Changes in the Isokinetic Strength Profile in Soccer Players Following Fatigue: A Systematic Review

Background: Soccer demands repeated high-intensity, powerful actions, which increase the risk of lower-limb injuries as fatigue accumulates in the working muscles. The hamstrings-to-quadriceps (H:Q) ratio is widely used to assess knee strength balance. Several studies have reported that fatigue can significantly decrease the H:Q ratio. Therefore, the objective of the present systematic review was to analyze the effect of simulated soccer-induced fatigue on the conventional and functional isokinetic H:Q ratios in soccer players. Methods: Eligibility required injury-free soccer players aged ≥ 18 years, 45- or 90-min running-based simulated soccer protocols, and pre-post H:Q data; injured samples and reports lacking relevant outcomes were excluded. MEDLINE (via PubMed), Web of Science, and SPORTDiscus were searched from inception to 6 April 2026. The following keywords were used: “soccer,” “muscle fatigue,” “hamstrings,” “H:Q ratio,” “isokinetic strength.” Quality was assessed using the 12-item National Heart, Lung, and Blood Institute tool for uncontrolled before–after studies. Findings were synthesized narratively. Results: Ten studies (155 participants) met the inclusion criteria, all assessing the H:Q ratio before and after a simulated soccer protocol. Most studies reported a significant decrease in the functional ratio of the dominant leg after different simulated soccer protocols. Conclusions: Simulated soccer-induced fatigue can reduce the H:Q ratios, but these changes reflect altered knee strength balance rather than direct evidence of increased injury risk. Future studies should use more advanced, field-simulated soccer protocols or real competitions to evaluate the H:Q ratio and whether its changes are associated with injury risk and develop rehabilitation strategies focused on delaying fatigue.