Blood Flow Restriction Training in the Young Athlete

Abstract

Blood flow restriction training (BFRT) is increasing in popularity in the rehabilitation setting due to its believed impact on mitigating muscle atrophy, maintaining muscle mass and improving muscle function after musculoskeletal injury. This intervention has shown to be an effective option for addressing muscle strength and atrophy during earlier phases of rehabilitation when higher loads are not tolerated after injury. Although this intervention appears to be a safe and effective approach in sports medicine environments, there is limited information on the young athlete population. The purpose of this study is to provide a detailed overview of mechanisms, safety considerations, and clinical applications specific to the young athlete after musculoskeletal injury.

With millions of adolescents and children participating in organized sports in the United States, young athletes are increasingly exposed to high volumes of training, sport specialization, and injury risk. Pediatric injuries can result in temporary or permanent disability, and in young athletes, the most common injuries are those to the lower extremity, particularly the knee and ankle. Although traumatic injuries such as ligamentous tears and fractures are prevalent, overuse injuries such as apophysitis and patellofemoral pain syndrome provide clinical challenges in sports rehabilitation.

Muscle weakness and atrophy are common after musculoskeletal injury and surgery in sports medicine patients. Six to 12 months after anterior cruciate ligament (ACL) reconstruction, quadriceps strength deficits range from 10% to 25% [29]. Similarly, many patients lack symmetrical strength, with differences ranging from 16% to 44% after patellar instability procedures [45], shoulder stabilization procedures [55], or lower-extremity fractures [5]. With adolescent sports becoming an increasingly high-stakes activity, reliable methods of returning patients to full baseline strength are needed to optimize recovery and prevent re-injury.

Typically, treatments for adults are modified for young athletes, but strength training has been controversial due to concerns about injury to the developing physis. Recent consensus statements on strength and conditioning in young athletes have endorsed resistance training as beneficial to health and injury prevention [11,30,50], indicating that strength training does not pose intrinsic risk to the growing skeleton.

Blood flow restriction training (BFRT) has become a popular tool in rehabilitation to enhance strength gains. It involves applying a pneumatic cuff on the proximal aspect of an extremity, with selected pressure providing vascular occlusion. Resistance exercises are completed at approximately 20% to 30% of a 1 repetition maximum. The addition of BFRT to low-load exercise has been shown to augment the adaptive training response [21,39,48]. The technique was initially popularized in Japan in the 1970s in fitness and geriatric populations, but its use has expanded to clinical applications after musculoskeletal injury.

Research has supported BFRT use for osteoarthritis in the elderly population, in postoperative adults after ACL reconstruction, and for osteopenia/bony metabolism [3,21,54]. Less is known about its use for the young athlete, but it has shown promise. This article discusses the proposed mechanisms of action of BFRT and provides safety considerations and clinical recommendations for its use in young athletes.

Exercise and Adolescence

Understanding the physiologic response to exercise in young athletes is essential to understanding the response to BFRT. In prepubescent children, resistance training has been shown to increase strength without hypertrophy, attributed to rapid myelination changes [44], and to increase the motor units used during muscle contraction [50]. A major contributing factor is high levels of neural plasticity, the nervous system’s capacity to change in response to a stimulus [58], which peaks at age 12 years [17]. Healthy children have shown the greatest neuromuscular gains performing low-volume, moderate-intensity, and high-movement-velocity resistance training 2 days per week for 8 to 12 weeks [44]. Resistance training may be beneficial in prepubescence, although some have suggested it is inappropriate during this phase of development due to possible injuries, especially to growth plates. However, these injuries have not been reported in prospective youth strength-training studies [11]. During adolescence, the mechanisms for strength gains change and they continue to do so until adulthood. Increases in testosterone, growth hormone, and insulin-like growth factors result in increased muscle size, contributing to increases in strength [53]. Until this phase of development, the number of skeletal muscle fibers remains similar to the number at birth. In adolescence, hypertrophy is caused by hormonal changes along with bone growth, increasing bodyweight loads, and activity [41].

