HFpEF Exercise: Why HIIT Did Not Beat Moderate Training

For people with heart failure with preserved ejection fraction (HFpEF), structured exercise can improve exercise capacity. Hard intervals do not appear to improve it more than moderate continuous training.

That conclusion comes from the largest head-to-head trial in the field: after three months, both high-intensity interval training (HIIT) and moderate continuous training (MCT) improved peak oxygen uptake compared with activity advice alone, while HIIT and MCT did not differ significantly from each other. The result challenges a familiar fitness rule—harder intervals usually produce larger aerobic gains—and points to a disease-specific constraint: many people with HFpEF cannot raise their heart rate normally during exercise.

HFpEF can leave ejection fraction normal while the heart still fills poorly and patients still become breathless with activity. Exercise intolerance often drives day-to-day disability. Exercise is therefore more than a pleasant add-on; it is one of the few interventions with randomized evidence for improving physical capacity and quality of life.

The trial that tested intensity directly

The OptimEx-Clin randomized clinical trial enrolled 180 sedentary adults with echocardiographically confirmed HFpEF at five European sites. Participants averaged 70 years old, and 67% were women. They were assigned in equal proportions to HIIT three times weekly for 38 minutes at 80% to 90% of heart-rate reserve; MCT five times weekly for 40 minutes at 35% to 50% of heart-rate reserve; or one session of guideline-based physical-activity advice.

After three months of supervised training, peak VO2 rose by 1.1 mL/kg/min in the HIIT group and fell by 0.6 mL/kg/min in the advice group. The adjusted between-group difference was 1.5 mL/kg/min (95% CI, 0.4 to 2.7). MCT increased peak VO2 by 1.6 mL/kg/min versus the same 0.6 mL/kg/min decline with advice, for an adjusted difference of 2.0 mL/kg/min (95% CI, 0.9 to 3.1).

Both exercise prescriptions therefore beat advice alone. But HIIT did not beat MCT: the direct difference was -0.4 mL/kg/min (95% CI, -1.4 to 0.6). Because that confidence interval crosses zero, the trial did not find a statistically significant intensity advantage.

Clinical importance deserves separate treatment. Neither active program reached the trial's prespecified minimum clinically important difference of 2.5 mL/kg/min. That threshold came from heart failure with reduced ejection fraction (HFrEF), not HFpEF, so it may not be the right cutoff for this population. Still, the measured gains were modest, not a license to promise a dramatic return of youthful stair-climbing powers.

Why harder intervals may not have won

A prespecified OptimEx-Clin analysis of chronotropic incompetence offers the most useful explanation. Chronotropic incompetence means the heart rate does not rise adequately with exercise. Among 175 participants assessed at baseline, 51% met the study definition; the analysis included 144 participants who completed follow-up.

Participants with chronotropic incompetence began at a meaningful disadvantage. Their mean peak VO2 was 16.5 versus 21.1 mL/kg/min among those without it, peak workload was 89 versus 116 watts, and their ventilatory efficiency slope was worse: 35.2 versus 32.6. The differences in peak VO2 and workload had P values below 0.001; the difference in ventilatory efficiency had a P value of 0.014.

Three months of HIIT or MCT did not improve chronotropic index, predicted peak heart rate, or the prevalence of chronotropic incompetence. All reported P values for those comparisons exceeded 0.500, including analyses adjusted for drugs that slow heart rate. Yet both exercise groups improved peak VO2 and peak workload versus guideline-based care.

This pattern supports, but does not directly prove, a peripheral explanation for the fitness gains. Exercise may have improved skeletal-muscle metabolism, vascular function, oxygen extraction, or some combination of these factors without increasing heart-rate reserve. The trials did not measure muscle biopsy findings or arteriovenous oxygen differences, so the mechanism remains an inference rather than a completed wiring diagram.

The parent trial also found no measurable improvement in diastolic-function markers or natriuretic peptides. Taken together with unchanged chronotropic measures, that result makes large central cardiac remodeling an unlikely explanation for the three-month peak-VO2 improvement.

The broader evidence supports exercise, not HIIT supremacy

The main conclusion fits the broader literature. A 2024 systematic review and meta-analysis by Baral et al. found that structured exercise improved peak VO2 and quality of life versus control in HFpEF. A 2023 meta-analysis by Lai et al. compared HIIT and MCT protocols and documented substantial variation among the small trials: patient selection, duration, sample size, and training design differed. A 2024 review by Callum et al. likewise found structured exercise better than usual care without a consistent HIIT advantage over MCT.

Those reviews should not be read as proof that every program works equally well for every patient. They do support a practical point: an HFpEF patient who can adhere to moderate training has no compelling evidence-based reason to assume that intervals at 80% to 90% of heart-rate reserve will produce more aerobic benefit.

The contradictory meta-analysis needs context

One analysis reached a different conclusion. A 2023 meta-analysis by Siddiqi et al. pooled three randomized trials totaling 150 participants and reported that HIIT improved peak VO2 by 1.46 mL/kg/min more than MCT (95% CI, 0.88 to 2.05; P<0.00001). It reported I²=0%, conventionally interpreted as no observed statistical heterogeneity.

That estimate conflicts with OptimEx-Clin's direct result. The smaller evidence base included very short trials—approximately 15 participants in one four-week trial and 19 in one 12-week trial—alongside OptimEx-Clin, which had 120 participants across its two exercise groups and followed patients through 48 weeks. Differences in trial duration, scale, and protocol make a pooled result less straightforward than its tidy I² value suggests. The Lai review's description of methodological heterogeneity reinforces that caution.

A 2023 network meta-analysis by Gomes-Neto et al. ranked HIIT near the top for aerobic-capacity improvement versus usual care. Network rankings are indirect comparisons, however, and can mix differences in training frequency, duration, and supervision with differences in exercise mode. They do not overturn the largest direct HIIT-versus-MCT comparison.

Benefits faded when supervision faded

OptimEx-Clin followed patients for 12 months, shifting from supervised training to telemonitored home exercise after the initial phase. By 12 months, no between-group comparison remained statistically significant. The study cannot establish exactly why the early differences disappeared, but it raises a practical concern: maintaining exercise gains may require more ongoing support than a handoff to home training provides.

Retention was strong for a year-long trial in an older population with multiple health conditions: 92% completed the three-month assessment and 86% completed 12 months. The study was registered prospectively and used blinded core-laboratory analysis for its primary outcome. Participants and evaluating staff were not blinded, which could affect subjective outcomes such as quality-of-life measures.

Safety data also warrant a measured reading. Acute coronary events occurred in 7% of HIIT participants, 5% of MCT participants, and 8% of guideline-care participants. The trial was not powered to establish safety differences, so those rates neither prove equivalence nor identify one program as safer. HFpEF exercise prescriptions belong in clinical care, particularly for people with symptoms, unstable disease, or complex medication regimens.

What this changes in practice

For stable, sedentary adults with HFpEF, the evidence favors structured aerobic exercise over advice alone. It does not favor HIIT over MCT for improving peak VO2 after three months. Preference, tolerance, supervision, access, orthopedic limits, and the ability to keep showing up should drive the choice between the two.

Chronotropic incompetence may explain why intensity has less payoff in this disease than in healthier populations. It is also a research target, not yet a treatment answer. Future trials need to test whether correcting an impaired heart-rate response, extending supervised programs, or tailoring exercise for common subgroups such as people with type 2 diabetes or iron deficiency changes the response.

Verdict: In HFpEF, consistent exercise has better evidence than chasing maximal intensity. The measurable benefit is modest, and the largest direct trial found that moderate training kept pace with HIIT.