High-Intensity Interval Training in Patients With Heart Failure With Reduced Ejection Fraction

The SMARTEX Heart Failure Study compared three exercise strategies head-to-head:

**HIIT (High-Intensity Interval Training):** Four 4-minute intervals at 90–95% of maximal heart rate, separated by 3-minute active recovery periods at moderate intensity. Total session time: 38 minutes (including warm-up and cool-down). Three supervised sessions per week.

**MCT (Moderate Continuous Training):** Continuous exercise at 60–70% of maximal heart rate for 47 minutes. Three supervised sessions per week.

**RRE (Recommendation of Regular Exercise):** Patients were advised to exercise at home according to current guidelines and attended one supervised moderate-intensity session (50–70% of maximal heart rate) every three weeks. This served as the control group.

The primary outcome was the change in left ventricular end-diastolic diameter (LVEDD) — a measure of how much the heart's main pumping chamber has enlarged (remodeled) due to heart failure. Secondary outcomes included peak oxygen uptake (VO2 peak), quality of life, and safety events.

**Sample size:** 261 patients randomized (88 to HIIT, 87 to MCT, 86 to RRE)

**Age:** Median 60 years

**Sex:** 19% women, 81% men

**Heart failure severity:** Left ventricular ejection fraction ≤35% (severely reduced pumping function), New York Heart Association class II–III (symptoms with mild to moderate activity)

**Cause of heart failure:** 59% ischemic (from blocked arteries), 41% non-ischemic

**Setting:** 9 European centers (Antwerp, Copenhagen, Leipzig, Luxembourg, Munich, Stavanger, Trondheim/Levanger, Veruno, and Ålesund)

**Recruitment:** Outpatient heart failure clinics, cardiac rehabilitation referrals, public announcements, hospital registries

**Medication:** All patients were on optimal pharmacological therapy (ACE inhibitors, beta-blockers, diuretics, etc.)

**Exclusion criteria:** Unstable heart failure, recent hospitalization, inability to exercise, significant valve disease, or other conditions that would interfere with exercise training

**Left ventricular end-diastolic diameter (LVEDD):** Measured by echocardiography at the tip of the mitral leaflet in the 2-dimensional parasternal long-axis view. All images were sent to a core laboratory in Trondheim, Norway, where two expert echocardiographers (blinded to group assignment) performed the analysis using EchoPAC software. Repeatability was tested with Bland-Altman analysis showing no bias (0.3 mm) and a coefficient of variation of 4.1%.

**Peak oxygen uptake (VO2 peak):** Measured via cardiopulmonary exercise testing using standard indirect calorimetry equipment. Patients exercised on a treadmill or bicycle ergometer with an incremental protocol (10–20 W increase per minute) until exhaustion. Tests were performed at baseline, 12 weeks, and 52 weeks.

**Quality of life:** Assessed using validated questionnaires (specific instruments not detailed in the abstract but mentioned in the full methods).

**Adverse events:** Serious adverse events were recorded throughout the 12-week intervention period and the 52-week follow-up.

**Training adherence:** Heart rate monitors were used during supervised sessions to verify that patients were exercising at their prescribed intensity. Training logs were kept.

**Study design:** This was a multicenter, randomized, controlled, parallel-group clinical trial (RCT) conducted at 9 European centers between June 2009 and July 2014. Patients were randomized 1:1:1 to HIIT, MCT, or RRE.

**Randomization:** Patients were stratified by study center and by pathogenesis (ischemic vs. non-ischemic heart failure). Stratification by center prevented bias from unobserved differences in how each center delivered care. Stratification by pathogenesis allowed post-hoc analysis of whether the cause of heart failure influenced the response to exercise.

**Blinding:** This was an open-label trial — patients and exercise supervisors knew which group they were in (you cannot blind someone to whether they are doing HIIT or MCT). However, the echocardiographers who measured the primary outcome (LVEDD) were blinded to group assignment, though they were not always blinded to whether the measurement was from baseline or follow-up.

**Duration:** The supervised intervention lasted 12 weeks. After that, all patients were encouraged to continue exercising on their own, with telephone follow-up every 4 weeks. Final assessments were at 52 weeks (one year from baseline).

**Statistical approach:** The primary analysis compared changes in LVEDD from baseline to 12 weeks between HIIT and MCT. The study was powered to detect a clinically meaningful difference between these two groups. Secondary analyses compared both HIIT and MCT to RRE. Results are reported with 95% confidence intervals and p-values.

