Vol. 7, Issue 2, Part C (2025)

Functional overreaching (FOR) and acute fatigue (AF) are distinct outcomes of intensified training periods in endurance athletes. While both conditions induce heightened fatigue, they differ critically in their performance impacts: FOR causes transient performance decline, whereas AF maintains performance capacity. Both conditions can lead to supercompensation and subsequent performance gains. However, FOR carries additional risks, including increased susceptibility to illness and injury.

Objective: To systematically review published biomarkers that distinguish FOR from AF in endurance athletes.

Method: We conducted a structured literature search across SPORTDiscus, PubMed, and PsycINFO databases from January 1, 2010, to April 5, 2023. Search terms included: (("Athletes"[Mesh]) OR athletes) AND (overload OR "intensified training" OR "increase in training") AND (("Overtraining Syndrome"[Mesh]) OR overreach* OR maladaptation).*

Results: From 270 identified articles, 12 studies met the inclusion criteria. The most frequently reported biomarkers were maximal heart rate (HR_max) and Profile of Mood States (POMS) (seven studies each), peak blood lactate concentration ([HLa]_peak) (five studies), maximal oxygen uptake (VO_2max) (four studies), and reaction time (RT) (three studies).

Conclusion: FOR is characterized by altered physiological responses including decreased VO_2max, HR_max, and [HLa]_peak, whereas AF markers are less definitive, with physiological variables remaining relatively stable while mental fatigue increases.