What Most People Think
"Overtraining" gets used for any stretch of feeling tired, flat, or unmotivated after hard training. People assume there's a clear line you cross, and probably a test (cortisol, testosterone, heart-rate variability) that proves you've crossed it. The belief is that overtraining is both common and measurable.
What the Evidence Actually Shows
It's a continuum, not a switch. The sports-medicine consensus (Meeusen 2013) describes three stages. Functional overreaching is a short performance dip from a hard block that then rebounds higher, which is the goal of a hard block, not a problem. Nonfunctional overreaching is stagnation that takes weeks to clear with no rebound. Overtraining syndrome is a long-term performance collapse with whole-body symptoms lasting months. STRONG MOD-HIGH
True overtraining syndrome is rare and a diagnosis of exclusion. There is no gold-standard test, so it can only be diagnosed by ruling out everything else (low iron, thyroid problems, infection, depression, under-eating) and confirming a long performance decline (Carrard scoping review 2022). STRONG HIGH
No single validated marker exists. Heart-rate variability (7 athletes), basal hormones and the testosterone-to-cortisol ratio (51 athletes), oxidative stress, blood viscosity and immune markers have all been floated. Each separates groups on average in a small study, then fails to give a reliable yes or no for the individual in front of you. STRONG HIGH
Performance decrement is the one consistent objective sign, and the only strategy with consensus support is tracking each athlete against their own baseline: performance, mood, sleep, resting heart rate and illness frequency, not a population cutoff. MODERATE MODERATE
A large share of "overtraining" is misdiagnosed under-fuelling. Overtraining and RED-S (low energy availability) overlap heavily, both starting from the same hormonal disruption driven by too little energy or carbohydrate. Many negative "overtraining" outcomes, including low testosterone in male athletes, are primarily under-eating (Stellingwerff and Heikura 2021). MODERATE MODERATE
The Debate
Is overtraining a detectable condition?
Older biomarker literature, 1990s–2010s
Overtraining is a discrete, detectable condition. Individual studies found altered cortisol, adrenaline (Fry 1994), blood viscosity (2006), oxidative stress (2010) and heart-rate variability (Mourot 2004) in overtrained athletes.
vs
2021–2022 scoping & systematic reviews
Overtraining has no validated marker and stays a diagnosis of exclusion. The Carrard scoping review (2022) mapped every proposed marker and found none reproduced or validated ahead of time.
The reviews have the stronger position. Decades of small single-marker studies each found a difference, but none replicated as an individual diagnostic. The field has moved from "find the one test" to monitoring each athlete against their own baseline.
Honest Limitations
LAB vs REAL WORLD
The lab: Controlled overload studies in elite endurance athletes detect overreaching with multi-test panels.
Real world: Almost all evidence is national-squad, triathlete or military samples under deliberately extreme load, and you can't ethically induce real overtraining, so samples are tiny (7 to 51 people).
Be MORE skeptical
LAB vs REAL WORLD
The lab: Markers separate overtrained from healthy groups.
Real world: Every usable marker only works as a change from that person's own baseline. Without prior data, a one-off reading tells you almost nothing.
Need a baseline
The Nuance
"Overtraining" in everyday speech and "overtraining syndrome" in the literature are not the same thing. The clinical syndrome is rare; the everyday tiredness is common and usually harmless.
The biggest practical trap is treating low testosterone in an under-fuelled male athlete with testosterone replacement. In that situation, low testosterone is usually a downstream effect of under-eating. Fix the energy first and refer before prescribing.
Published prevalence figures (roughly 7 to 20 percent, and higher in some endurance groups) are unreliable, because they mix ordinary overreaching, burnout and the true syndrome under inconsistent older definitions. Treat any specific prevalence number with suspicion.
