Blood Flow Restriction Training

Original Editor - Vidya Acharya

Top Contributors - Vidya Acharya and Kim Jackson  


Introduction

Muscle weakness is highly prevalent among the most clinical musculoskeletal conditions worldwide. To improve muscular strength and hypertrophy the American College of Sports Medicine recommends moderate to high load resistance training. However, the use of moderate to high loads is often not feasible in clinical populations. Furthermore, a number of disease states result in frailty including HIV/AIDS, cancer, sepsis, COPD, and diabetes. This frailty is largely due to the loss of muscle mass. Treatments that prevent muscle wasting or stimulate muscle growth would improve the quality of countless lives[1]. Therefore, the emergence of low load blood flow restriction (BFR) training as a rehabilitation tool for clinical populations is becoming popular[2]. Training with low loads in combination with venous blood flow occlusion from the working muscle and arterial blood flow restriction to the working muscle (BFR) may be beneficial for such populations[3].

Blood flow restriction (BFR) training

Blood flow restriction training was originally conceived and developed in Japan in the late 1960’s by Yoshiaki Sato and termed KAATSU training[4]. Blood flow restriction (BFR) as a modification to traditional exercise modalities, such as resistance training or walking has become an area of research interest. This technique utilizes the application of pneumatic cuff, similar to a blood pressure cuff, on the proximal aspect of an upper (i.e. distal to the deltoid muscle) or lower extremity (i.e. inguinal crease). A selected pressure is used to provide partial arterial and complete venous occlusion to the distal aspect of the limb. The patient then performs resistance exercises at relatively low intensities (i.e., 20–30% of 1 repetition maximum [1RM]), high repetitions per set (i.e., 15–30), and short rest intervals between sets (i.e., 30 seconds). Research has commonly examined single joint exercises (e.g., plantar flexion, elbow flexion, leg extension) to minimize the complexity of the movement pattern with the blood flow partially restricted[5].

BFR and Strength training

Understanding the physiology of Muscle Hypertrophy.

Muscle hypertrophy is characterized by an increased in diameter and in total protein content of fibres and occurs as a result of an enhanced rate of protein synthesis. Muscle atrophy is induced by a decrease in activity and load; catabolic loss of muscle mass is a decrease in the size of pre-existing muscle fibers, resulting from a dramatic increase in protein degradation and turnover[6]. A strong relationship exists between the cross-sectional area of the muscle and its strength. Greater the cross-sectional area of the muscle greater is the strength of that muscle. The two primary factors which are important for muscle hypertrophy are mechanical tension and metabolic stress. And when both are combined an environment for muscle hypertrophy is created. Both are necessary to get the effects.  

1. Mechanical tension: Growth and repair of adult skeletal muscle require the action of a population of normally quiescent myogenic precursors called satellite cells[6] which reside between the basal lamina and the plasma membrane of the myofibres.  In response to muscle injury or increased muscle tension, these cells become activated, start proliferating and are responsible for the repair of damaged muscle fibres and the growth of muscle fibres. Thus, setting a building block for greater muscle hypertrophy and increased cross-sectional area. According to the American College of Sports Medicine (ACSM), optimising muscular strength and hypertrophy can be achieved through moderate to high intensities of resistance exercises, A sufficient load must be placed in the muscle to induce adaptive changes. The target muscles must be subjected to substantially increased load. The ACSM recommends that the load should exceed 70% of the one repetition maximum to achieve maximum hypertrophy.[7] 

2. Metabolic Stress: Additionally, under appropriate mechanical loading stress, anabolic hormone concentration levels elevate. The combination of myogenic stem cell activation and anabolic hormone elevation results in protein metabolism and muscle growth. This sets up an environment for muscle hypertrophy. Researchers have suggested that metabolic stress has an important impact on hormonal release, hypoxia, cell swelling and production of reactive oxygen species (ROS). All of these components can initiate anabolic signalling for muscle growth and adaptations on energy metabolism.[8] 

