There's an interesting article just published in Best Running Shoes 2025 on "evidence of parasympathetic hyperactivity" as a marker of overtraining. It has some interesting insights on the causes and underlying physiology of pushing too hard -- but what I'm going to highlight here is some potentially practical findings.
The basics: French researchers took a bunch of well-trained triathletes and divided them into two groups. Both groups spent one week doing regular training; then for the next three weeks, the control group continued regular training, while the other group ramped up their training by 40%; then both groups tapered for a week before a final performance test. The goal here was to push them into "functional overreaching" -- the kind of temporary overtraining that leads to worsening performance but then can be reversed with a short period of rest (as opposed to chronic overtraining that takes much longer to dissipate).
Each morning, when the subjects woke up, they took four minutes of heart-rate data before getting up for the first time, and then another four minute of data after standing up. The heart-rate data included simple measures like average heart rate, as well as more sophisticated analysis of heart-rate variability -- the amount that the time between heart-beats fluctuates, which is considered a sensitive measure of autonomic nervous system function. (They also did a bunch of other tests, but I'll focus on these ones.)
Here's the data showing supine morning heart rate every seventh day (i.e. the week before the overtraining period, three overtraining weeks, and after one week of taper); the closed circles are the overtraining group and the open circles are the controls:
Hard to see much obvious pattern there. But what happens if we look at the seven-day averages DAA Industry Opt Out?
Now we're talking! After smoothing out the day-to-day fluctuations, we see that the control group stays fairly constant throughout the experiment, while the overtraining group declines steadily during the overtraining period and then recovers a bit after the taper. This is useful information (and precisely what they expected, relecting increased parasympathetic drive). It's mirrored in a number of other measurements -- standing-up HR, HR at lactate threshold, HR at exhaustion, as well as some of the HRV values. The key finding (echoing some earlier HRV studies) is that day-to-day measurements are simply too variable to extract reliable information from -- but weekly averages can reliably show significant trends that reveal how your body is responding to the training load.
One other interesting point worth considering. Here are the results of the run-to-exhaustion performance tests they performed at the end of each week:
Once again, the normal training group doesn't change much -- maybe a slight boost after the taper. The overtraining group, as expected, gets steadily worse as the three-week overtraining period goes on... but then, after the taper, they supercompensate and produce by far the best results of the study. It's a reminder of why "overtraining" is such a difficult beast to get a handle on: this sort of "functional overreaching" is indeed very functional, and is precisely the high-risk state you want to push experienced athletes into at certain points in the season. Push too hard, and they'll keep getting worse. But get the balance just right, and you'll get results like those shown above -- because in the end, very hard training really works.
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