What's your heart rate after the first 20 or so?

If it's beyond 70% max heart rate, you are likely going anaerobic, muscles and heart start switch from fat to glycogen to ATP as the demand increases.

It's like a runner doing "speed work" on a track. First 800 relatively "easy". By the eight repeat, not so much.

You’re absolutely right. It surprised me the first time I wore a heart rate monitor for a near max effort deadlift. Maxed out my heart rate in a mere five reps.

On the other hand I’ve done high rep air squats and while I wasn’t wearing a monitor, it felt very much like the cardio zone.

I need a new heartrate monitor. I typically do about 100 reps for a given muscle group, and I think it would be good to see how my heart rate responds.

This is going to seem like an odd recommendation if you are a casual athlete but to save you time/money/satisfaction I strongly recommend a HRM strap instead of wrist optical and even more so if you can justify it, one that has self-storage and can download afterwards. Turns out monitoring via bluetooth and ANT+ has dropout at high heartrates.

I think the Wahoo ticker-x is the only one that meets all those requirements and I think it can work without a watch too. I have the garmin hrm-pro which can save/download but it needs the watch to be connected to start working which is bad design. Check DCrainmaker website for latest reviews.