A Critical Occluding Tension Phase Transition Occurs Between 30% and 40% 1RM in Dynamic Knee Extension Exercise.

Oxygen delivery and utilization to working muscle(s) are essential for sustained exercise performance. Higher contractile loads can occlude intramuscular blood vessels, limiting oxygen perfusion, while lighter loads minimize occlusion and allow perfusion. This study explored how external load impacted relative muscle endurance, work capacity, and muscle oxygenation in free-flow and cuff-occluded states to estimate the load at which a muscle occludes itself. Twenty-nine participants (10 untrained (UT), 9 endurance-trained (ET), 10 resistance-trained (RT); 14 females, 15 males) completed repetitions to task failure at 15%, 20%, 25%, 30%, 40%, and 80% of their unilateral knee-extension 1RM in free-flow and cuff-occluded states, with muscle oxygenation (SmO₂) measured using near-infrared spectroscopy (NIRS). Compared to cuff-occlusion, greater work capacity and SmO₂ were observed in the free-flow state at 15%-30% 1RM (p < 0.05), but no differences were seen at 40% and 80% 1RM (p ≥ 0.05). In the free-flow state above 40% 1RM, RT participants performed more work than UT and ET (p < 0.05), with no differences between UT and ET (p ≥ 0.05). At 25% and 30% 1RM, ET and RT participants performed more work than UT (p < 0.05), with no differences between ET and RT (p ≥ 0.05). At 15% and 20% 1RM, ET participants performed more work than UT and RT (p < 0.05). These results suggest a load range of > 30% but ≤ 40% 1RM may represent a phase transition in critical occluding tension of the quadriceps, with contractile induced occlusion occurring above this range, and perfusion occurring below.