She took out the pulse oximeter, and the screen read 70. YOUNGER founder Verna was in a hotel room in Shangri-La, Yunnan, having just finished a full day of acclimatization — walking Dukezong Old Town, eating a local dinner with butter tea. She felt only mildly off; everything else seemed normal. Sophie, traveling with her and a former intensive care nurse, stared at the number in disbelief: "If this were one of my old patients, someone would have called a code."
Seventy is a blood oxygen level that in clinical terms is close to requiring emergency care. A healthy person at sea level sits at 98 to 99. And what Verna was about to do the next day was shoulder her gear and run a 55-kilometer trail race across grassland at 3,900 meters. At the time she felt something was not quite right but did not dwell on it — the training plan was set, the coach was there, and ten weeks of preparation could not simply be thrown away. She took a tablet, went to bed early, and set off as planned the next morning.
This is more than a race report; it is a real experiment in body data — and the part most worth unpacking is not race day but what happened in the two weeks afterward. Her HRV, resting heart rate and wearable recovery score all bounced back to normal. She briefly believed she was fully restored and even started planning her next training block. Then the blood report came back: CPK 5018, muscle damage at the level of rhabdomyolysis. That was the moment she genuinely understood that the "I feel fine" signal the body sends and the actual extent of the damage are two entirely different things.
Why does "feeling recovered" not mean "actually recovered"?
From a functional medicine perspective, this is the cognitive trap endurance athletes most easily overlook and the most dangerous one. Altitude and prolonged endurance exercise place acute stress on the body, which triggers a cascade of compensatory mechanisms to maintain homeostasis — but those compensatory responses run on completely different timelines. The autonomic nervous system rebounds far faster than muscle tissue repairs, and regulation of the hormonal axes runs on yet another schedule entirely. Three systems recover at three different rhythms, and all of it gets compressed into one sentence: "I feel recovered."
The illusion of autonomic rebound
After altitude training, sympathetic and parasympathetic tone gradually return to baseline, which makes HRV and heart rate data look normal. But that only means the tension in the nervous system has temporarily lifted; it does not mean muscle fibers, mitochondrial function or the hormonal axes have finished repairing. Returning to high-intensity training purely because a wearable says recovery is good risks layering fresh damage onto tissue that has not healed, creating a hidden cumulative injury you cannot feel in the short term but which erupts all at once in some future session or race.
Underestimating muscle damage
A spike in CPK means muscle cell membranes have been damaged and creatine phosphokinase has leaked into the blood — an important marker of rhabdomyolysis risk. The value often peaks 24 to 72 hours after training, far later than perceived fatigue or HRV rebound, which is why at the very moment you feel fine the blood report may be telling a completely different story. More troubling still, rhabdomyolysis does not only affect muscle: the myoglobin released has to be cleared through the kidneys, and under a prolonged high load that can place additional strain on kidney function. This is precisely why post-race follow-up cannot be as simple as asking whether you have your strength back.
Disruption of the hormonal axes
Prolonged stress combined with insufficient energy intake suppresses the hypothalamic-pituitary-gonadal axis, and both estrogen and androgens fall. For female athletes, whether the physical signs associated with ovulation have disappeared reflects the true load on the hormonal axes better than whether periods arrive on time — an irregular cycle is itself a signal that the body has reached the limit of what it can carry. This mechanism also explains why so many athletes with high long-term training volumes and no matching increase in energy intake develop fatigue, low mood or a performance plateau that seems to have no cause: behind it, the hormonal system is quietly calling for help.
Two markers most likely to look falsely normal
In functional medicine logic, the genuinely dangerous situation is rarely an abnormal number. It is a number that looks normal while hiding a problem. These two markers are where endurance athletes are most easily misled.
| Marker | Conventional focus | The YOUNGER functional medicine view |
|---|---|---|
| HRV / recovery score | Whether the score is high and meets the wearable's threshold | Reflects only the short-term rebound of the autonomic nervous system; it needs cross-checking against CPK, cortisol and other markers to avoid wrongly concluding you have recovered |
| Menstrual cycle regularity | Whether it comes, and how heavy it is | Whether the physical signs associated with ovulation have disappeared reflects the true state of the hormonal axes better than whether the cycle is on time |
Why are endurance athletes especially prone to missing these warnings?