Neuromuscular function continues to affect strength development in adolescence, although to a lesser extent than in prepubescence. Increased motor unit recruitment contributes to strength gains, as it does throughout the lifespan [9]. Although neuroplasticity and myelination are no longer at their peaks, significant changes can still be made throughout puberty [17]. Recommendations for strengthening of adolescents are performing 1 to 2 sets with moderate intensities and velocities 3 times per week for 8 to 12 weeks [44]. Recommendations are made based on chronological age, but an understanding of the training age of children and adolescents can also be used to progress to higher loads and more difficult exercises [50].

Mechanisms of Blood Flow Restriction Training

Recent evidence suggests that muscle adaptation is determined by the effort exerted by muscle, commonly measured by the proximity to failure [34]. Traditional exercise achieves a high effort through an increase in external loads. Blood flow restriction training achieves a high effort by expediting fatigue in the exercising muscle through the manipulation of blood flow. Although exact mechanisms are unknown, several have been proposed.

Metabolite Accumulation

Through partial arterial occlusion and full venous occlusion, BFRT creates an anaerobic environment that leads to an accumulation of metabolic byproducts, including blood lactate, deoxygenated hemoglobin, phosphate, and hydrogen ions, as well as an acute release of anabolic hormones. This combination facilitates muscle fatigue with relatively few repetitions compared with a similar exercise intensity without occlusion. Motor unit recruitment thresholds are lowered when an exercise is taken to the point of fatigue, which allows for activation of higher threshold motor units [20]. Yasuda et al showed when individuals performed biceps curls and triceps extensions with elastic bands, the BFRT group had a significantly greater increase in motor unit recruitment than the free-flow group who performed the same number of repetitions [56].

Muscle Protein Synthesis

To stimulate muscle hypertrophy, a positive shift in the net protein balance of the muscle is essential. This is accomplished by upregulating muscle protein synthesis, down-regulating muscle protein breakdown, or a combination of both. Following an acute bout of knee extensions with BFRT, muscle protein synthesis was upregulated by 46% in young adults and by 56% among older adults compared with no change in the free-flow group [14,15]. In addition to this acute increase in muscle protein synthesis, BFRT was able to increasing the resting rate of muscle protein synthesis by approximately 42% in a 6-week study [46].

Myostatin Suppression

Blood flow restriction training can increase muscle hypertrophy and strength by suppressing myostatin, a growth factor that negatively regulates muscle growth, inhibits muscle protein synthesis, and upregulates muscle protein breakdown. Recent evidence has shown that 8 weeks of lower-extremity strengthening with either high or low loads and BFRT significantly reduced myostatin levels, whereas traditional low-load exercise failed to do so [28]. In addition to the negative impact on muscle growth, it is also a significant contributor to fibrosis in skeletal muscle after injury. In the presence of an acute ACL tear, myostatin levels were upregulated 2-fold in the vastus lateralis of the ACL-torn limb compared with the contralateral, indicating a potential avenue for BFRT before and after ACL reconstruction [25].

Vascular Endothelial Growth Factor Upregulation

Blood flow restriction training assists in the upregulation of vascular endothelial growth factor (VEGF), an essential angiogenic growth factor that stimulates proliferation, migration, and survival of endothelial cells. Its presence in muscle is required to increase capillarity, which is important to maintaining perfusion to the hypertrophying muscle. Recent evidence has shown a 5-fold upregulation of VEGF 2 hours post-BFRT exercise and over 6.5-fold 4 hours post-BFRT exercise [12]. These changes led to a 24% increase in capillarity after 7 training sessions, which corresponded to a 290% increase in muscle stem cells [36]. Increasing the stem cell pool is a key component to the muscle’s response to injury and has been shown to explain 49% of the variance of hypertrophy [2].

Myocyte/Cell Swelling

The final mechanism believed to contribute to muscle adaptation with BFRT is myocyte swelling. Although the effects have not been fully elucidated, it is believed to involve a plasma volume shift, in which plasma is pushed into the cell from the vascular system. Resistance training–induced cell swelling may contribute to an increase in muscle protein synthesis and a decrease in proteolysis. Researchers have shown that in the absence of exercise, cycling periods of arterial occlusion with reperfusion can attenuate the atrophying of muscle [24,51]. Intracellularly, this appears to be driven by the suppression of markers of muscle protein breakdown rather than stimulating muscle protein synthesis [24,37].

