Physiological Responses to Treadmill HIIT with Fixed vs Self-Selected Recovery without Feedback in Runners

This dataset contains cardiopulmonary exercise data (gas exchange and heart rate) processed to 5-second averages and
blood lactate values collected during a treadmill high-intensity interval training (HIIT) protocol performed under
three recovery strategies:

- HIIT 1:0.5 — 2 min work / 1 min passive rest
- HIIT 1:1 — 2 min work / 2 min passive rest
- Self-selected — passive rest duration chosen by the runner, without time or heart-rate feedback

Design:
- Within-subject crossover: each participant completed all three conditions in counterbalanced order (1–2 weeks between visits).
- Each session: 6 × 2-min work bouts at constant speed, 1% incline, with passive standing rest between bouts.
- Gas exchange + HR recorded continuously; exported here as 5-second means.
- Lactate measured discretely during recovery time points (see variables below).