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Previous research has indicated that short-duration, high-intensity work intervals performed at velocities associated with maximal oxygen uptake (vJOURNAL/jscr/04.02/00124278-200702000-00034/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max) combined with active recovery intervals may be effective in eliciting improvements in endurance performance. This study was designed to characterize selected physiological responses to short-duration (≤60 seconds) interval work performed at velocities corresponding to 100% of vJOURNAL/jscr/04.02/00124278-200702000-00034/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max. Twelve men participated in 3 randomized trials consisting of treadmill running using work (W)/recovery (R) intervals of 15 seconds W/15 seconds R (15/15); 30 seconds W/15 seconds R (30/15); and 60 seconds W/15 seconds R (60/15). Work intervals were performed at 100% of vJOURNAL/jscr/04.02/00124278-200702000-00034/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max, whereas R intervals were performed at 50% of vJOURNAL/jscr/04.02/00124278-200702000-00034/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max. A fourth trial consisting of continuous work (C) at 100% of vJOURNAL/jscr/04.02/00124278-200702000-00034/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max was also performed. All subjects completed the 15/15 and 30/15 trials; however, only 5 of the 12 completed the 60/15 trial. The percentage of JOURNAL/jscr/04.02/00124278-200702000-00034/ENTITY_OV0312/v/2017-07-20T235329Z/r/image-pngO2max (mean ± SD) during 15/15 (71.6 ± 4.2%) was significantly lower (p ≤ 0.05) than the percentages during 30/15 (84.6 ± 4.0%), 60/15 (89.2 ± 4.2%), or C (87.9 ± 5.0%). Similar results were found for heart rate and perceived exertion. Blood lactate concentrations following exercise were significantly lower (p ≤ 0.05) in 15/15 (7.3 ± 2.4 mmol·L−1) than in the other trials. No significant differences (p > 0.05) existed among 30/15 (11.5 ± 1.8 mmol·L−1), 60/15 (12.5 ± 1.8 mmol·L−1) or C (12.1 ± 1.8 mmol·L−1). High intensity, short-duration 2:1 W/R intervals appear to produce responses that may benefit both aerobic and anaerobic energy system development. A 4:1 W/R ratio may be an upper limit for individuals in the initial phases of interval training.

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