I’m sure this falls into the bucket of ‘I’m overthinking it’, but here goes:
I’m following the prescribed ‘0:20 on, 1:40 off, repeat’ HIIT protocol. I aim for 155 BPM, right in the middle of Zone 4. I typically hit the bottom of Zone 4 within the last 5 seconds of the 20s effort. My heart rate continues to climb and remains at or above 155 BPM for roughly 50 seconds, then settles back to the bottom of Zone 3 by the end of the rest.
So, my question: Does it matter at all that most of the time at the target heart rate is spent during the rest period?
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For these Sprint Intensity Training (SIT) intervals, I don’t usually prescribe or track heart rate that closely. Instead, I’m looking at maintaining near-max pace, power output, etc. for each interval. If people can’t do that due to fatigue, I’ll typically have them extend rest periods.
In your case, I wouldn’t worry about HR either.
Thank you for the explanation, Coach! I own several older templates that all call for 20s sprint @ 10, 100s rest. Per recent advice I adjusted the sprints to be more like RPE 8-9. I also own a newer template that calls for more conservative high intensity work. This new protocol is more in line with the HIIT/SIIT information you’ve put out recently.
So in the future if I run my older templates I’ll be more creative with the HIIT slots. BBM just sent out an article that laid out some good options that sound a lot more fun than what I’ve been doing.
Thanks! 
Yep, some of that older stuff still works obviously, though I’d refine the recommendation a bit.
Thanks for the kind words!