Originally Posted by Heathpack

Interval 1: First 8 min feel good, somewhere around the 8 min mark I start to think things like "only 12 min left," then the rest of it requires concentration to stay on the power numbers, I notice my legs and they feel tired but I know I can finish it, last 3 min no problem to pour it on as needed to make the target.

Interval 2: Takes concentration right from the very beginning, I notice my legs, my breathing etc right away and the whole interval is like that, I have to consciously stay on the pedals all through the pedal stroke, take note of my resp pattern & adjust effort continuously, my legs feel tired but not really painful. But without concentration/focus, I'd find myself 10-15% under target. I'll have something left in the tank for a final 1-2 min push if necessary but it will be really hard & I know my best chance of making the power target on the 2nd is to not get behind.

For much of the first interval, the effort is automatic. Towards the end of the first & for all of the second interval, the effort is deliberate.

I would not describe a 105% effort as painful as much as I'd call it difficult.