HIIT vs LISS: The Cardio Debate Settled Once and For All

HIIT or LISS? The short answer is: both. The right answer is: it depends on exactly where you are in your training and what you're trying to achieve. Kayla Itsines built BBG around this distinction — and the evidence supports her framework.

What Each Actually Does

HIIT (High-Intensity Interval Training)

Short bursts (20–40 seconds) of maximal or near-maximal effort, followed by brief recovery. Elevates EPOC (post-exercise oxygen consumption — the "afterburn"), improves cardiovascular fitness quickly, and is time-efficient. A 20-minute HIIT session can deliver equivalent cardiovascular adaptation to 35–40 minutes of moderate steady-state work.

Best for: Time-crunched individuals, those with a good fitness base, cardiovascular improvement

Not ideal for: Beginners, anyone post-injury, people doing heavy strength training on the same day

LISS (Low-Intensity Steady-State)

Sustained moderate effort — brisk walking, cycling, light swimming — for 30–60 minutes. Burns fat preferentially as a fuel source, is easier to recover from, and has a lower injury risk. In prep phases, it preserves muscle mass better than HIIT.

Best for: Fat loss during cutting phases, beginners, active recovery, mental decompression

Not ideal for: Maximum cardiovascular efficiency gains

The BBG Recommendation

Beginners: start with LISS exclusively. Build your aerobic base, your joints, and your movement confidence before introducing HIIT. Two to three 30-minute brisk walks per week is genuinely enough to start.

Intermediate: two LISS sessions + one HIIT per week is the sweet spot for most people training for general health and body composition.

The Rule That Always Holds

The best cardio is the one you'll actually do consistently. If you hate HIIT and always quit halfway, it's not delivering the result the literature promises. If you enjoy a 45-minute walk and do it five days a week, that consistency compounds into significant results over a year. Consistency over optimisation, every time.