**What this design can prove:**

This is a gold-standard design for comparing interventions — the RCT can establish causality (exercise caused the changes, not some other factor).

The multicenter design increases generalizability across different populations and settings.

The 52-week follow-up allows assessment of whether gains are maintained after supervised training stops.

**What this design cannot prove:**

Because it was open-label, patients who preferred HIIT might have been disappointed to get MCT (or vice versa), potentially affecting motivation and adherence.

The study cannot tell us about long-term effects beyond one year.

The study cannot tell us about different HIIT protocols (different interval lengths, intensities, or frequencies) — only this specific 4x4-minute protocol.

The study was not powered to detect differences in safety events (serious adverse events), so the numeric differences in adverse events (39% HIIT vs. 25% MCT vs. 34% RRE) should be interpreted cautiously.

**Major methodological weakness:** Adherence to prescribed intensity was poor — 51% of HIIT patients exercised below their target heart rate, while 80% of MCT patients exercised above their target. This means the actual exercise stimulus delivered was closer between groups than intended, which could explain the lack of difference. The authors acknowledge this as a key limitation.

**Primary outcome — Left ventricular end-diastolic diameter (LVEDD) change at 12 weeks:**

HIIT vs. MCT: No significant difference (P = 0.45)

HIIT vs. RRE: −2.8 mm (95% CI: −5.2 to −0.4 mm; P = 0.02) — HIIT produced a small but statistically significant reduction in LVEDD compared to doing nothing

MCT vs. RRE: −1.2 mm (95% CI: −3.6 to 1.2 mm; P = 0.34) — MCT was not significantly different from doing nothing

**Secondary outcome — Peak oxygen uptake (VO2 peak) at 12 weeks:**

HIIT vs. MCT: No significant difference (P = 0.70)

Both HIIT and MCT were superior to RRE (specific numbers not provided in the abstract, but both showed improvement)

**Durability at 52 weeks:**

None of the improvements in LVEDD or VO2 peak were maintained at the 52-week follow-up. This means that once supervised training stopped, patients regressed to baseline.

**Serious adverse events at 52 weeks:**

HIIT: 39%

MCT: 25%

RRE: 34%

P = 0.16 (not statistically significant — the study was not designed to detect safety differences)

**Adherence:**

51% of HIIT patients exercised below their prescribed target heart rate (i.e., they couldn't sustain 90–95% of max HR)

80% of MCT patients exercised above their prescribed target (i.e., they naturally pushed harder than instructed)

The primary finding is a null result — HIIT was not superior to MCT. However, translating the absolute numbers:

**LVEDD reduction:** The HIIT group showed a 2.8 mm reduction in left ventricular diameter compared to the control group. To put this in perspective, a normal left ventricle is about 45–55 mm in diameter. A 2.8 mm reduction is roughly a 5–6% change — noticeable on an echocardiogram but unlikely to be felt by the patient as a dramatic improvement in symptoms.

**VO2 peak improvement:** Both HIIT and MCT improved aerobic capacity compared to doing nothing, but the difference between them was negligible. The magnitude of improvement was modest — typical gains in cardiac rehabilitation are in the range of 10–20% improvement in VO2 peak, but these gains were not sustained.

**The "real-world" effect:** The most striking finding is that neither group maintained their gains after supervised training ended. This suggests that the supervised program itself was the driver of improvement, not the specific type of exercise. Once patients went back to unsupervised exercise, they stopped doing enough to maintain the benefit.

**Adherence problem:** The fact that half of HIIT patients couldn't reach their target heart rate and 80% of MCT patients exceeded theirs tells us that in practice, people naturally gravitate toward a moderate intensity — HIIT is hard to sustain, and MCT is easier to push a bit harder. The actual exercise intensity delivered was probably similar between groups.

**What the authors acknowledge:**

Poor adherence to prescribed intensity (51% of HIIT below target, 80% of MCT above target) — this is the elephant in the room. The intended exercise stimulus difference between groups was not achieved.

The study was not powered to detect differences in safety events, so the numeric trend toward more adverse events in HIIT (39% vs. 25% in MCT) cannot be interpreted as evidence of harm.

The 52-week follow-up showed loss of all gains, indicating that unsupervised exercise was insufficient to maintain benefits.

Open-label design (patients and trainers knew which group they were in) could have introduced bias in motivation and adherence.

**What a critical reader would note:**

**Sample size:** While 261 patients is reasonable, the study was powered to detect a difference between HIIT and MCT. The null result could reflect insufficient power to detect a small but real difference, or it could reflect that no meaningful difference exists.

