NEURAL ADAPTATIONS FROM BFR

Whether or not blood flow restriction exercise (BFR) elicits neural adaptation is an important component in determining its overall utility for the rehab or exercise professional. One might consider neural adaptation is implied given we say rather confidently that BFR performed repeatedly in combination with a low load resistance exercise increases strength. Others might attribute an increase in force production following a BFR training block to be a result of the biomechanical adaptation hypertrophy. Although, the association between strength and muscle size seems to be loosening (Jessee et al., 2021). A better understanding of the neural adaptations elicited via BFR could provide valuable information regarding the mechanisms which lead to improvements in skeletal muscle strength, as well as provide support for BFR’s use and refining how it is deployed.    

A recent review paper by Christoph Centner and Benedikt Lauber tackled the complex task of assessing the existing literature to determine how neural adaptations from BFR exercise compared to those achieved via light (LL) or heavy load (HL) resistance exercise without blood flow restriction.(Centner & Lauber, 2020) They performed their review in accordance with PRISMA guidelines, have made their data available, and only included studies that passed their risk of bias and quality assessments.   While examining the acute neural responses to exercise of any sort provides insight into mechanisms or the potential for adaptation, the main objective of this review was to “summarize the long-term (≥4 weeks) effects of LL-BFR exercise on neural markers (peripheral vs. central) compared to HL and LL training without BFR”. The authors were able to include 10 studies in their qualitative synthesis and 4 studies into a meta-analysis. Based on the data available, the authors were able to draw much stronger conclusions regarding peripheral adaptations than central adaptations. What data there is on central nervous system adaptations does not look to be negative in any way. It’s merely questionable whether or not low-load BFR exercise elicits similar responses centrally to heavy load resistance exercise when used chronically.  

LL-BFR vs LL (Muscle Excitation and Central Activation)

This review was able to conclude via meta-analysis that LL-BFR does in fact lead to greater excitation in muscle than LL. However, because there’s always a “however”, despite this conclusion aligning with many of the studies assessing acute EMG, differences between the two appear to be attenuated when the LL cohort exercises to volitional failure. Thus, highlighting both a key concept in BFR and an imperative for muscle size increase; that BFR accelerates fatigue and exercise must elicit high levels of fatigue respectively. The authors took extra care to highlight that while important, using an elevation in EMG as a surrogate for motor unit (MU) recruitment is problematic. They directed the reader to works by Fatela that have taken steps to estimate MU recruitment and firing rates, while Krustrup in 2009 used biopsies to assess creatine phosphate levels.(Fatela et al., 2019; Krustrup et al., 2009) Taken together these findings point to early recruitment of higher threshold motor units as a result of the addition of BFR to low load resistance exercise.   Evaluating central responses between LL-BFR and LL resistance exercise is much more difficult given the paucity of work in this area. There are only 2 studies looking at long-term changes in response to LL exercise with or without BFR. Those studies looked at entirely different outcomes so they were unable to compare them in any way. Briefly, Colomer showed no changes in V-wave or H-reflex following 4-weeks of LL-BFR or LL resistance exercise.(Colomer-Poveda et al., 2017) While Moore 2004 noted depressed resting twitch torque resulting in an increased post-activation potentiation finding.(Moore et al., 2004) The authors note the finding by Moore potentially helps explain the adaptation of strength increase in response to LL-BFR training, but state clearly that no one has in fact examined this in a long-term trial.  

LL-BFR vs HL (Muscle Excitation and Central Activation)

Sorting out whether or not there is a difference in muscle excitation between LL-BFR and HL is much more elusive. There are only two long-term studies to consider. An eight-week trial by Ramis 2020 found no difference in muscle excitation between their HL and their LL-BFR group while a six-week trial by Sousa 2017 actually found a 20% greater excitation in their LL-BFR group compared to their HL group.(Ramis et al., 2018; Sousa et al., 2017) The authors note a number of potential reasons for this unusual finding in light of the fact that it also contradicts the vast majority of acute EMG studies examining differences between LL-BFR and HL. Ultimately they conclude “a more strict and standardized normalization of the data (e.g., to maximal M-wave amplitude or maximal EMG activity of the post-test) in future studies is therefore warranted for an adequate interpretation of these variables.”   Much like in the comparison between LL-BFR and LL on central activation, there are only two papers but they did assess the same outcome; electrically evoked super-imposed twitches. A 6-week trial by Cook found no significant changes in either a HL or LL-BFR cohort while Kubo noted significant increases in a HL group that were not realized in a LL-BFR group. (Cook et al., 2018; Kubo et al., 2006)  

