Purpose: During very short (≤15 s) high-intensity (∼100% maximum rate of oxygen uptake [VO2]) intervals, defined here as microintervals, VO2 and lactate concentrations can remain near the first lactate threshold (LT1). However, no research has yet identified microinterval combinations that maintain acute responses entirely within the low-intensity zone below LT1, accounting for oscillations caused by pulsating power output.

Methods: Thirteen endurance athletes cycled for four 45-minute exercises: continuous low-intensity exercise (CON; power: 88% LT1) and 3 microinterval exercises alternating 111% power associated to maximum rate of VO2 and ∼60 W, with cycles of 4-6, 7-23, and 10-20 s.

Results: Compared with CON, heart rate and VO2 did not exceed LT1 more during microintervals (P ≥ .26, effect size ≤ 0.44), and rating of perceived exertion was greater only during 10-20 s (3.1 [0.8] vs 2.4 [0.7], P = .01). Blood lactate and mean respiratory exchange ratio did not differ between exercises (analysis of variance P ≥ .36). All mean responses of 4-26 and 7-23 were equivalent to CON (P ≤ .02). Oscillation of the examined variables during microintervals was largely greater than CON only for heart rate (>100% greater; P ≤ .001, effect size ≥ 1.7), while oscillation of VO2 was at most 25% greater (P ≥ .02, effect size ≤ 0.70).

Conclusions: Microintervals could be classified as oscillating low-intensity exercises as they fulfill the traditional low-intensity criteria-responses of almost all internal variables did not exceed those of CON, not even fat oxidation. Microinterval exercises form a unique exercise class in which high-intensity muscular work blends with mild metabolic perturbation.

Keywords: VO2max; low-intensity exercise; microinterval exercise; short high-intensity exercise.