Introduction
Skeletal muscle mass is a crucial factor for health, longevity, and maintaining the ability to complete activities of daily living, ambulate, and avoid falls. Muscle quantity and quality also directly impacts individuals in fitness and sport performance. A disuse of skeletal muscle can rapidly lead to atrophy of muscle tissue, resulting in reduced oxidation capacity, sarcomere shortening, and reduced muscle compliance. This in turn reduces exercise capacity, impairs the immune system, and decreases sensitivity to insulin.
Typically, high-intensity resistance training at 70-8% of a 1 repetition maximum (1-RM) is recommended to increase muscle mass and strength. However, this recommendation is often very challenging and even contraindicated for certain individuals such as those with underlying chronic medical conditions, rehabilitation patients, recovering athletes, or a post-operative population. Several studies have proposed the idea that Blood Flow Restricted (BFR) low load exercise (20-30% maximal capacity) may stimulate significant muscular adaptations, through use of an external constriction device. There has been a fast-growing body of literature for BFR training and the aim of this review is to systematically assess studies and identify which BFR training methods have the greatest results in strength and hypertrophy.
Methods
47 studies were identified to fit inclusion criteria of BFR with exercise stimulus compared to non BFR with exercise. Studies were required to include at least one of two outcomes: muscle strength or muscle size. These studies included all healthy participates with a mean age of 34. Studies were categorized into two exercise groups: aerobic and resistance training.
Results
Muscle Strength
The mean improvement in strength grains of experimental group with BFR aerobic exercise was 0.4nM above the strength changes in that of the control group. Training >6 weeks increased this mean difference to 0.6nM, compared to 0.2 nM in training <6 weeks. Typically, muscle strength in the BFR aerobic group was increased by 5-8 nM.
The mean improvement in strength gains of experiment group with BFR resistance exercise and control group was an additional 0.3 kg force. Gains in muscle strength were significantly greater when intensity of exercise was >20%1 RM versus <20% 1 RM. When comparing 20% 1 RM to 30% 1 RM, training at 30% 1 RM resulted in a much greater improvement in muscle strength. The mean difference between the experimental and control group were relatively small. Cuff pressure of >150 mmHG caused a greater increased in strength compared to cuff pressure <150 mmHg., 0.2 kg and 0.1 kg respectively.
Muscle hypertrophy
Aerobic training had a mean increase of post training muscle size of 0.32 cm^2 between control and experimental groups. The increase in size of muscle as a result of BFR Training was 0.41 cm^2 greater than that seen in the control groups.
Studies considering both modalities of exercise combined with BFR showed an increase in muscle size of 2-5 cm. Muscle size differences between the experimental and control group did vary when training took place 3 days per week compared to 2 days per week, 0.34 cm versus 0/29 cm respectively.
Discussion
The current research suggests BFR with low load exercise training is effective improving muscle strength and size. This was true for both aerobic and resistance training. When performing BFR aerobic exercise, training durations >6 weeks produced greater strength increases, which is the generally accepted adaption period for standard resistance training. Greater strength gains with BFR resistance training may be expected at an intensity >20% 1 RM.
Conclusion
This systematic review provides evidence of greater increases in muscle size and strength when exercise is combined with BFR, compared to low load exercise alone. This type of training offers potential benefits for populations recovering from orthopedic or other conditions that require rehabilitative care, for which higher load training is contraindicated. However, this study reveals there is a gap in the evidence in regards to optimal training methods and protocols, which would be important for future research.
Practical PTF Take-Aways
- Lighter load BFR Training may be effective to increase muscle size and strength when traditional high load training in contraindicated
- Adaptations and benefits are greater at 30% 1 RM compared to 20% 1 RM
- Training durations >6 weeks offer greater returns in strength adaptions
- BFR training can be applied to a range of populations who seek to progress strength while reducing loads on associated muscle, connective, tendinous, and bony tissues
Reference
Slysz. J, Stultz, J., Burr, J (2016). The efficacy of blood flow restricted exercise: A systematic review & meta-analysis. Journal of Science and Medicine in Sport. 19: 669-675.