Chronic knee pain and acute or traumatic knee injury is a major cause of time loss in many competitive sports. A recent injury surveillance study identified knee injury as a leading cause of injury burden across all the top four professional sports codes in the United States and North America, including: basketball, baseball, football and ice hockey. Not surprisingly, the search for rehabilitation strategies that optimize knee health and improve performance remains a top priority within the sports medicine world. Blood flow restriction training is fast becoming a common part of gold-standard management in many different forms of knee injury rehabilitation including:
- ACL reconstruction
- Patellofemoral joint pain
- Total knee arthroplasty
- Patella tendinopathy
- Knee arthroscopy
- Meniscal/joint surface injury
BFR is typically used as a strategy to improve skeletal muscle mass and strength in load compromised or injured individuals. However, research is emerging to demonstrate the proposed benefits may also apply to bone, tendon, cardiovascular health, cognitive function, neuromuscular performance and pain mitigation. The purpose of this blog is to explore the different ways in which BFR training is currently being used to improve clinical outcomes and athletic performance in acute and chronic knee injury.
Acute Knee Injury:
The use of BFR methods in acute knee injury dates back more than 20 years when Takarada and colleagues demonstrated that the use of BFR as a passive strategy had the capacity to diminish muscle atrophy following ACL
reconstruction surgery. Since then, more than 50 publications have explored the use of BFR both passively, and in combination with low-intensity exercise in multiple forms of knee injury. A recent review from Hughes et al (2018) described the use of BFR following ACL surgery and identified four key phases through which BFR may be implemented.
1 – Post-operative BFR:
The objective of this phase is preventing muscle disuse atrophy and strength loss, minimising joint effusion and pain management. Passive application of BFR can stimulate muscle protein synthesis and has been shown to minimise muscle atrophy when applied as soon as 2-days post ACLR surgery. The most common protocol is 5 sets of 5 minutes occlusion with 2-3 minutes of rest. Higher occlusion pressures, perhaps full limb occlusion (100% LOP) may be required to provide a sufficient stimulus to trigger muscle adaptation. Combining passive BFR with voluntary muscle contraction or neuromuscular electrical stimulation has also been demonstrated as an effective strategy to promote muscle hypertrophy and optimize early stage return of muscle function post traumatic knee injury.
2 – Early Loading with BFR:
The objective of this phase is to further prevent muscle atrophy, normalize gait patterns and improve quadriceps muscle function. Combining BFR with walking and low-intensity cycling has been shown to increase muscle size, strength and function. BFR application may also promote muscle deoxygenation and enhance aerobic and strength endurance adaptations. When using BFR in conjunction with aerobic exercise higher pressures between 60-80% LOP are typically required to optimize improvements in strength. Anecdotally there is often concerns that using BFR can exacerbate joint effusion. However, evidence from Hughes et al (2019) demonstrated that the application of BFR in post-operative ACL reconstruction can reduce joint effusion and reduce pain during this stage of rehabilitation.
3 – Low-load Resistance Training with BFR:
During the phase the application of BFR to low-intensity resistance exercise is commonly used to maximize muscle hypertrophy. The benefits of BFR combined with resistance exercise are well documented, with improvements in muscle hypertrophy being comparable to heavy traditional resistance exercise, and improvements in strength superior to an exercise matched control. Programming within this phase should be consistent with principles of progressive overload and specificity. Metabolic stress and mechanical load should be systematically increased through changes in limb occlusion pressure/time under occlusion and intensity respectively. Training loads between 20-40% 1RM combined with cuff pressures between 40-80% LOP are recommended for improvements in muscle strength and hypertrophy.
4 – Heavy Resistance Training with Low-load BFR Training:
The end goal of any rehabilitation program it to progress athletes to be able to tolerate high intensity strength and plyometric training. Heavy strength training is more effective than low-load BFR at improving maximal muscle strength, and plyometric training is essential at re-training leg power and reactive strength qualities. During this phase traditional high-intensity training methods can be supplemented with on-going to continue to target improvements in muscle hypertrophy with a low-cost training intervention.
