So here is the exercise today,
4x 1min @80~90% HRmax

Feelings:
The idea of this type of exercise is to work your speed sustain. I read that professionals can do it for 12min intervals. I think I'll need some years more :)
You should not override the 90% of your maximum heart rate for a long time because you are working in anaerobic area. This exercise helps to improve your anaerobic dependency, which means that with the time your body will get a better tolerance to it or in other words you get a wider range of aerobic. A sufficient recovery between your intervals is very important giving your body time to "deal" with the acid produced.
None of this important specs were a problem to me! First I can not reach 90%HRmax :) second I need bigger intervals because I get totally wasted :) As we can see in the chart, the recovery intervals seem to increase exponentially. LOL
1min sprint + 2min recovery + 1min sprint + 6min recovery + 1min sprint + <20min>
Doing it on road can be sometimes a limitation, because you can not break your rhythm so you have to find the right conditions.
Its quite fun though, it is like hunting the right spot for sprint. Once you have even terrain its very pleasant because you can see your speed rising, kind of a motivation! But if you're out just for training performance, then don't hunt it!, instead do it uphill, it gives a much faster response from your heart.
But for me biking its not about performance, but a pleasure, a trip, ...


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