REF-001 · Lorenzo et al., 2010 · Journal of Applied Physiology, Vol. · Paper

Heat acclimation improves exercise performance in cool and hot conditions

B+moderate evidenceMeasured on trained to highly trained athletesWritten before October 2026, being re-checked

Participants

n=20: 12 trained cyclists (heat acclimation group) + 8 matched control subjects

Protocol

Heat group: 10 days, ~50% VO2max in 40°C

Control: 10 days of identical exercise in a cool (13°C) environment

Tests: VO2max, time trial, lactate threshold — in cool (13°C, 30% RH) AND hot (38°C, 30% RH) conditions, before and after

VO2max and lactate-threshold tests were preceded by warm (41°C) or thermoneutral (34°C) water immersion to induce hyperthermia (0.8-1.0°C) or sustain normothermia

Key Results

Parameter	Cool	Hot
VO2max	+5% (66.8→70.2 ml/kg/min, p=0.004)	+8% (55.1→59.6, p=0.007)
Time trial (work done)	+6% (879.8→934.7 kJ, p=0.005)	+8% (718.7→776.2 kJ, p=0.014)
Power at lactate threshold	+5% (3.88→4.09 W/kg, p=0.002)	+5% (3.45→3.60, p<0.001)
Plasma volume	+6.5 ± 1.5%	—
Max cardiac output	+9.1 ± 3.4%	+4.5 ± 4.6%

Control group: no changes in VO2max, time-trial performance, lactate threshold or any physiological parameter.

Mechanism

Inference, not tested: plasma volume expansion (+6.5%) → larger stroke volume → higher maximal cardiac output (measured, +9.1% in cool) → greater oxygen delivery. The authors' conclusion: heat acclimation improves aerobic performance in temperate-cool conditions, i.e. the benefit is not confined to racing in the heat.

Practical Application

Finding: in 12 trained cyclists, 10 days of ~50% VO2max exercise at 40°C raised VO2max by 5%, time-trial work by 6% and power at lactate threshold by 5% in cool conditions (and by 8%, 8% and 5% in the heat); 8 matched controls doing the same work at 13°C changed nothing. Heat acclimation is a training aid for cool races too, not only heat preparation.

Dose tested: 10 consecutive days, ~50% VO2max in 40°C — a chamber protocol. Overdressing or sauna were not tested here.

Not from this paper; the base's own practice: heat exposure 3-4×/week in a build block, then 2×/week to maintain; no entry in this base measures how fast the adaptation decays.

Limitations

Small sample (n=12 acclimated); trained cyclists only; matched rather than randomized controls; 10-day duration optimality unknown; 40°C chamber may not equal overdressing (protocol difference).

How this entry was checked and changed

Why this level

regraded 2026-09-27 from A: parallel controlled trial, n=20 (12 acclimation + 8 matched controls)

Content correction 2026-09-27

Type "RCT" -> controlled trial with matched controls (abstract does not describe randomization); level A -> B+ (parallel controlled trial, n=20).

Practical bullets "+5% FTP", "3-4×/week via overdressing or sauna", "2×/week minimum (Périard: degradation within 48h)" -> FTP was not measured (power at lactate threshold was); the frequency advice is relabelled as the base's own practice; the 48-hour claim is not in REF-002's abstract and was removed.

Lead bullet now states the study's own result (n, protocol, +5/+6/+5% in cool) instead of a training prescription.

Resync 2026-09-28: Pointed to REF-002/REF-061 for decay of adaptations; REF-002 no longer carries decay -> says no entry measures decay.