30 second sprints and HR

I did a workout yesterday that was very simply 12 x 30 second “sprints” with 3 minute recovery between each.

Some information that may help in the reply:
I do not currently have a Power Meter on my bike. I know that this may make a reply to this post a bit difficult.
It was cold, at least I was! The average temperature my Karoo read was 31 degrees
I did a short warmup (20 minutes) as I’ve read that all-out type workouts are better executed from a physiological standpoint when a short warm up is used (maximize O2 deficit).
I soft pedaled and even coasted down the hill I did the intervals on. I wanted to maximize the effort on the intervals themselves. I followed essentially the same pattern for all of the intervals.

All that stated, what I found was that my HR popped pretty good for the first 4 intervals but then began to peak at a lower HR.

My TH HR is 175 BPM with a max of around 190. In the first 4 intervals I was seeing 170 but from intervals 4-9 I noticed that I didn’t see a HR value above 166 or so. I cut the workout short as I saw/and felt that I was no longer getting the intended physiological benefit from the session.

Is what I was experiencing an acute cardiac response where my body was essentially telling me I was toast? If this indeed what was going on then it was corroborated by the fact that I was not able to stand for the entire 30 seconds on intervals 8 and 9.

Thanks in advance for your response.

The analogy I’d would use is that the first intervals were like when a car engine is cold, it’s not that efficient and it burns more fuel. Once the engine warms up it’s able to run more efficiently and produce the same output with less fuel.

Your body takes time to respond to demands you place on it. This is often why a longer warm up enables you to perform better in a workout. Those first intervals your cardio vascular system was cold and warming up. The latter intervals it’s started to make the physiological changes necessary to the demand being placed on it.