November 2016

Sex and BFR: Too many studies exclude young women

In the moment: Rehabilitation


By Cary Groner

The problematic trend of physiological studies in which young women are underrepresented now extends to research on blood flow restriction (BFR) training, according to a paper epublished in October by Clinical Physiology and Functional Imaging.

BFR training, which involves partially occluding blood flow to the limbs in combination with low-load resistance exercise, is experiencing a recent surge in popularity (see “Blood Flow Restriction Training: The slow-flow movement is fast becoming rehab’s hottest trend,” July 2016, page 20).

The authors of the new paper reported, for example, that in Japan, where the technique originated and is most widespread, 55% of those using BFR are women, but that of existing BFR research, only 29% and 17% of short- and longer-term studies, respectively, included young women.

“This issue isn’t exclusive to blood flow restriction; it’s rampant in physiology studies,” said Jeremy Loenneke, PhD, one of the paper’s authors and an assistant professor of exercise science at the University of Mississippi in Oxford. “People will do a study only in men, then extrapolate it to the whole population. But you really shouldn’t do that, and it’s a problem given that women represent about half the world.”

Loenneke and his coauthors considered the possible reasons for the discrepancy and eliminated one—namely, that women have more fatty tissue than men, thus complicating surface electromyography (EMG) signals.

“Women do have more adipose tissue in general, but many men have a lot of it as well,” Loenneke said. “If you normalize the EMG signal to itself, you address that problem.”

The researchers noted a curious aspect of the issue: namely, that older women are represented in studies just as often as older men. As LER reported, for example, BFR shows promise for leg strengthening in female patients with osteoarthritis. The most likely reason young women are excluded, then, is that researchers don’t want to deal with the metabolic variables introduced by the menstrual cycle. But, according to Loenneke, this is a poor reason to leave women out.

“Studies don’t usually last a week; they typically last two to three months,” he said. “So the women would be training across both the follicular and luteal phases of their cycles; they don’t train half the month and then take the other half off. Mechanistically speaking, the distinction between BFR’s effects in different parts of the cycle might be interesting, but pragmatically, for muscle size and strength, it probably wouldn’t matter.”

Loenneke suggested that practical solutions to the problem would strengthen research rather than compromise it.

“The ideal would be to have the same number of each sex and then see if there was any difference between them in the end,” he said. “But we understand that sometimes that’s not possible. You may be unbalanced in your study population and end up with more men than women. One thing you could do in that situation is run the group results and get the mean—do traditional stats the way we always do, in other words—but then plot the individual responses to see if there’s a similar pattern in men and women. That would be something to investigate.”

Loenneke and his colleagues have recruited women for their own BFR studies and have seen them respond to the technique, so they admit to frustration with the slow rate of change across the field.

“Once in a while you hear researchers say that we need more women in the studies, but then nothing ever comes of it,” he said. “If there’s something you need to control for in the menstrual cycle, consider it when you plan the study, but at least try to include them. And if for some reason you don’t, explain why and acknowledge that it’s a limitation in your study.”


Counts BR, Rossow LM, Mattocks KT, et al. Let’s talk about sex: Where are all the young females in blood flow restriction research? Clin Physiol Funct Imaging 2016 Oct 11. [Epub ahead of print]

Segal NA, Williams GN, Davis MC, et al. Efficacy of blood flow-restricted, low-load resistance training in women with risk factors for symptomatic knee osteoarthritis. PM R 2015;7(4):376-384.

Bryk FF, Dos Reis AC, Fingerhut D, et al. Exercises with partial vascular occlusion in patients with knee osteoarthritis: a randomized clinical trial. Knee Surg Sports Traumatol Arthrosc 2016;24(5): 1580-1586.

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