + 71 s respectively for the CCT, CI, HCT and HI trials (H vs. C, P < 0 05; I vs. CT, P < 0 01). These results demonstrate that for the same mean power output, intermittent exercise provided a greater thermal strain than continuous exercise at both 10 and 35 'C.