🏃 Want to Improve Your Cardio Fitness? Two HIIT Sessions a Week May Be Enough

High-intensity interval training (HIIT) has exploded in popularity—and for good reason. It's time-efficient and proven to boost cardiovascular fitness. But here's the question recreational runners keep asking: How many sessions per week do I actually need?

A 2025 study from the University of Würzburg finally put this to the test—and the answer might free up your schedule.

🧪 The Study

Researchers led by Mascha Lenk and colleagues published their findings in Physiological Reports (September 2025). They recruited 26 recreationally active adults and randomly assigned them to perform the classic 4×4-minute HIIT protocol either:

Once per week

Twice per week

Three times per week

Each session involved four 4-minute intervals at 85–95% of maximum heart rate, with 3-minute recovery periods between. The intervention lasted 6 weeks, with testing before and after.

📊 The Results

All groups improved. But the gains weren't equal:

VO₂max (aerobic capacity):

1×/week: Essentially no change (effect size d = 0.09)

2×/week: Moderate improvement (d = 0.48)

3×/week: Larger improvement (d = 0.61)

Time-to-exhaustion:

1×/week: No meaningful change (d = 0.02)

2×/week: Moderate improvement (d = 0.49)

3×/week: Large improvement (d = 0.77)

Here's the kicker: the difference between twice and three times weekly wasn't statistically significant. Both produced meaningful gains—but adding that third session didn't deliver proportionally greater benefits.

🧠 Why It Works

HIIT drives cardiovascular improvements through two main pathways:

Central adaptations: Increased stroke volume (your heart pumps more blood per beat)

Peripheral adaptations: Mitochondrial biogenesis (your muscles get better at using oxygen)

Training once weekly appears insufficient to trigger these adaptations. But twice weekly crosses a threshold—providing enough stimulus for your body to adapt without excessive recovery demands.

👟 What This Means for Runners

If you're a recreational runner trying to improve your cardio fitness:

✅ Two HIIT sessions per week appears to be the minimum effective dose for meaningful VO₂max and endurance gains

✅ Three sessions work too—but don't expect dramatically better results

🚫 Once weekly probably isn't enough to move the needle on aerobic capacity

✅ Adherence was highest in the 2×/week group (96% vs. 91% for 3×/week)—suggesting it's also more sustainable

⚠️ Important Caveats

This was an exploratory study with limitations:

Small sample size (26 participants) means limited statistical precision

All confidence intervals crossed zero—findings are hypothesis-generating, not definitive

Six weeks may be too short to detect changes in running economy or ventilatory threshold

Participants were already recreationally active—results may differ for sedentary individuals or elite athletes

No true control group (though frequency comparison partially addresses this)

✅ The Bottom Line

Train smarter, not just harder. For recreational runners, two weekly sessions of 4×4-minute HIIT may deliver most of the cardiorespiratory benefits—with better sustainability than cramming in a third session.

Science suggests: find 30 minutes twice a week, push hard, and watch your fitness climb.