BFRT Device Considerations

Limb Occlusion Pressure

When determining the pressure to use during exercise, it is important to consider the limb occlusion pressure (LOP) or the pressure required to achieve full arterial occlusion. In the upper extremity, evidence suggests that 40% to 50% LOP is optimal; 1 study found no significant differences in muscle activation after elbow flexion exercise or in muscle strength and cross-sectional area after 8 weeks of resistance exercise between low pressure (40%) and high pressure (90%) [8]. With standard rotator cuff shoulder exercises, diminished returns of strength have been shown with occlusion pressures less than 25% or greater than 75% [42]. At 50% occlusion, which appears to be optimal in the upper extremity, significant increases in lean mass in the upper quarter were seen after 8 weeks of training [27].

In the lower extremity, higher pressure is needed to achieve venous occlusion and allow metabolite accumulation. Recently, Ilett et al showed that 20% maximal voluntary isometric contraction (MVIC) on an isokinetic dynamometer with 40% LOP produced similar changes in lactate as the same intensity contraction in the non-BFRT group [22]. A greater increase in metabolites accumulated with LOP increased to 60%, but exercise with 80% LOP showed a similar accumulation of metabolites as exercise with 80% MVIC.

Optimal LOP maximizes the benefits and minimizes the risks of exercise. The latter could include pain, numbness/paresthesia, and undesired hemodynamic responses related to pressure applied by the cuff. Hemodynamic responses (change in blood pressure, heart rate, and cardiac output) to high-load resistance exercise were seen when biceps curls were performed with high pressure (150 mm Hg), whereas exercise performed with low pressure produced fewer hemodynamic changes [4]. Blood flow restriction training must be performed as safely as possible, especially if higher pressure adds no benefit.

Devices

The pressure used should be relative to the individual’s LOP, not an absolute value or a perceived compression tightness [39]. This means that accurate assessment of LOP is essential. There are many cuff systems on the market, yet accurate LOP measurements appear to be inconsistent [26]; a validated device should be used. Finally, a wide tourniquet (>10 cm) will allow a given level of occlusion at a lower pressure compared to narrow cuffs.

Safety

The safety and tolerance of BFRT in adolescents remain important considerations, due to inconsistencies in treatment methodology and implementation and the many BFRT devices on the market [1]. Major possible adverse effects include venous thrombosis, nerve injury, muscle damage, and rhabdomyolysis, whereas minor effects include pain or discomfort with exercise, muscle soreness, dizziness and fainting, and cardiovascular stress [1].

Studies have demonstrated that the risk of side effects is no greater than that occurring during traditional methods of training [31]. Two surveys of more than 12 000 patients carried out by Japan’s KAATSU Training Society found no serious side effects, such as cerebral hemorrhage, cerebral infarction, thrombosis, or rhabdomyolysis [35,57]. In adolescents, no major adverse events (deep vein thrombosis, subcutaneous hemorrhage, or fainting) were reported in over 500 treatment sessions in a small cohort of patients enrolled in a larger clinical trial that utilized BFRT after ACL reconstruction [37]. Mild temporary side effects were reported at time of occlusion, including itchiness of the occluded limb (7.85%), lower-extremity paresthesia (2.81%), and dizziness (0.75%). Finally, BFRT in adolescents appears to be well tolerated, as 1 study found them able to finish the prescribed exercise 89% of the time and required a reduction of LOP 3.55% of the time. Several studies have found no increased markers for thrombus formation [35,38] after BFRT with a short-duration tourniquet [6,33].

Improving patient experience and minimizing risk involves the type of cuff used, LOP, duration of inflation, and cuff width. Variable-contour cuffs enhance patient comfort, decrease risk of mechanical shearing, and allow for occlusion at lower pressures [31,40]. A higher rate of adverse effects was reported with narrower diameter cuffs and higher pressures [10]. Personalizing tourniquet pressure plays a role in patient comfort, as does gradually increasing LOP from 60% to 80% between sessions. If the LOP is too high, blood supply to the limb can be eliminated. If LOP is too low, the limb may become painful due to venous congestion [31]. Dizziness and fainting have been reported, but infrequently [21].

