The Anterior cruciate ligament (ACL) is one of the key ligaments within the knee joint. It connects the femur (thigh bone) to the tibia (shin bone), and it is responsible for maintaining stability in the knee with lateral movements and/or sudden changes in direction. ACL tears are often regarded as one of the more severe types of knee injuries due to their lengthy recovery time.
An ACL reconstruction (ACL-R) surgery typically takes 6 to 9 months of recovery, but symptoms can still linger upwards of 12 months. Following ACLR surgery, patients experience significant loss of lower limb strength due to muscle atrophy. Knee extensor and knee flexor muscle weakness is substantial during the first 12 weeks following surgery, impairing lower limb function and quality of life. Muscle weakness can persist for years after ACLR surgery, and is associated with chronic reductions in function, a high re-injury risk, and joint degeneration. Therefore, targeting muscle weakness early in the rehabilitation process is imperative.
Current ACL rehabilitation protocols recommend heavy load resistance training (HL-RT) using external loads of 65–70% of an individual’s one repetition maximum in order to stimulate skeletal muscle hypertrophy and strength adaptations. Whilst these loads may be required to increase strength to a satisfactory level, issues such as meniscal damage, increased quadriceps arthrogenic muscular inhibition, and bone bruising may contraindicate HL-RT in load-compromised ACLR patients. So finding alternatives to promote muscle hypertrophy while minimizing potentially detrimental side effects is important to accelerating rehabilitation and improving performance after surgery.
Blood flow restriction (BFR) is a new tool in rehabilitation that can be used to promote similar gains in muscle performance as heavier load strength training. BFR uses a cuff calibrated by technology to restrict arterial flow and completely occlude venous return. This restriction increases muscle oxygen demand, causing the muscles to work harder to generate a contraction, allowing lighter weights to be used to promote muscle mass and strength gains. Restrictive pressures are commonly set relative to the individual (which is known as the individual’s limb occlusion pressure, or LOP) which increases safety and allows for an objective measurement of how much blood flow is being restricted during exercise.
The randomized controlled trial by Hughes et al. (2019) compared the effectiveness of BFR resistance training (BFR-RT) and standard-care traditional heavy-load (HL-RT) at improving skeletal muscle hypertrophy and strength, physical function, pain and effusion in 28 ACLR patients following surgery. Participants were block randomized to either HL-RT at 70% repetition maximum (1RM) (n=14) or BFR-RT (n=14) at 30% 1RM. Participants completed 8 weeks of biweekly unilateral leg press training on both limbs, totaling 16 sessions, alongside standard hospital rehabilitation. Resistance exercise
protocols were designed consistent with standard recommended protocols for each type of exercise. Scaled maximal isotonic strength (10RM), self-reported function, Y-balance test performance and knee joint pain, effusion (swelling) and range of motion (ROM) were assessed at pre-surgery, post-surgery, mid-training and post training.
Results from the 8 week study showed that:
- Both BFR-RT and HL-RT elicited comparable increases in skeletal muscle hypertrophy and strength.
- BFR-RT resulted in greater improvements in physical function and ROM.
- BFR-RT resulted in a greater reduction in pain and effusion.
- There were no adverse events or effects on knee joint laxity with either intervention.
These findings have important implications for post-surgery ACLR rehabilitation. The greater reduction in pain with BFR-RT can be attributed to the lighter load used (30% vs 70% 1RM) compared to HL-RT. The end goal of ACLR rehabilitation is for patients to be able to return to heavy loading and their pre-injury strength and activity level. Thus during the progressive limb loading phase of rehabilitation, the advantages of BFR-RT over HL-RT are that it can be used to allow a greater reduction in pain and effusion and improve physical function to a superior extent compared with HL-RT, importantly without any detrimental effect on muscle hypertrophy and strength improvements. Therefore BFR-RT may be a superior tool during the early stages of this phase of rehabilitation, particularly in patients with a high degree of pain and/or effusion.