REF-711 · Lei, Stannard, Perry, Schlader, Cotter & Mündel, 2017 · The Journal of Physiology, 2017;595 · Paper · PubMed
In well-trained women, cycle phase did NOT affect performance in the heat
Humidity did, costing 7% of work done
Protocol
n=10 well-trained eumenorrheic women, unacclimated. Early-follicular versus mid-luteal phase, each in two environments matched for WBGT (27°C): dry heat versus humid heat — four trials. Each trial: rest and cycling at fixed intensities (125 and 150 W) to assess autonomic regulation (rectal and skin temperature, local sweat rate, forearm blood flow), then a self-paced 30-min work trial to assess behavioural regulation; outcome = total work done (kJ).
Key Findings
| Comparison | Work done | Verdict |
|---|---|---|
| Early-follicular vs mid-luteal | 257 vs 255 kJ | no difference (P = 0.62) |
| Dry vs humid heat (WBGT-matched) | 263 vs 248 kJ | dry 7 (±9)% higher (P<0.01) |
At rest and fixed-intensity exercise, rectal temperature was ~0.2°C higher in the mid-luteal phase, independent of environment — the offset was present, and it did not cost performance.
Local sweat rate and/or forearm blood flow showed a phase × environment interaction; mean skin temperature did not differ between phases but was higher in dry heat. In the work trial, dry heat gave equivalent autonomic regulation and thermal strain but higher evaporative cooling (by 16 W·m⁻²).
Authors' conclusion: behavioural responses (self-selected pace) nullified the thermodynamic and autonomic differences associated with menstrual phase and with dry vs humid heat.
The environment beat the calendar. Two environments with the same WBGT produced a 7% performance gap, while two cycle phases produced none.
Practical Application
In n=10 well-trained, unacclimated women in WBGT-matched heat (27°C), 30-min self-paced work did not differ between early-follicular and mid-luteal phases (257 vs 255 kJ), but was 7% higher in dry than humid heat (263 vs 248 kJ) — do not attribute a bad hot session to cycle phase by default; check humidity first, that is where the measurable performance effect was.
WBGT-matched does not mean equally hard. At the same 27°C WBGT, humid heat cost work because evaporative cooling was lower. Not from this paper; the base's own practice: plan pacing and cooling around humidity, not just the thermometer.
Inference, not tested: training status may change the answer (REF-710 found a luteal deficit in recreationally active women in hot-humid air; this study found none in well-trained women — but protocols also differed: time to fatigue vs self-paced work). When advising, state which population the advice comes from.
Not from this paper; the base's own practice: a ~0.2°C luteal offset is real but small in a trained athlete — worth knowing, not worth restructuring a week around.
Limitations
n=10. A null result in a sample this small is weak evidence of no effect; it cannot exclude a small phase effect.
Unacclimated women — an acclimated athlete's response may differ.
Self-paced work-done protocol at a fixed WBGT: informative and ecologically reasonable, but a single exercise model.
Two phases only, in eumenorrheic women; hormonal contraceptive users not represented (REF-338).
The dry-versus-humid comparison is the strong finding here; the phase comparison is the underpowered one.
How this entry was checked and changed
Content correction 2026-09-27
Type "randomised crossover" -> within-subject crossover (randomisation not stated in the abstract); Tier 3-4 -> Tier 3 (well-trained, VO₂max 57); "hormone status verified" removed (the abstract says eumenorrheic only).
Added the abstract's protocol (fixed 125/150 W bouts, 30-min self-paced work trial), P values, the 16 W·m⁻² evaporative-cooling difference and the authors' behavioural-thermoregulation conclusion; skin temperature did not differ between phases.
Lead ▶ now states the study's numbers; "training status changes the answer" labelled as inference (protocols also differ); pacing/cooling and "not worth restructuring a week" labelled as the base's own practice.