Blood Flow Restriction (BFR) Works in Broad Cuff Pressure Range: Lower (40%) & Higher (80% AOP) Similarly Effective

"Go super tight or go home"? Nope, we're not talking 'bout NC-17 rated websites... this provocatively phrased statement about how to wear your #BFR cuffs at the gym is probably as faulty as the notion that you can judge the effectivity of a workout by how much it hurts.

Based on personal observation I can say with certainty that #BFR, i.e. Blood Flow Restriction during resistance training is becoming mainstream. Even in German mainstream gyms you'll see guys many of you would probably ask "do you even lift" if you met them outside of the gym cuffing themselves up to the point where their arms are close to start necrotizing...

That's not just potentially dangerous but, as a recent study from the Brigham Young University in UT suggests, it's also unnecessary.
Why? It may sound astounding, but a higher cuff pressure doesn't translate to practically relevant changes in blood flow restriction. That's the main finding of a new study some of you may have heard about in the @SuppVersityFacebookNews, when the data was first presented at a conference. Data representing the results of an experimental investigation by Kent W. Crossley, and colleagues, who studied the relationship between blood flow restriction (BFR) cuff pressure and blood flow at rest and during exercise. Their goal: to answer the practically (and potentially health-)relevant question:

Will lower cuff pressures provide an ischemic stimulus comparable to higher pressures?

The relationship between blood flow and cuff pressure at rest was determined by measuring blood flow (Doppler Ultrasound) through the superficial femoral artery (SFA) in 23 adults across a range of pressures (0-100% Arterial Occlusion Pressure at rest, rAOP vs. eAOP which denotes testing during exercise, i.e. plantar flexion exercise).

Figure 1: Overview of the study design; the exercise was basically "calf training" (Crossley 2019).

Somewhat to their own surprise the cuff pressure / blood flow relationship was not linear, but plateaued as early as with 40% (see Figure 2 further below in this article). What this means is that...

...you can well apply more than 40% of the arterial pressure to the cuff and still won't see much more actual blood flow occlusion than at this rather moderate level.

Ok, the blood flow wasn't 100% identical but the differences were - within the statistical and experimental margins of error so small that you will not be able to argue based on Figure that they were statistically significant (in fact, p = 1.0) different when he external pressure was increased from 40 to 80% of the previously determined rAOP.
In the absence of significant effects of AOP in the 40-80% range, the authors conducted the final exercise test (eAOP) only at 40% AOP; and while Crossley et al. observed in this calf-raise(-ish) test that "eAOP was greater than rAOP (229±1.5 vs. 202±1.5 mmHg, P<0.01)", i.e. that...

...the real-world blood pressure building up in the arteria was 20% higher when the subjects exercised compared to the sedentary condition.

The actual blood flow [the amount of blood per minute] did "not significantly differ (P=0.49) between 40% rAOP or 40% eAOP". This means that the resting vs. exercise condition at a given external cuff pressure of 40% of the pre-determined e- and rAOP were identical. From a mechanistic standpoint it is likely that the greater pressure normalizes the amount of blood that's bypassing your calves and reaching your feet to what previous research suggested to be "optimal" levels, i.e. 60-70% of the unrestricted blood flow (~ 70-80 ml/min vs. 120 ml/min without cuff) whenever you apply cuffs at a pressure in what I deemed following the use of the phrase in astronomy the "goldilocks zone" (cf. Figure 2, green mark-up).

Figure 2: Blood flow through the superficial femoral artery at different cuff pressures as %-age of resting Arterial Occlusion Pressure (rAOP); mind the non-linear plateau in the "goldilocks zone" (my term added to Crossley 2019)

Overall, and most prominently in view of the previously discussed non-linear nature of the effects on actual blood flow, it does thus seem logical to follow the authors' conclusion that "BFR interventions opting for lower (e.g. 40% AOP), more comfortable pressures will likely provide an ischemic stimulus comparable to that of higher (80% AOP), less-comfortable pressures" (Crossley 2019).
References:

• Crossley, Kent W., et al. "Effect of Cuff Pressure on Blood Flow during Blood Flow–restricted Rest and Exercise." Medicine & Science in Sports & Exercise (2019). Ahead of print.
• Mouser, J. Grant, et al. "A tale of three cuffs: the hemodynamics of blood flow restriction." European journal of applied physiology 117.7 (2017): 1493-1499.