Quick Aerobic Shift

I feel like this has happened to me in the past as well. Its like all of the sudden, one workout made me faster, and it was like an overnight thing.

You may know this already, but its worth brining up. You should think about how long the training effect takes for each workout, its a lot longer than you'd think. It could be something that you did weeks ago that you're finally realizing fitness gains from. I'm not saying that its a specific time period, its obviously a gradual thing, but if you see you had a good week of training two weeks ago, chances are you might be seeing those benefits now.

https://runnersconnect.net/coach-corner/how-long-before-you-benefit-from-a-workout/

There is likely some placebo or psychological effects rather than a physical improvement. My guess is that after a good MP session, your easy days feel easier and you are running with less tension because of new found confidence. That relaxed feeling likely accounts for the lower heart rate.

Psychological

Accumulation of prior co sister training

Weather

Terrain

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Psychological

Accumulation of prior co sister training

Weather

Terrain

Psychological

Accumulation of prior co sister training

Weather

Terrain

hydration

sleep quality

overall stress level

Being tired can suppress heart rate. It doesn’t mean you are faster immediately, just tired. You can run the same easy paces at a lower heart rate. Athletes that are doing a ton of swimming and bike on top of running are more likely to notice this especially in a final build. You may not think you are tired, but most likely that is why.

Yes. It happens and it's not a placebo. You also get your resting heart rate drop a couple of beats.

I second the opinion that this probably has to do with being tired and this somehow makes you more economical at your easy paces. The effect is short-lived, usually the next day you're almost back to the baseline. Not sure what are the underlying reasons, but this effect is quite real.

Darby wrote:

Yes. It happens and it's not a placebo. You also get your resting heart rate drop a couple of beats.

I second the opinion that this probably has to do with being tired and this somehow makes you more economical at your easy paces. The effect is short-lived, usually the next day you're almost back to the baseline. Not sure what are the underlying reasons, but this effect is quite real.

I wonder if this is a matter of your heart temporarily beating harder/moving more volume per beat. So, it doesn't require as many beats at RHR or at easy pace. I'm not sure if that is even possible.

Thanks for all the thoughts everyone and it's good to know it's potentially a thing.

I'm running 105-115 mpw, so I'm quite in tune with my running so I'd be aware if it was terrain/weather related. Psychological is possible.

I think it could be some sort of post-exercise alteration in cardiac function, but I suppose I'm interested as to why 65-70mins average 164bpm causes that where as something like 10 x 1k at 175-185bpm doesn't.

Another possible cause is glycogen depletion of intermediate and faster twitch fibers.

You're running your easy pace with decreased contribution of less economical fibers, making you more economical for a limited time.

I have also experienced that, a jump in "fitness" after a single session of long tempo or a fast long run. "Fitness" being measured as the relationship between HR and pace, and not being related to fatigue, because I would still get the same HR values as always during the faster workouts a couple days later.

I would assume that it's mainly caused by plasma volume expansion (GIYF), that's a training adaptation that you can get within a few hours of your workout.