REF-1586 · Kjertakov et al., 2025 · Journal of Thermal Biology · Paper · PubMed

Sitting 30 min in 42 °C water after each of 9 HIIT sessions over 3 weeks gave 14 male cyclists no added gain over 34 °C water: the 20-km time trial improved 1.5% vs 1.7% (between-group p = 0.99, effect size 0.0), and lactate threshold, peak power, VO2max and muscle mitochondrial markers did not change significantly in either group; from the abstract

Main caveat: 7 per group; allocation method not stated; men only; training status not given.
limited, mixed evidenceMeasured on trained athletesRead from the abstract only

Kjertakov and colleagues asked whether hot-water immersion after interval sessions adds to the endurance gains of the intervals. Fourteen male cyclists did nine HIIT sessions in 3 weeks; straight after each, one group sat in 42 °C water and the other in 34 °C water for 30 min.

Both groups rode the 20-km time trial faster afterwards, by amounts that did not differ between groups. Lactate threshold, peak power, VO2max and muscle mitochondrial measures did not change significantly in either group. The authors conclude that adding hot water after HIIT gave no extra performance benefit.

Participants

n = 14 male cyclists, 7 per group; age and training status are not given in the abstract (MEDLINE indexing: adult, young adult).

Protocol

Training: 9 HIIT sessions over 3 weeks (session content not given in the abstract).
Immersion: immediately after each session, 30 min at 42 °C (hot) or 34 °C (thermoneutral control).
Tests before and after, at 18-20 °C: incremental test to exhaustion; 20-km time trial.
Muscle biopsies (vastus lateralis, at rest): mitochondrial respiratory complexes I-V, maximal citrate synthase activity, mitochondrial respiration.
Interpretation: by p-value and effect size.

Key Results

OutcomeHot water, 42 °C (n = 7)Control, 34 °C (n = 7)Between groups
20-km time trial1.5% faster (p = 0.04)1.7% faster (p = 0.03)F(1, 12) = 0.00, p = 0.99; effect size 0.0
Lactate threshold, peak power, VO2maxno significant changeno significant change—
Mitochondrial content and respirationno significant changeno significant change—

Practical Application

Hot bath after hard intervals: in 14 male cyclists, 30 min in 42 °C water after each of 9 HIIT sessions neither added to nor took away from the 3-week time-trial gain compared with 34 °C water (1.5% vs 1.7% faster), so this trial shows no endurance benefit and no cost; it tested hot water, not sauna, with 7 riders per group.
Inference, not tested here: VO2max, threshold and mitochondrial markers did not move significantly in either group in 3 weeks, so the trial could not show whether heat changes those adaptations over a longer block.
The comparison was warm water, not dry rest, so both groups had the water and only the temperature differed (from the design).
Practice, not from this paper: the base's recovery guide does not list sauna or hot water after training as a recovery tool (SYNTH-70); longer, more frequent passive heat (5 × 45 min a week for 5 weeks) is the protocol that raised haemoglobin mass in REF-135.

Limitations

7 per group; allocation method not stated; men only; training status not given.
Three weeks and nine exposures only; the abstract reports no heat-tolerance outcome.
Written from the abstract (the full text was not read; no open-access copy in Europe PMC).
How this entry was checked and changed
Why this level

a parallel trial with 7 per arm is B by design; held one step lower because it is an abstract-only small study whose allocation method is not given. The main outcome is a time trial (performance); the mitochondrial results are markers