The question of what is happening to the tendon with BFR has been a hot topic of discussion since the early days of military application. This is an area where there has been scarce information available. A study by Centner et al. was published at the end of this last year and it helps to add information to knowledge base on BFR and tendon.
This study aimed to compare changes in Achilles tendon properties (size and stiffness) after performing BFR with light load (LL-BFR) or heavy load (HL). There was also a non-exercising control group. Standing and seated heel raises were performed 3 days per week for 14 weeks and 1 RM was assessed and the loading was progressed every 4 weeks. The HL group performed exercise at 70% of 1RM for the first four weeks and progressed the load by 5% every four weeks, utilizing an 85% load for the final 2 weeks. The LL-BFR group performed exercise with 20% of 1RM for the first four weeks and progressed the load by 5% every four weeks, utilizing a 35% load for the final 2 weeks. The HL group performed 3 sets of 6-12 reps per exercise and the LL-BFR group performed the standard 30,15,15,15 for each exercise. A 60 second rest period was used between sets for both groups and the LL-BFR group used a 50% limb occlusion pressure (LOP) that was measured every session in the standing position.
The main findings of the study were that the HL and LL-BFR protocols produced similar changes in muscle size and strength as well as similar changes in tendon CSA and mechanical properties (stiffness).
This article is really fascinating for many reasons. For starters, it shows similar changes in tendon adaptation when comparing BFR to heavy load. It's obviously a mistake to draw any strong conclusions from just one paper, but these findings certainly help counter the concern that BFR may cause adaption in muscle that outpaces tendon. Second, the study was designed with a standardized progression of loading for both groups which can and probably should be carried over into our exercise prescription in rehab. Re-assess load every four weeks to establish a new rep max and apply a greater percentage of load. Third, the study applied 50% of LOP which is below the 60% threshold we've seen in other articles and typically teach in our courses for LE exercise. This lower percentage being effective may be related to the measurement being taken in standing. Hughes et al. demonstrated a significant difference in arterial occlusion pressure when comparing measurements taken in supine to seated and standing. The average pressure in supine was 187mmHg and the average in standing was 241mmHg. (Hughes 2018) Based on the numbers from this paper by Hughes, 50% pressure from the standing measurement (120mmHg) would be equal to 64% of the pressure measured in supine. This would bring it up into the 60-80% range we think of as being effective.
This seems to be another valuable piece to add to the BFR puzzle. The big takeaways are that BFR is affecting more than just muscle and we need to progress the load!
Centner C, Lauber B, Seynnes OR, et al. Low-load blood flow restriction training induces similar morphological and mechanical Achilles tendon adaptations compared to high-load resistance training. J Appl Physiol. November 2019. doi:10.1152/japplphysiol.00602.2019
Hughes L, Jeffries O, Waldron M, et al. Influence and reliability of lower-limb arterial occlusion pressure at different body positions. PeerJ. 2018;6:e4697.