**Population:** Only 19% women — results may not generalize equally to women with heart failure. The median age was 60, so results may not apply to younger or older patients.

**Single HIIT protocol:** Only one specific HIIT protocol was tested (4x4 minutes at 90–95% max HR). Other protocols (e.g., 1-minute intervals, 30-second sprints, different work-to-rest ratios) might produce different results.

**Duration:** 12 weeks of supervised training is relatively short. Some cardiac remodeling effects might require longer training periods.

**Echocardiography limitations:** LVEDD is a one-dimensional measure. More sophisticated measures of cardiac function (e.g., strain imaging, 3D echocardiography, MRI) might have detected differences that LVEDD missed.

**Industry funding:** The study was investigator-initiated, but potential conflicts of interest are not detailed in the abstract. The authors declare no direct industry funding for this specific trial.

**Generalizability to healthy people:** This study was in heart failure patients with severely reduced ejection fraction. Results cannot be directly applied to healthy individuals or those with preserved ejection fraction.

For someone running their own n=1 experiment (assuming you are a generally healthy person, not a heart failure patient — if you have heart failure, consult your doctor before starting any exercise program):

### What to test

Compare moderate continuous exercise (e.g., brisk walking, jogging at a conversational pace) versus high-intensity interval training (e.g., 4 minutes hard effort, 3 minutes easy, repeated 4 times). The key question: does HIIT produce meaningfully better results for YOUR body than simply doing moderate exercise for slightly longer?

### Minimum meaningful duration

**For initial effects:** 4–6 weeks. The SMARTEX study saw changes at 12 weeks, but smaller studies have shown adaptations in as little as 4 weeks.

**For maintenance testing:** 12 weeks minimum to see if you can sustain the regimen.

**For durability:** Continue measuring for 4–8 weeks after stopping the structured program to see if gains persist.

### What to measure (specific metrics)

**Primary metric:** Choose one that matters to you:

- Resting heart rate (measure first thing in the morning before getting out of bed)

- Heart rate recovery (how much your heart rate drops in 1 minute after stopping exercise — a marker of cardiovascular fitness)

- Time to complete a fixed distance (e.g., 1 mile walk/run time)

- Submaximal heart rate at a fixed workload (e.g., heart rate while walking at 3 mph on a 5% incline)

**Secondary metrics:**

- Blood pressure (resting, measured at the same time each day)

- Body weight and waist circumference

- Subjective energy levels (rate 1–10 daily)

- Sleep quality (subjective rating or using a sleep tracker)

- Mood (using a simple 1–10 scale or a validated questionnaire like the PHQ-9)

### Key confounds to control for

**Total exercise volume:** The SMARTEX study attempted to match energy expenditure between HIIT and MCT. In your experiment, either match total time (e.g., 38 minutes of HIIT vs. 47 minutes of MCT as in the study) or match total distance/calories. Do not compare 20 minutes of HIIT to 20 minutes of MCT — that's comparing different doses.

**Time of day:** Exercise at the same time each day to control for circadian effects on heart rate and performance.

**Pre-exercise nutrition:** Eat the same meal at the same time before each session. Carbs, caffeine, and hydration status all affect performance.

**Sleep:** Track sleep quality and duration. Poor sleep will blunt exercise adaptations.

**Stress:** Note major life stressors. Cortisol interferes with cardiovascular adaptation.

**Medication:** If you take any medications (blood pressure, beta-blockers, etc.), keep them constant. Beta-blockers in particular will blunt heart rate response and make HIIT difficult.

**Temperature and humidity:** Exercise in similar conditions. Heat increases heart rate and perceived effort.

### What a positive result would look like

**Resting heart rate drops by 3–5 bpm** over 4–6 weeks in one condition compared to the other.

**Heart rate recovery improves by 5–10 bpm** (faster drop in the first minute after exercise).

**Time to complete a fixed distance improves by 5–10%** more in one condition.

**Submaximal heart rate at a fixed workload drops by 5–10 bpm** — this is the most reliable marker of improved cardiovascular efficiency.

**Subjective energy and mood improve** by at least 1–2 points on a 10-point scale.

**Important caveat:** The SMARTEX study found that HIIT was NOT superior to MCT in heart failure patients, and adherence to HIIT was poor. For a healthy person, HIIT might produce faster gains in VO2 max, but the difference may be smaller than fitness influencers claim. The real-world lesson from this study is that the best exercise is the one you can actually do consistently — if HIIT feels miserable and you skip sessions, MCT will win in the long run.