While some of that is true, it is only half the puzzle, not to mention hugely misleading and taken out of context the way you presented it.
It is really, really important to understand the underlying mechanisms of why
HIIT simply isn't the end-all-be-all for cardiovascular training.
As far as your heart is concerned, the two main adaptations it has in response to exercise are:
- increased ejection fraction (% of blood in your heart that gets pumped out in one beat)
- increased ventricular chamber volume (how much blood your heart holds)
In order to increase the ventricular volume, you have to put a full stretch on the ventricles. That means you need full ventricular filling. There's a known HR range where this happens:
110 BPM to 150 BPM. This is known as the "zone of linearity," and it is called that because there is basically a perfectly linear increase in the cardiac output.
Here's what that means: below this rate, the ventricles have not been fully stretched out because blood is not falling into the ventricles fast enough to do so. We just aren't pumping the blood fast enough through the body, because we don't HAVE to yet. Every extra beat per minute is causing more blood to move, which is increasing the amount of blood entering the heart per second, which is increasing the amount of blood in the ventricle.
So, from resting heart rate up to about 110 BPM, every increase of one BPM (say 94 BPM to 95 BPM) actually adds a little MORE blood to circulation than the last. This is not a linear increase, because each increment of 1BPM does not add the same amount as the last.
From 110BPM to 150BPM, every beat of the heart adds a nearly identical amount of blood to the cardiac output, because the ventricles are already fully stretched. You're just getting one extra beat of a full ventricle with each 1 BPM increase within this range.
This is the optimal range for stretching the ventricles.
Now, once we get past 150 BPM we are actually requiring the heart to beat just a bit BEFORE the ventricle is completely stretched out. This still gives us more blood flow at 180 BPM than we had at 150 BPM,
but we are not stretching the ventricles anymore (which means this heart rate range will NOT increase ventricular filling volume) and each 1 BPM increase within this range actually reduces the amount of blood we move per beat because the ventricle is less full.
The total blood flow per minute will increase, but the blood per stroke will actually decrease.
According to the research I'm familiar with,
it takes at least a 30-minute session in that 110-150 zone to noticeably increase volume. Basically - long, slow distance (LSD).
So, what does that mean for HIIT in terms of getting a heart that can hold more blood?
If your intervals don't take you above 150 BPM, or if your heart rate stays at 110-150 BPM for the majority of the time then congratulations! Your intervals will absolutely stretch your ventricles quite effectively. But then again, that's not HIIT.
Now, HIIT does have a place in training.
High-intensity training absolutely DOES increase stroke volume. One thing that is a guarantee is that you will be
increasing ejection fraction by increasing the contractility of the heart, which is essentially training it to squeeze harder (indeed,
"strength training for your heart"). This forces a higher percentage of the blood out with each beat, up to around 90%.
This is a much faster adaptation to achieve than stretching the ventricles because it is changing communication... not actual physical dimensions (for the most part). Over time you WILL develop thicker muscle around the ventricles as well, but that doesn't increase the amount of blood they will hold.
HIIT is also important because it increases your body's ability to process lactate.
OK, now that we understand all of that, let's put HIIT and LSD head to head.
Maximum increase in ejection fraction = up to around 90 % during exercise, 70 % at rest
Maximum increase in ventricular volume = over time, 2x the original size
Let's look at the table below to see the effects of HIIT vs LSD in action:
| Baseline | Best case increasing EF (HIIT) | Best case increasing VV (LSD) |
Ejection fraction (rest) | 55 % | 70 % | 55 % |
Ejection fraction (exercise) | 75 % | 90 % | 75 % |
Ventricular volume | 130 mL | 130 mL | 260 mL |
Heart rate (rest) | 75 bpm | 75 bpm | 75 bpm |
Heart rate (exercise) | 180 bpm | 180 bpm | 180 bpm |
Stroke volume (rest) | 71.5 ml | 91 ml | 143 ml |
Stroke volume (exercise) | 97.5 ml | 117 ml | 195 ml |
Cardiac output (rest) | 5,4 L/m | 6,8 L/m | 10,7 L/m |
Cardiac output (exercise) | 17.5 L/m | 21,1 L/m | 35,1 L/m |
Stroke volume = how much blood leaves the heart each time it beats
Cardiac output = SV * Heart Rate (in Beats Per Minute, or BPM), in liters per minute
Of course, this is only an example, EF and VV are not developed in isolation, but you can clearly see which adaptation actually drives cardiac output more.
In practice, you would ideally do both to get all the benefits from both adaptations. In a training week, it could look something like this:
- 2-3x 30-60 minute LSD sessions (110-150 bpm)
- 2-3x 5-10 minute HIIT sessions (150+ bpm)