Cause of declining power and heart rate during an interval

What is the cause of when power starts declining and heart rate goes down during an interval? Underfueling, (lactic) acid build up, something else?



So lactic acid build up?

@ArHu74 depends on the interval. If it’s a 90 minute Tempo interval (colloquially known as Vaughters interval :rofl: ) I’d say that’s just normal fatigue exacerbated by poor hydration.

If it’s a VO2max interval I’d say check your HR strap & if that seems OK see a doctor.


Also, as you get further into an interval you’ll likely start getting into bigger motor units which are less aerobically well-trained. Keep going and they get better trained, eventually. :slightly_smiling_face:

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No, most probably you just don’t have the aerobic endurance to maintain the workload.

To remedy that, do slightly less hard work next time.

Other factors can be sleep, nutrition, health.


I agree with this. Fatigue.

Happens to me in VO2 Max Intervals where I come out too hot, or if I’m not fully recovered going in. By the last interval I’m not able to maintain the same power nor can I get the same heart rate response.

Conversely, come in well rested, and/or pace it better, and I’m able to build power (and heart rate) through to the last interval.

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You can simply say power declines, I got a little confused at first.

Perhaps something else to consider - what is the cause of increasing power (or duration) over a set of intervals? Being serious.

Sometimes power starts declining but heart rates starts increasing, I figured this was caused by fatigue/lactic acid build up. Ending intervals like that feels much more tiring but also satisfying