The American College of Sports Medicine states that in order to gain strength, one must lift a load 65% of a 1 rep max, or 65% of a load heavy enough that a person is only able to lift it one time. This is a problem in Physical Therapy. When we see a patient for the first few months after an injury or surgery, typically they are limited to significantly less than 65% of their 1RM. This may be due to soft tissue limitations such as after a tendon repair, or muscle strain. Or, it may be due to structural limitations such as after a fracture, or surgery that requires partial, or non-weight bearing. Regardless of the cause, the fact is that we are limited in how PTs can load a patient and build strength without disruption of the injury or surgical repair.
This question was of increased significance with the onset of the wars in Iraq and Afghanistan. As our in-field medical care improved, soldiers were surviving much higher level trauma, which lead to more wounded warriors entering the rehab side of the military health care system. These individuals were highly motivated to get back into the field, but could not push their loads heavy enough to 65% 1RM to achieve strength gains. This is where Blood Flow Restriction or BFR started in the United States. BFR is a technique in which a tourniquet is applied to a limb to restrict blood flow. This can be manipulated in various ways depending on the treatment. The majority of the time, the blood flow is not restricted fully, but enough to restrict venous return, but allow a lower pressure of arterial blood to perfuse the limb. The research has found that, under conditions of BFR, an individual is able to build muscle strength at loads of 20-30% of 1RM. This sounds too good to be true, but there have been nearly 200 rigorous studies that have found just that.
BFR works on the body in a few ways. Without delving too far into exercise physiology, understand that muscles don’t fire all their contractile fibers all at once. Unless under extreme duress, like when a mother lifts a car off a child, muscles only contract the bare minimum number of fibers to do the job. In BFR, the muscles are cut off from the rich oxygen supply of the blood flow which forces them to work in a hypoxic state. This switches the muscle metabolism from aerobic (using oxygen) to anaerobic (no oxygen). This causes the buildup of muscle metabolites with the primary metabolites being lactic acid and hydrogen ions. In the presence of these metabolites, muscle fibers quickly fatigue which signals the body to activate more muscle fibers. Studies have found with BFR, the biceps muscle activation is nearly 50% higher than with control at the same resistance. This increased activation is part of what leads to increased strength gains at lower levels of resistance.
There is also a hormonal component to BFR training. Studies have found that with the spike in lactate and H+ ions, there is following spike of Growth Hormone (GH). Despite what has been said about Human Growth Hormone in the media and with various sports stars, there are very few studies to actually show that it is a significant muscle builder. However, it does play a very important role in collagen synthesis. This collagen synthesis is what is needed for tendon repair, bone healing, and cartilage repair. With safe, low loads, we are able to manipulate GH in ways that will build tendon strength and help with tendon healing. As this GH spike is systemic, we can use low loads with BFR in the legs, to help heal tendon issues in the upper body. The last and most complex effect of BFR is with how the body uses two different muscle compounds. IGF-1 is a compound the body produces that is associated with muscle growth, and Myostatin is a compound that is associated with reducing muscle growth. Animals that have been found to be lacking in Myostatin have enormous muscle growth. Why would a body have a compound that hinders muscle growth? It is possible to have too much of a good thing. BFR has been show to up regulate IGF-1 and down regulate Myostatin. The result is increased muscle hypertrophy at low loads compared to the loads needed to achieve the same thing without BFR.
Is it safe? The literature to date has shown no significant risks with BFR to a healthy individual. The primary concern is with blood clotting as the limb is under a state of reduced blood flow. In fact, the release of the tourniquet at the conclusion of the exercise has show to have a fibrinolytic effect which actually reduces the risk of DVT. There are also some cardiovascular effects to be aware of, as there is a change in blood volume through the heart due to the restriction. This decrease in stroke volume is countered by the heart by increasing heart rate. If this is an issue to a patient, BFR may be contraindicated. The questions to ask are: Is this patient appropriate for surgery? (as a tourniquet would be used in surgery) Is this patient appropriate for exercise? And lastly; Is this patient appropriate for high intensity exercise? BFR will stress the body, as that is how adaptation and improvement happen. However, with BFR, we are able to elicit positive responses to exercise at a lower level than has been seen before. This results in faster recovery from surgery, improved functional outcomes, reduced muscle loss from disuse atrophy, and better surgical repair.