Ironman run - HR drop off

Ok - last 3-4 Ironman runs the piano has hit me as normal at the 27-32km mark. Whilst a slowing in pace, my HR rate drops considerably and a dizziness hits me - pace drops 30-40 sec per k/ HR 20 bpm.
Am I just running out of energy - but wouldn’t Hr try to run higher? Am I mentally just dropping off and not pushing through the wall? Interested in how others bodies respond when hitting the wall. All the literature I work through talks more of higher HR and slowing pace- indicating I am just mentally not there.

Your heart is basically a pump and because you have slowed down your muscles no longer require the same volume of blood to function. There are other factors such as fatigue and dehydration but ultimately because of the exhaustion your muscles can no longer function at a high enough level to demand ‘more’ from your heart.

If you’ve ever done a HM for example and really blown up having gone to hard you can’t run the second half anywhere near as quick and HR typically drops (although is probably high relative to the speed being run)


HR only increases if O2 demands increase, this can happen as you fatigue as you need to recruit less efficient muscles/fibres to maintain the same output, but doesn’t happen where you simply throwing more oxygen at the problem doesn’t work. You know that fatigue isn’t just throwing more oxygen at it, 'cos otherwise you’d run your marathon at the same heart rate as your 5km.

Generally you would see a lower HR in almost any fatigue situation other than being very dehydrated, as fatigue stops you being able to put out the same effort, the HR slowly drifting up through the event is more normal of course.


The technical term is that you have run out of steam…


Learn to pace/fuel better…
Train more to allow that pace to take you further into the race…
Adjust your training so that the desired pace/energy availability is manageable…


…& start eating Pro Plus tablets as soon as you experience your drop-off in pace.

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Thanks for the answers - stepping away and reading again it makes perfect sense. I can sometimes get in a circular reference with myself - pacing no doubt the issue as it tends to be for most things in IM.