I’m doing my intervall workouts always in the evening.
In the morning I’m going for fasted runs.
But I have realised the mentioned effect first on the indoor training and only sometimes during the outdoor training. I think during outdoor trainings there are to many changing factors so that I might not even realise the effect after 35 minutes into the training.
Back when I was doing Polar Bear a lot this phenomenon with heart rate became a mainstay of all my long duration sweet spot workouts. You notice it a lot more with workouts like Polar Bear because there aren’t any low intensity breaks…you just crank away at sweet spot. After the sweet spot work starts you’ll see heart rate go down, down, down until about the 40 minute mark where there is a shallow inflection point. Then heart rate starts to climb again.
Don’t know why.
I did read some stuff about VO2 priming after the fact & wondered if some VO2 priming would make the 40 minute downdrift in HR go away. Never tried, though.
I think it’s psychological. The work is no longer a shock to your system, yet you still have enough work capacity so that the work isn’t painful. Therefore, around 30 minutes is where you feel at your best.
Here is a more tangible example of a moderate intensity workout having lower heart rate in the 2nd half of the workout than in the first half. Three 20 minute intervals…the first two average heart rate is higher than the third and final interval. During the third interval, average heart rate in the 2nd 5 minute portion of the interval is 161bpm & average HR for the final 5 minutes of the interval is 158bpm.
This is something that pretty consistently happens among a certain portion of cyclists. It’s not unusual at all.
