Low-load blood flow restriction training as a viable alternative to high-load eccentric training for improving Achilles tendon properties: the superiority of a calf-specific regimen

Abstract Objective To compare the effects of 2 different blood flow restriction (BFR) rehabilitation protocols versus classic high-load eccentric resistance training on the morphology and biomechanical properties of the Achilles tendon in healthy adults, and to provide alternative training strategies for preventing Achilles tendon injuries in military training. Methods​ A randomized controlled trial was conducted. Fifty-five healthy adults from a university in Xi’an were enrolled between December 20 and 23, 2024, and randomly assigned to one of three groups using a random number table: a high-load eccentric resistance training group (HL, n=18), a low-load eccentric resistance training with calf-applied BFR group (C-BFR, n=16), and a low-load eccentric resistance training with thigh-applied BFR group (T-BFR, n=19). All groups completed an 8-week training program. Achilles tendon cross-sectional area was assessed using color Doppler ultrasonography at baseline, 6 weeks and 8 weeks. Achilles tendon stiffness and Young’s modulus were evaluated using color Doppler ultrasonography and a wireless microFET3 dynamometer. The maximal voluntary isometric contraction (MVIC) of the medial gastrocnemius was simultaneously measured using the wireless microFET3 and surface electromyography. Countermovement jump (CMJ) height and drop jump (DJ) ground contact time were measured using a three-dimensional force platform. Results After 8 weeks of intervention, color Doppler ultrasonography assessment revealed that the cross-sectional area of the Achilles tendon significantly increased in all three groups compared to baseline (P<0.01, η²=0.847), with a greater increase in the C-BFR group than in the T-BFR group. Tendon stiffness significantly increased in all three groups compared to baseline (P<0.01, η²=0.722), and the increase was greater in the C-BFR group than in the HL and T-BFR groups. For Young’s modulus, no statistically significant change was observed in the T-BFR group compared to baseline (P>0.05), while both the C-BFR and HL groups showed significant increases (P<0.05, η²=0.181), with a greater increase in the C-BFR group than in the HL and T-BFR groups. The MVIC of the medial gastrocnemius significantly increased in all three groups compared to baseline (P<0.01, η²=0.775), with a greater increase in the C-BFR group than in the T-BFR group. CMJ height significantly increased in all three groups (P<0.01, η²=0.569), with a greater increase in the C-BFR group than in the T-BFR group. DJ ground contact time significantly shortened in all three groups (P<0.01, η²=0.518), and the shortening was greater in the C-BFR group than in the T-BFR group. Conclusion​ Both C-BFR and T-BFR combined with low-load eccentric resistance training can achieve improvements in Achilles tendon morphology, biomechanical properties, and kinematic performance similar to those achieved with classic high-load eccentric resistance training. Furthermore, compared to the T-BFR protocol, the C-BFR protocol demonstrates superior benefits.