Clinicians using BFRT should employ training specifications, consider training progressions, use a personalized tourniquet system, and screen patients for contraindications. Blood flow restriction training’s ability to improve strength and function with only mild discomfort makes for an appealing tool for use during the early postsurgical phases of rehabilitation.

Clinical Implementation

Clinical use of BFRT has focused on improving muscle quality and quantity in those unable to train with higher intensities, but it can be advantageous to young athletes in other circumstances.

Hypertrophy and Strength

The most common application of BFRT is for muscular hypertrophy and strength [8]. Low-load BFRT has been shown to be superior in improving muscle strength and size when compared with low-load training without BFRT, and low-load BFRT has yielded similar results to high-load training [8]. Achieving similar strength and hypertrophy effects with a lower load makes BFRT an attractive option for young athletes following injury or surgery. This typically involves the completion of 2 to 4 exercises during a rehabilitation session with full deflation of cuff between exercises (Table 1).

Table 1.

Clinical application guidelines of BFRT in young athletes.

Cuff Size	Width: >10 cm for improved comfort [10] and reduce risk of injury [18] Length: variable sizes for different limb sizes, long enough to fully close without significant overlap
Limb Occlusion Pressure	Lower extremity: 60 to 80%, with greatest benefit at 80% Upper extremity: 40 to 50%
Volume	4 sets with 30 seconds rest between sets (1 × 30 and 3 × 15 repetitions) with cuff inflated for duration of repetitions Exercise quantity: 3 to 4 exercises in succession Perform 2 to 3 days/week [7,52]
Load	20% to 40% of 1 repetition max [7] and/or 2 to 3/10 on a perceived exertion scale to control intensity [7,16,52]
Exercise Selection	When using BFRT in rehabilitation, consider the intent of the exercise. Single limb exercises can ensure stress to the targeted muscle. While multi-joint exercises can be used, they may reduce effectiveness on the intended targeted muscle

Aerobic Conditioning

Blood flow restriction training with endurance interventions can be beneficial to high-performing young athletes. Combining it with low-intensity aerobic exercise may be comparably beneficial to high-intensity training on markers of aerobic fitness [13]. This could be useful for adolescent athletes who want to maintain endurance mid-season and limit intensity due to an elevated in-season workload. High-intensity cycling intervals with BFRT have demonstrated increased oxygen transport capacity, potentially delaying fatigue and improving performance.

Chronic Pain

Blood flow restriction training may induce hypoalgesia comparable to that of heavier loaded resistance, making it a potential intervention for adolescents in pain who cannot tolerate strength training [49] or for chronic pain patients who cannot tolerate recommended loads. Adolescents with common overuse injuries, such as patellofemoral pain syndrome, apophysitis, or lower-extremity tendinopathy, may benefit from BFRT for its heightened analgesic effect with low-load resistance.

Postoperative Conditions

Possibly the most common application of BFRT in adolescents is in acute postoperative rehabilitation. Passive BFRT (requiring no external load) has been shown to significantly attenuate atrophy and loss of strength following a period of disuse, mainly by slowing the breakdown of muscle proteins; when combined with electrical stimulation during 14 days of immobilization, passive BFRT preserved lean leg mass [47]. For those who can bear weight/perform resistance exercises shortly after surgery, low-intensity BFRT exercise has been shown to result in significantly greater gains in muscle strength compared with traditional therapeutic exercise [21]; similar improvements in strength have been shown as those who are able to perform high-load training [19].

Although there is little evidence on the effects of BFRT after surgery in adolescents, its use in this population has become more common after surgeries for meniscal tears, patellar instability, and cartilage damage. Utilizing BFRT after ACL reconstruction is one of the few areas where adolescents are represented in the literature [37], but it has also demonstrated benefits in the adult population [32]. In young adults, BFRT has demonstrated preservation of lean leg mass and attenuated loss of bone mass in the operative leg 6 and 12 weeks postoperatively [23]. Recent evidence has also shown isometric and isokinetic strength changes of more than 20% after a BFRT protocol following ACL reconstruction compared with traditional rehabilitation [43].