Figure 1 – Programming considerations for BFR in acute knee injury.
Chronic Knee Pain:
Chronic and degenerative knee conditions such as patellofemoral joint pain or patella tendinopathy have the potential to cause pain for many years. Experimental pain studies have demonstrated that localized joint pain leads to a decrease in muscle function, impaired motor control and fear avoidance behavioural patterns (avoiding painful activities). Movement strategies that avoid loading the painful knee may trigger a vicious spiralling decline in physical function characterised by progressive muscle weakness and decreased joint stability, which may in-turn expose the athlete to more significant injuries. While traditional resistance exercise is commonly considered the most effective rehabilitation strategy this form of training has the potential to increase load through the injured area and potentially aggravate symptoms
Recent research has shown that performing light BFR exercise may have a pain modulation effect. A landmark study from Korakakis (2018) demonstrated that subjective pain scores while performing functional tasks (single leg squat and step down) were reduced by more than 60% following BFR exercise in subjects with anterior knee pain. Hughes et al (2019) speculated that there may be several mechanisms by which BFR might influence pain:
Conditioned pain modulation:
The simplest way to describe conditioned pain modulation is with the expression 'pain cures pain'. Within the conditioned pain modulation paradigm, a painful conditioning stimulus, may inhibit the perceived pain of a secondary stimulus. While BFR is characterised by low-intensity training, perceptions of pain and discomfort comparable to high intensity exercise are consistently reported in the literature.
Opioid and endocannabinoid systems:
BFR exercise causes an increase in metabolites such as lactate within the muscle. These metabolites stimulate the production of opioids and endocannabinoids. Opioids are a family of neuropeptides produced within the nervous system. Specifically, beta-endorphine is thought to play a key role in exercise-induced pain relief. Recent evidence from Hughes et al (2020) demonstrated an increase in beta-endorphine and 2-arachidonoylglycerol levels following high-pressure BFR exercise, and that these changes were associated with increased post-exercise pain relief.
While the evidence supporting BFR as a pain relief strategy is relatively new, the implications for states of chronic knee pain are significant. Not only does BFR have the potential to alleviate pain which may allow for more traditional rehabilitation loading strategies, evidence indicates a moderate effect of BFR exercise on increasing muscle strength in individuals suffering musculoskeletal weakness. This mean BFR exercise can target changes in function, pathology and pain, the three hallmark symptoms of chronic knee injury. There are three primary use cases by which BFR is currently being implemented:
1 – Pre-loading strategy: Heavy resistance training is still viewed as the gold standard rehabilitation strategy in multi-factorial anterior knee pain; however, pain often contraindicates many people from being able to lift sufficient loads. Pre-loading with BFR exercise may enable individuals to successfully complete high-intensity resistance exercises to improve muscle strength. Hughes et al. (2020) demonstrated that the pain-relieving effects of BFR can last up to 24 hours and are likely dependent on the intensity of the BFR stimulus. It appears higher occlusion pressures up to 80% LOP may provide the most potent pain relief strategy.
2 – Pre-competition strategy: While higher occlusion pressures may optimize pain relief, using such a fatiguing stimulus prior to competition may not be desirable. Occlusion pressures as low as 40% LOP have been shown to be effective at providing significant pain relief, while also providing an effective post-exercise potentiation stimulus (Zheng et al 2022).
The capacity to relieve pain and improve leg power performance makes the use of BFR prior to training or competition a desirable strategy.
3 – A stand-alone rehabilitation tool: BFR in combination with resistance exercise has the capacity to increase skeletal muscle strength, increase bone cell turnover, stimulate changes in tendon morphology, and improve conditions of chronic pain in individuals with complex anterior knee pain. When using BFR as an isolated rehabilitation tool it should be noted that BFR is not able to increase muscle strength to the same extent as traditional heavy resistance training. Users are encouraged to systematically increase mechanical load as a part of their progressive overload protocols.