Hunter, BenMeyler, SamuelMaunder, EdCox, Tobias HMuniz-Pumares, Daniel2024-12-092024-12-092024-07-24International Journal of Sports Physiology and Performance, ISSN: 1555-0265 (Print); 1555-0273 (Online), Human Kinetics, 19(9), 963-972. doi: 10.1123/ijspp.2024-01011555-02651555-0273http://hdl.handle.net/10292/18436Purpose: Training characteristics such as duration, frequency, and intensity can be manipulated to optimize endurance performance, with an enduring interest in the role of training-intensity distribution to enhance training adaptations. Training intensity is typically separated into 3 zones, which align with the moderate-, heavy-, and severe-intensity domains. While estimates of the heavy- and severe-intensity boundary, that is, the critical speed (CS), can be derived from habitual training, determining the moderate–heavy boundary or first threshold (T1) requires testing, which can be costly and time-consuming. Therefore, the aim of this review was to examine the percentage at which T1 occurs relative to CS. Results: A systematic literature search yielded 26 studies with 527 participants, grouped by mean CS into low (11.5 km·h−1; 95% CI, 11.2–11.8), medium (13.4 km·h−1; 95% CI, 11.2–11.8), and high (16.0 km·h−1; 95% CI, 15.7–16.3) groups. Across all studies, T1 occurred at 82.3% of CS (95% CI, 81.1–83.6). In the medium- and high-CS groups, T1 occurred at a higher fraction of CS (83.2% CS, 95% CI, 81.3–85.1, and 84.2% CS, 95% CI, 82.3–86.1, respectively) relative to the low-CS group (80.6% CS, 95% CI, 78.0–83.2). Conclusions: The study highlights some uncertainty in the fraction of T1 relative to CS, influenced by inconsistent approaches in determining both boundaries. However, our findings serve as a foundation for remote analysis and prescription of exercise intensity, although testing is recommended for more precise applications.© 2024 Human Kinetics. Free access.endurance trainingexercise prescriptionintensity domainsmonitoringtesting32 Biomedical and Clinical Sciences3202 Clinical Sciences42 Health SciencesClinical Research1106 Human Movement and Sports Sciences1116 Medical Physiology1701 PsychologySport Sciences3202 Clinical sciences3208 Medical physiology4207 Sports science and exerciseHumansRunningPhysical EndurancePhysical Conditioning, HumanAthletic PerformanceAdaptation, PhysiologicalHumansAdaptation, PhysiologicalPhysical EnduranceRunningAthletic PerformancePhysical Conditioning, HumanHumansRunningPhysical EndurancePhysical Conditioning, HumanAthletic PerformanceAdaptation, PhysiologicalJournal ArticleOpenAccess10.1123/ijspp.2024-0101