Centner et al. (2019) published what could be considered a landmark study that calls into question our current understanding of how tendons adapt to stress. This study supports the idea that blood flow restriction at low-load protocols could be used to increase the physical properties of the Achilles tendon similar to traditional high-load protocols.
Let’s look at what this study entailed..
Centner et al. investigated the effects of low-load BFR (20-35% 1RM) on Achilles tendon (AT) properties (stiffness, CSA and Young’s Modulus – a measure of the stress/strain of the tendon) and muscle adaptations (Gastrocnemius CSA and plantarflexion torque) and compared them to traditional high-load (70-85% 1RM) resistance training.
Fifty-five healthy individuals who reported no resistance training experience (1-2 hours/week) were recruited for this 3-times per week, 14-week study. Each participant was randomly allocated to either LL-BFR, HL or CON (no exercise) group. HL group performed bilateral sitting and standing calf raises for 3×6-12 repetitions with 1 minute interset rest and 3 minutes of between-exercise rest. Loads were adjusted every 4 weeks with periodic 1RM testing to accommodate for strength increases. The LL-BFR protocol had similar rest period allocations. LL-BFR was performed at 50% limb occlusion pressure with 20% 1RM and increased by 5% every 4 weeks until 35% 1RM using a 30-15-15-15 protocol and a Zimmer Biomet cuff (12cm wide) on each thigh. The cuff was inflated throughout the duration of each exercise and only deflated between exercises.
Thirty-eight participants completed the program (14 = HL, 11=LL-BFR, 13 = CON) with no drop-outs related to the side effects of the training. AT CSA showed greater changes in HL and LL-BFR vs. CON with AT CSA at 25% AT length increasing from 70.3 ± 17.7 mm² to 73.5 ± 17.2 mm² in HL (+4.6%) and from 68.2 ± 11.4 mm² to 73.5 ± 14.4 mm² (+7.8%) in LL-BFR with no changes in CSA in CON. AT stiffness increased in HL (401.5 ± 102.6 N/mm to 564.8 ± 157.6 N/mm; +40.7%) and LL-BFR (388.7 ± 76.9 N/mm to 529.2 ± 142.8 N/mm; +36.1%) with no changes in CON group. Young’s Modulus was unchanged in any group. Gastrocnemius muscle CSA increased in HL (14.3 ± 4.5 cm² to 15.4 ± 4.5 cm²; +7.7%) and LL-BFR (16.5 ± 3.2 cm² ± 18.0 ± 4.5 cm²; +9.1%) with no changes in CON group. Maximum voluntary torque increased in both HL (189.0 ± 83.1 Nm to 214.6 ± 86.1 Nm; +13.5%) and LL-BFR (226.7 ± 47.6 Nm to 248.9 ± 48.5 Nm; +9.8%) with no changes in CON.
What this means…
In individuals with tendinopathies, rehabilitation is focused predominantly on heavier resistance training (> 70% 1RM) since lighter load training (< 40% 1RM) has been shown ineffective at stimulating positive morphological and functional outcomes. This study raises questions about our current understanding of tendon physiology and provides a potential new avenue to utilize low-load resistance training in the clinical setting by using blood flow restriction to elicit these positively desired outcomes. Based on the results of this study, individuals with painful Achilles tendinopathies who cannot tolerate heavier loading protocols could utilize low-load blood flow restriction in the short-term and obtain similar physical and muscle changes as those lifting heavier loads. Blood flow restriction should ultimately still be viewed as a bridge modality in those that have the capacity to eventually lift heavier weights, as the benefits afforded to the cardiovascular, musculoskeletal and neurological system (motor unit recruitment etc.) are superior when compared head-to-head.
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