Summing things up:

While there are a large number of papers examining how BFR influences the neuromuscular system. Its influence has not been examined in a systematic way so that strong conclusions can be drawn regarding mechanisms of adaptation or how best to facilitate desired adaptations in the nervous system. “From a practical standpoint, the findings from this systematic review and meta-analysis indicate that neural adaptations, especially on the EMG level, can be augmented when applying BFR to LL exercise. These results seem to support the notion that besides structural, also neural changes might contribute to the observed strength increases following LL-BFR training.”  

1. Centner, C., & Lauber, B. (2020). A Systematic Review and Meta-Analysis on Neural Adaptations Following Blood Flow Restriction Training: What We Know and What We Don’t Know. Frontiers in Physiology, 11, 887.
2. Colomer-Poveda, D., Romero-Arenas, S., Vera-Ibáñez, A., Viñuela-García, M., & Márquez, G. (2017). Effects of 4 weeks of low-load unilateral resistance training, with and without blood flow restriction, on strength, thickness, V wave, and H reflex of the soleus muscle in men. European Journal of Applied Physiology, 117(7), 1339–1347.
3. Cook, S. B., Scott, B. R., Hayes, K. L., & Murphy, B. G. (2018). Neuromuscular Adaptations to Low-Load Blood Flow Restricted Resistance Training. Journal of Sports Science & Medicine, 17(1), 66–73.
4. Fatela, P., Mendonca, G. V., Veloso, A. P., Avela, J., & Mil-Homens, P. (2019). Blood Flow Restriction Alters Motor Unit Behavior During Resistance Exercise. International Journal of Sports Medicine. https://doi.org/10.1055/a-0888-8816
5. Jessee, M. B., Dankel, S. J., Bentley, J. P., & Loenneke, J. P. (2021). A Retrospective Analysis to Determine Whether Training-Induced Changes in Muscle Thickness Mediate Changes in Muscle Strength. Sports Medicine . https://doi.org/10.1007/s40279-021-01470-5
6. Krustrup, P., Söderlund, K., Relu, M. U., Ferguson, R. A., & Bangsbo, J. (2009). Heterogeneous recruitment of quadriceps muscle portions and fibre types during moderate intensity knee-extensor exercise: effect of thigh occlusion. Scandinavian Journal of Medicine & Science in Sports, 19(4), 576–584.
7. Kubo, K., Komuro, T., Ishiguro, N., Tsunoda, N., Sato, Y., Ishii, N., Kanehisa, H., & Fukunaga, T. (2006). Effects of low-load resistance training with vascular occlusion on the mechanical properties of muscle and tendon. Journal of Applied Biomechanics, 22(2), 112–119.
8. Moore, D. R., Burgomaster, K. A., Schofield, L. M., Gibala, M. J., Sale, D. G., & Phillips, S. M. (2004). Neuromuscular adaptations in human muscle following low intensity resistance training with vascular occlusion. European Journal of Applied Physiology, 92(4-5), 399–406.
9. Ramis, T. R., Muller, C. H. de L., Boeno, F. P., Teixeira, B. C., Rech, A., Pompermayer, M. G., Medeiros, N. da S., Oliveira, Á. R. de, Pinto, R. S., & Ribeiro, J. L. (2018). Effects of Traditional and Vascular Restricted Strength Training Program With Equalized Volume on Isometric and Dynamic Strength, Muscle Thickness, Electromyographic Activity, and Endothelial Function Adaptations in Young Adults. Journal of Strength and Conditioning Research / National Strength & Conditioning Association. https://doi.org/10.1519/JSC.0000000000002717
10. Sousa, J., Neto, G. R., Santos, H. H., Araújo, J. P., Silva, H. G., & Cirilo-Sousa, M. S. (2017). Effects of strength training with blood flow restriction on torque, muscle activation and local muscular endurance in healthy subjects. Biology of Sport / Institute of Sport, 34(1), 83–90.