Anatomy of the 15 minute VO2max interval

I’ve noted when I up the intensity that initially my HR climbs but after a while it can drop even though the intensity hasn’t. It’s almost as though it takes a short while to get to the optimum operating conditions when it comes to oxygen demands of the muscle.

I wonder if it’s to do with muscle motor unit recruitment. That as you spend longer at an intensity the motor unit recruitment changes. If fresh units are brought in to replace or supplement fatigued ones that oxygen consumption requirements go down. So in the ramp test although more power is being demanded it changes the motor unit firing and thus oxygen demand. Thus they can make more optimal use of units available from the oxygen that can be supplied.

You can imagine a signal from brain to muscles going “Right this is as much oxygen as you’re going to get, so I’m going to change things a little to eek just that little bit more power for a bit longer”.

I see your 15 minute VO2max interval, and raise you a 20 minute VO2max interval. :see_no_evil:

Anyone that has gone out too hard during a 10 mile TT then tried to hang on for dear life knows this feeling! (Pretty much everyone!?)


@PhilW that is VERY interesting! I’ve only ever experienced that at intensities below ~lactate threshold…above that intensity the ‘long component’ of VO2 holds primacy for me. HR always goes up…the only question is how fast. Ha!

I’m not sure why you see the HR response that you do but it’s fascinating.

Search for “VO2max verification.” You’ll find this paper as well as dozens more stretching back to the 1990s.

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I have read that the body is always trying to optimise things. What’s the least amount of energy it needs to use to complete a task?

Now it maybe when when my brain receives the signal from the muscles “We need more oxygen etc” it employs a series of strategies. So first of all it tells the heart to beat faster. But then I start breathing more efficiently and bringing more oxygen in per breath. It means my heart doesn’t need to beat as fast to deliver more oxygen.

Does the brain ultimately know how much oxygen the lungs can exchange per breath given the rate and force of the diaphragm etc? We clearly have multiple inputs signalling the brain. I ultimately see a sub conscious calculation going on somewhere , where it is measuring the feedback loops on muscle oxygen use, oxygen / blood exchange in lungs, and HR and stroke volume. Then it decides what to increase or decrease.

A bit like the ventilatory thresholds. You know where your breathing changes? Initially just increasing HR was enough and then you really need to ramp the breathing up. Well as those there’s also those times where you start breathing harder initially when you up the intensity but then the breathing settles down. Maybe as the breathing settles it gets more efficient, and therefore Har can decrease and the muscle still gets some amount oxygen.

Other thoughts. :grinning: