I often read it is recommended to either train in HIT or LIT, because that training leads to growth in volume or density of mitochondria. See e.g. https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/EP085319.

It is said that training inbetween HIT or LIT does hardly any benefit for the performance. I would assume that the metabolism changes and the signal cascades, leading to mitochondria growth, are interrupted and it takes time for the body to trigger the signal cascades again(?)*

Question: How do the following examples affect the mitochondria growth?

Lets say my heart rate for basic endurance (BE) is 95-123 and 150+ for HIT intervals. And lets take Example 0 as a benchmark training session for the other 5 examples listed below.

  • Example 0: I train for 2.5 hours in BE (pulse < 123).

  • Example 1: I train for 2.5 hours in BE (pulse < 123). Then my puls is 130+ for the next and final hour. (so 3.5 hours in total).

  • Example 2: I train for 1.5 hours in BE (pulse < 123). Then my puls is 130+ for the next hour. Finally, i train another hour as BE (pulse < 123).

  • Example 3: I start with an hour of harder training (puls 130+) then I train for 2.5 hours in BE (pulse < 123).

  • Example 4: I train for 3.5 hours in BE (pulse < 123), but the terrain has a lot of minor hills in between. a) My pulse is 125-135 for 3-5 sec for 15 times in between. b) My pulse is 125-135 for 10-20 sec for 15 times in between. c) My pulse is 125-135 for 1-2min for 15 times in between.

  • Example 5: I train a HIT session for an hour. Then i do 3 hours with a pulse of 80-105.

So basically i try to figure out what actions can harm the mitochondria growth, triggered by basic endurance training.

What i tried so far:

My intuition would tell me that having higher pulse in between for 10-20+ seconds could stop the signal cascades, but having an hour in adittion at the beginning or end would not do any harm. I did not find any studies. Only an interview of Jan Frodeno where he tells that he does short term sprints during his BE training to loosen up the muscels. I would doubt that he does it while risking the training effect. So i would guess short interruptions (<10sec) with higher pulse are not that bad. But it is only guessing and i would try to get a „more scientific view“ on it.

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