Cardiovascular Drift During Continuous And Interval Exercise In The Heat

During continuous (CONT) and interval (INT) exercise in the heat, rectal temperature (Tre) and heart rate (HR) can increase over time while V̇O2max can temporarily decline. It is unknown if CONT or INT exercise differentially affects Tre, HR, or V̇O2max when average work rate and duration are matched during exercise in the heat. PURPOSE: To test the hypotheses that during INT compared to CONT exercise, Tre will increase by similar magnitudes, HR will increase by a larger magnitude, and V̇O2max will decrease by a greater magnitude in the heat when average exercise intensity and duration are matched. METHODS: Six subjects (5 men) performed a graded exercise test (GXT) on a cycle ergometer to determine V̇O2max in a temperate (~22 °C) environment and 4 experimental trials in a hot (~35 °C) environment immediately followed by a GXT to determine V̇O2max. For 2 trials, subjects cycled at 60% V̇O2max for 15 or 43 min (15CONT and 43CONT). The 2 remaining trials began with 8 min at 60% V̇O2max followed by either 1 (15INT) or 5 (43INT) rounds of intervals (4 min at 75% V̇O2max and 3 min at 40% V̇O2max). RESULTS: Tre was higher at the end of the 43-min trials (38.0 ± 0.1 °C) compared to the 15-min trials (37.3 ± 0.1 °C; p = <0.001) but was not different between conditions (p = 0.13). Change in Tre from min 15 to min 43 of the 43-min trials was also not different between CONT and INT (p = 0.70). HR at the final minute of exercise was higher (p = 0.006) at min 43 of the 43-min trials (163 ± 4 bpm) compared to min 15 of the 15-min trials (146 ± 6 bpm) but was not different between conditions (p = 0.43). V̇O2max was lower after the 43-min trials (2.9 ± 0.4 L/min; p = 0.02) compared to the 15-min trials (3.2 ± 0.4 L/min) but was not different between conditions (p = 0.22). CONCLUSIONS: During INT and CONT exercise in the heat, HR increased by similar magnitudes (17 bpm); and HR was similar at the end of the 43-min trials (163 bpm). While V̇O2max decreased by 9% from 15 to 43 min, the decrease was not different between CONT and INT exercise. Similarly, Tre increased over time, but the increase was not different between conditions. This suggests there are similar cardiovascular and thermoregulatory responses to INT and CONT exercise in the heat. Supported by: This study was funded by New Mexico State University College of Health, Education, and Social Transformation.