REF-1744 · Moore et al., 2026 · Journal of Strength and Conditioning Research, · Paper · PubMed

Average and peak power in a HIIT session did not differ significantly between a low- and a high-hormone phase in 35 women (randomised crossover, 4 trials each); the work-rest ratio mattered instead

20 s on/10 s off gave about 37 W more average power than 1 min on/1 min off — and the within-session power drop was larger in the low-hormone phase (by 11.2 W·s⁻¹ across both protocols); how phase was determined, and whether contraceptive users took part, is not in the abstract

Main caveat: One acute session per condition; power and a derived fatigue index only.
limited evidenceMeasured on non-athletes to trained athletesRead from the abstract only

Moore and colleagues asked whether hormonal phase changes performance and fatigue in a HIIT session, and whether the answer depends on the work-rest ratio.

Thirty-five women (mean age 24) each did four work-matched HIIT sessions: ten 1-min efforts with 1-min rests and twenty 20-s efforts with 10-s rests, each once in a low-hormone and once in a high-hormone phase, in randomised order. Average and peak power were higher with the 20:10 format and did not differ significantly between phases.

The power drop within the session, peak minus minimum power over time, was larger in the low-hormone phase for the 20:10 format and when both formats were combined, but not for the 1:1 format.

The authors conclude that power performance was barely affected by phase, that fatigue may vary with phase, and that choosing the work-rest ratio may help manage fatigue across the cycle, particularly in the low-hormone phase.

Participants

n = 35 women, age 24.3 ± 6.1 years.
Not in the abstract: training status, sport, menstrual-cycle regularity, how phases were confirmed, hormonal contraceptive use.

Protocol

Four trials per woman in randomised order of phase and protocol: HIIT 1:1 (10 rounds of 1 min on, 1 min off) and HIIT 2:1 (twenty 20-s efforts, each followed by 10 s of rest), work-matched, each in the low-hormone phase (LHP) and the high-hormone phase (HHP).
Outcomes: average power (AP), peak power (PP) and power drop (PD), defined as peak minus minimum power divided by time.
Not in the abstract: the ergometer or exercise mode, the intensity of the work intervals, the phase definitions and timing, hormone measurements, the statistical model, and whether ± values are SD or SE.

Key Results

ComparisonDifferenceP
Average power, HIIT 1:1 minus HIIT 2:1−36.6 ± 2.3 W< 0.001
Peak power, HIIT 1:1 minus HIIT 2:1−34.1 ± 4.0 W< 0.001
Average and peak power, LHP vs HHPno significant difference (no values given)—
Power drop, HIIT 2:1, LHP minus HHP11.4 ± 4.7 W (unit as printed)0.020
Power drop, HIIT 1:1, LHP vs HHPno significant difference (no values given)—
Power drop, both protocols, LHP minus HHP11.2 ± 4.9 W·s⁻¹0.031
The abstract gives no absolute power values, no effect sizes and no confidence intervals.

Practical Application

Do not move or soften a HIIT session because of cycle phase: in 35 women, average and peak power did not differ significantly between a low- and a high-hormone phase, while the choice of work-rest ratio changed power by about 35 W. This supports SYNTH-37 ("the phase is not the lever"), with one reservation: the abstract does not say whether phase was confirmed by hormones, counted from days, or was a pill cycle, so it cannot say which kind of "phase" was tested.
The one phase signal is a fatigue index. Power faded more within the 20:10 session in the low-hormone phase (by 11.4 units; 11.2 W·s⁻¹ across both formats), with no significant difference in average or peak power between phases. Inference, not tested here: this is not a reason to change the plan by phase; if a woman reports that short-rest intervals fade badly on certain days, the session's own numbers and her symptoms are what to act on, as SYNTH-37 says.
Inference, not tested here: the authors suggest longer rests (1:1) may help hold power in the low-hormone phase; no training outcome was measured, so that stays a suggestion.
Not settled by this design: whether any of this adds up over a training block, whether symptoms explain the power-drop difference (not reported), and whether it applies to trained endurance athletes or contraceptive users.

Limitations

One acute session per condition; power and a derived fatigue index only.
Phase definitions, verification and contraceptive status are not in the abstract; "low-" and "high-hormone phase" may cover natural cycles, pill phases or both.
The power-drop result is significant in one protocol and in the pooled analysis but not in the other protocol; units are printed as W for one and W·s⁻¹ for the other.
No absolute values, effect sizes or confidence intervals; whether ± is SD or SE is not stated.
Training status, sport and the exercise mode are not in the abstract.
Written from the abstract and the PubMed record; full text not read.
How this entry was checked and changed
Why this level

a randomised crossover of 35 sits in the rubric's B+ band for crossovers of 15 or more; held at B because the abstract does not say how the phases were identified (hormone measurement, day counting or a contraceptive pill cycle) or whether hormonal contraceptive users were included, which is the method that decides what "phase" means here, and gives no phase-by-phase power values