Background <p>This study investigated the effects of adding blood flow restriction (BFR) to high-load isokinetic resistance training on muscle strength and local muscular endurance.</p> Methods <p>Forty-two middle- and long-distance runners were randomly assigned to an isokinetic training group with BFR (BFR group; <i>n</i> = 21) or an isokinetic training group without BFR (non-BFR group; <i>n</i> = 21). The training protocol consisted of concentric knee flexion and extension exercises performed at angular velocities of 60°/s (3 sets of 10 maximal repetitions) and 180°/s (3 sets of 30 maximal repetitions), twice weekly for 8 weeks. BFR was applied at 80% of arterial occlusion pressure (AOP) in the BFR group. Muscle strength (peak torque [PT]) and local muscular endurance (fatigue index [FI]) were evaluated before and after the intervention.</p> Results <p>Both groups demonstrated significant increases in muscle strength for both knee flexors and extensors at angular velocities of 60°/s and 180°/s. However, no significant between-group differences were observed for strength gains (<i>p</i> &gt; 0.05 for all comparisons). Regarding local muscular endurance, the magnitude of improvement in FI was significantly greater in the BFR group compared to the non-BFR group for both flexors (<i>p</i> = 0.003, η²= 0.102) and extensors (<i>p</i> = 0.005, η²= 0.091).</p> Conclusions <p>This study demonstrates that adding BFR to high-load isokinetic resistance training does not enhance muscle strength beyond high-load isokinetic resistance training but significantly improves local muscular endurance, as evidenced by greater reductions in FI in the BFR group. These findings suggest that BFR may be a valuable tool for optimizing endurance-specific adaptations in athletic populations requiring sustained performance. Future research should explore optimal BFR protocols for endurance-focused training across diverse athletic disciplines.</p> Trial registration <p>Registration date: 04/11/2024, ClinicalTrials: NCT06678009.</p>