Acute Musculoskeletal Responses to Isometric Blood Flow Restriction Training of the Patellar Tendon in Healthy, Active Young Adults: A Randomized Controlled Trial

Objective: This study aimed to investigate the acute patellar tendon (PT) responses to different isometric resistance training regimens in healthy individuals, with a particular focus on low-intensity blood flow restriction training. Design: Randomized controlled experimental study. Methods: Sixty-six participants (22.3 ± 3 years) were randomly assigned to one of three groups: high-intensity isometric exercise (HI; 70% maximal voluntary isometric contraction [MVIC]), low-intensity isometric exercise with sham blood flow restriction (LI-Sham; 30% MVIC + 20 mmHg), or low-intensity isometric exercise with blood flow restriction (LI-BFR; 30% MVIC + 80% arterial occlusion pressure [AOP]). All participants performed five sets of 30-s isometric knee extensions. Patellar tendon thickness was assessed using ultrasonography at three sites (25%, 50%, and 75% of tendon length) before and immediately after exercise. Vastus medialis thickness and MVIC were also assessed. Results: Patellar tendon thickness decreased significantly over time, with a significant Group × Time interaction (F(2, 315) = 27.94, p < 0.001). When averaged across measurement sites, the estimated reductions were −0.586 mm in HI, −0.545 mm in LI-BFR, and −0.242 mm in LI-Sham. Reductions were greater in HI and LI-BFR than in LI-Sham (both Holm-adjusted p < 0.001), whereas no statistically significant difference was detected between HI and LI-BFR (Holm-adjusted p = 0.416). Vastus medialis thickness showed significant Time and Group × Time effects (both p < 0.001), with greater increases in HI and LI-BFR than in LI-Sham. No significant Time or Group × Time effects were observed for MVIC (p = 0.100 and p = 0.409, respectively). Conclusions: A single session of LI-BFR induced an acute reduction in ultrasound-measured PT thickness that was greater than that observed with LI-Sham. No statistically significant difference in tendon-thickness change was detected between LI-BFR and HI; however, this finding should not be interpreted as evidence of equivalence. These findings are limited to immediate responses in healthy young adults and should not be interpreted as evidence of long-term tendon adaptation or clinical benefit.