Metabolic Stress: It Burns So Good!
Metabolic stress appears to be a key driver for many of the potential benefits of BFR training. Metabolic stress refers to the build-up of by-products of muscle contraction and is responsible for the burn and hypoxic environment that we all know and love. “Metabolite accumulation and the hypoxic environment causes greater muscle activation, fatigue, and anabolic signaling when compared to the same intensity of exercise done without BFR.” (Lorenz et al., 2021)
Metabolic stress has been shown to:
Cause earlier recruitment of type 2 fibers and high threshold motor units
Stimulate group III-IV afferents to induce fatigue and recruit additional motor units
Trigger angiogenesis- development of new blood vessels.
Increase mitochondrial density
Improve buffering processes.
Evidence supporting the benefits of local ischemic exercise that reduces arterial flow has been known as early as 1990. This graph, from Kaijser et al. (1990) is the result of a 4x per week, 4-week training program where 8 males exercised one leg in ischemia to exhaustion at the maximal tolerable ischemic pain and the other leg did the same workload to serve as the control.
While this is an exaggerated example of the benefits of ischemic exercise, the results showed a ~25% increase in exercise duration to failure in the ischemic leg during local ischemia and a ~19% increase in duration when exercising in normal conditions. While both groups improved in their time to failure, the ischemic exercise leg improved significantly more in ischemic exercise and was trending towards a higher improvement in normal exercise conditions. Further, citrate synthase activity was higher post-training in the ischemic leg indicating improved mitochondrial function. Pair that with a reduction in lactate dehydrogenase activity in the ischemic leg post-training and the authors conclude greater local muscle oxidative capacity. Interesting to note there was a degree of specificity in the adaptations…
Resources
Kaijser, L., Sundberg, C. J., Eiken, O., Nygren, A., Esbjornsson, M., Sylven, C., & Jansson, E. (1990). Muscle oxidative capacity and work performance after training under local leg ischemia. Journal of Applied Physiology, 69(2), 785–787. doi:10.1152/jappl.1990.69.2.785
Lorenz D, Bailey L, Wilk K, Mangine B, Head P, Grindstaff TL, Morrison S. Current Clinical Concepts: Blood Flow Restriction Training. J Athl Train. 2021 Jan 22. doi: 10.4085/418-20. Epub ahead of print. PMID: 33481010
****Remember, the decision to use BFR, or any treatment for that matter, should be based on the pillars of evidence-based practice.
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