Endurance training culture carries a widespread belief that getting through it is progress — fatigue is treated as evidence of effort rather than a signal that needs addressing. Add the ubiquity of wearables, and athletes get used to judging whether they can train today from a single number such as a recovery or sleep score. Very few people schedule extra blood work on a day they feel good, just to confirm the real state of their muscles and hormones. This is not because athletes are careless; it is because perception and wearable feedback are inherently more sensitive to neural relaxation than to tissue damage. Different systems in the body simply do not repair in sync — which is why even Verna, who considers herself attuned to her own body, got this one wrong.
What the body actually needs to repair
While CPK has not returned to normal, feeling recovered does not mean you can resume high-intensity training. During this period the body needs enough protein to support muscle repair, enough antioxidants to lower the inflammatory response, and sleep that is not cut short — sleep is the golden window for repairing both the autonomic nervous system and the hormonal axes, and after a heavy training load it should be the last thing sacrificed. At the same time, moderating training intensity and substituting low-intensity aerobic work for hard sessions allows the body to free up compensatory capacity for the tissue that genuinely needs it, without stopping training altogether.
On supplements, magnesium supports autonomic relaxation and sleep quality, omega-3 helps regulate the inflammatory response, and vitamin D supports both the hormonal axes and bone repair — but the specific type and form are still best advised precisely by a professional based on your own test data, rather than guessed at.
How to build your own body-signal tracking system
For an endurance athlete, a genuinely safe judgment has never come from a single number but from reading HRV, CPK, hormones and the menstrual cycle together. In practice, schedule a blood test for CPK and cortisol after every hard session or race, alongside continuous HRV tracking from a wearable. Female athletes are advised to additionally record their cycle and ovulation-related signs and watch whether these change with training load. When these signals turn abnormal at the same time, that is the point to pause training and prioritize repair. The body's warnings usually arrive earlier and more honestly than perception does — and learning to read them is the key to sustaining both performance and health over the long term.
1. Meeusen R, et al. (2013). Prevention, diagnosis and treatment of the overtraining syndrome. Medicine & Science in Sports & Exercise, 45(1), 186-205.
The questions people ask most about HRV, CPK and overtraining
How long does HRV take to return to normal?
After a hard session or race, HRV usually rebounds to near baseline within one to two days of the stressor being removed. That only reflects the immediate regulatory state of the autonomic nervous system, though — it does not mean muscle or the hormonal axes have finished repairing, and HRV rebound alone cannot tell you that you have fully recovered.
What is a normal CPK, and what does a spike mean?
For the normal reference range for CPK (creatine phosphokinase), go by the laboratory's figures printed on your own report, as methods vary. A spike means muscle cell membranes have been damaged and creatine phosphokinase has leaked into the blood, an important marker of rhabdomyolysis risk, and it usually peaks 24 to 72 hours after training.
What are the signs of overtraining syndrome?
Common signs include HRV and resting heart rate departing from baseline over a sustained period, changes in the menstrual cycle or ovulation signs, fatigue that rest no longer relieves, and performance plateauing or declining. Any one of these alone does not necessarily mean overtraining, but several appearing together and persisting is worth confirming through blood work and tracked data.
Is an irregular cycle related to training volume?
It may be. A sustained high training volume without matching energy intake suppresses the hypothalamic-pituitary-gonadal axis and causes ovulation-related signs to disappear. In that situation an irregular cycle is itself a signal that the body's stress load is too high — not simply a matter of periods being late.
Replace "I feel recovered" with data you can actually read
If you train hard over the long term and want to know what state your HRV, CPK and hormonal axes are genuinely in —
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