Substituting Low-Intensity Endurance Exercise With High-Intensity Microintervals: Responses to Acute Exercise
Participants
n=13 endurance athletes (abstract); cycling exercise. Regional- to international-level athletes (5 triathletes, 4 cyclists, 2 dog skijorers, 1 runner, 1 rugby player; 6 women, 7 men), all spending at least 40% of their training time cycling; approved by the University of Jyväskylä ethics committee, Finland (accepted-manuscript full text, checked 2026-09-28: Methods, Table 1).
Protocol
4 sessions of 45 minutes in randomized crossover:
| Condition | Description |
|---|---|
| CON | Continuous low-intensity work at 88% LT1 power (classic Z1) |
| MI 4-26 | Microintervals, work:rest 4 s:26 s (a work interval starts every 30 s): alternating 111% of the power associated with VO2max and ~60 W (accepted-manuscript full text, checked 2026-09-28: Methods) |
| MI 7-23 | Microintervals, work:rest 7 s:23 s: same intensities |
| MI 10-20 | Microintervals, work:rest 10 s:20 s: same intensities |
Key point: work phase at 111% of power at VO2max — very high muscular intensity, but so brief (the paper defines microintervals as ≤15 s at ~100% VO2max) that the systemic metabolic response stays near LT1.
Key Findings
| Measure | CON vs MI 4-26 | CON vs MI 7-23 | CON vs MI 10-20 |
|---|---|---|---|
| VO2 | Did not exceed LT1 more than CON (p≥.26, ES≤0.44) | Same | Same |
| HR | Did not exceed LT1 more than CON | Same | Same |
| Lactate | No differences (ANOVA p≥.36) | No differences | No differences |
| RER (mean) | No differences | No differences | No differences |
| Fat oxidation | Did not exceed CON | Same | Same |
| RPE | No difference | No difference | Higher: 3.1±0.8 vs 2.4±0.7 (p=.01) |
Equivalence: all mean responses of MI 4-26 and MI 7-23 were statistically equivalent to CON (equivalence test p≤.02).
Variable oscillation:
Mechanism
Inference, not tested in this paper: high-intensity muscular work in ultra-short work phases does not raise systemic VO2 or lactate above LT1 because phosphocreatine and oxygen stores cover the work-phase demand and the ~60 W recovery phases allow rapid restoration; fast-twitch fibres are recruited per sprint, yet the whole-body metabolic response remains aerobic. The authors describe the class as "high-intensity muscular work blended with mild metabolic perturbation".
Main Conclusion
Microintervals = "oscillating low-intensity exercise" (authors' term). They fulfil the traditional low-intensity criteria: responses of almost all internal variables — including fat oxidation — did not exceed those of continuous work at 88% LT1. Classifying them as HIT by systemic response would be incorrect, despite the high muscular power.
Practical Application
Limitations
Only acute responses — unknown whether chronic microinterval training produces adaptations beyond standard Z1. Single work-phase intensity (111% of power at VO2max). Cycling only, 45-min sessions only. n=13. Randomized longitudinal research needed for chronic effects.