  • During resistance training, muscle contractions compress blood vessels in active muscles, and this occlusion can lead to a reduction of oxygen levels and, consequently, resulting in a hypoxic environment. Intramuscular hypoxia during exercise can increase the necessity of anaerobic lactic metabolism by activation of hypoxia-inducible factor (HIF-1α) that regulates the expression of glycolytic enzymes. Thus, an exercise that produces high levels of lactate can be associated with hypoxia[8].
  • Scientific evidence shows that load, number of repetitions, and reset between intervals are important factors to induce metabolite accumulation. Gonzalez et al found that acute resistance training with moderate repetitions combined with short rest intervals (70% 1RM, 10-12 repetitions and one minute rest interval) shows an increase in blood lactate, serum concentration of lactate dehydrogenase, growth hormone and cortisol when compared to higher loads, low repetitions combined with longer rest intervals (90% 1RM, 3-5 repetitions and three minute rest intervals). Concerning these findings, duration of rest intervals may reflect directly on the magnitude of metabolic stress. In a review study, researchers demonstrated that short interval sets (less than one minute) are essential in increasing blood lactate and growth hormone production, mainly because of insufficient recovery of phosphocreatine and H+ accumulation.
  • Exercise is a potent physiological stimulus for growth hormone secretion, and both aerobic and resistance exercise results in significant, acute increases in growth hormone secretion.[9] Accumulation of lactate and hydrogen ions within the muscle results in an augmented growth hormone release. According to Doessing et al growth hormone plays a direct role in increased collagen synthesis after exercise[7].
  • Lifting heavy loads or doing powerful activities such as sprinting forces our body to switch from slow twitch oxidative fibres to fast twitch anaerobic fibres. Anaerobic metabolism produces very strong contractions is very short lived and creates subsequent byproducts. The byproduct of this action lactate and hydrogen ions create the subsequent burn you feel in your muscle
  • Additionally, Nishimura et al demonstrated higher effects of muscle hypertrophy when resistance training is performed during hypoxia, possibly because of the strong influence of hormonal release, the recruitment of fast-twitch muscle fibres, ROS production and cell swelling[8]
  • Additional traits during muscle hypertrophy during resistance training or high-intensity training are upregulation of insulin-like growth factor and downregulation of myostatin. Myostatin has an inhibitory effect on myogenic stem cells, so decreased myostatin is a key for muscular hypertrophy[7].

Effects of Blood Flow Restriction on Muscle Strength

BFR training is designed to take advantage of these normal physiological adjustments/adaptations to exercise. With BFR training, high-intensity exercise is simulated by artificially reducing blood flow to active skeletal muscle during periods of low-intensity exercise. Blood flow is decreased mechanically by placing flexible pressurizing cuffs or elastic-bands around the active limb proximal to the exercising muscle. This technique selectively reduces the outflow from the muscle, thereby causing pooling of capillary blood of low oxygen tension, which markedly enhances the production of protons and lactic acid. The intent of the manoeuver is to mimic the metabolic environment necessary to stimulate muscle growth while concomitantly recruiting muscle fibres possessing the greatest force-generating capacity.

Loenneke et al. postulated that low-intensity(LI) BFR results in increased water content of the muscle cells, which induces a cascade of anabolic intracellular signalling to occur. This postulation is supported in part by Fry et al. who observed greater increases in muscle size (measured by circumference) with LI-BFR compared with low-intensity resistance exercise without BFR. The authors suggested that this acute swelling might mechanistically explain part of the increase in muscle protein synthesis observed after LI-BFR. According to Haussinger et al., cell swelling shifts protein balance toward anabolism and thus induces hypertrophy[10].

In addition, BFR hastens the recruitment of fast-twitch muscle fibres. As a result, the functional and metabolic adjustments known to occur during high-intensity exercise without BFR are reproduced during low-intensity exercise with BFR[11]. Research has demonstrated that BFR exercise training resulted in increased muscular strength, hypertrophy, localized endurance, and cardiorespiratory endurance[5].

Hypothetically speaking, the potential mechanisms for these adaptations may include[5]

  • hypoxia-induced additional or preferential recruitment of fast-twitch (FT) muscle fibres,
  • greater duration of metabolic acidosis via the trapping and accumulation of intramuscular protons (H+ ions) and stimulation of metaboreceptors, possibly eliciting an exaggerated acute systemic hormonal response,
  • external pressure-induced differences in contractile mechanics and sarcolemmal deformation, resulting in enhanced growth factor expression and intracellular signalling,
  • metabolic adaptations to the fast glycolytic system that stem from compromised oxygen delivery,
  • production of reactive oxygen species (ROS) that promotes tissue growth,
  • gradient-induced reactive hyperemia after removal of the external pressure, which induces intracellular swelling and stretches cytoskeletal structures that may promote tissue growth, and
  • activation of myogenic stem cells with subsequent myonuclear fusion with mature muscle fibres.

Relatively short-duration (4–6 wk) low-intensity BFR training has been associated with a consistent, 10–20% relative increase in muscle strength compared with baseline, with differences noted based on age, sex, and muscle group being studied. Of importance, the increases in muscle strength were generally comparable to high-intensity exercise without BFR, suggesting that similar gains can be achieved with lower loads using Kaatsu techniques.[11].

Another study [12]aimed to analyse the effects of six weeks of strength training with and without blood flow restriction (BFR), on torque, muscle activation, and local muscular endurance (LME) of the knee extensors. 37 healthy young individuals were divided into four groups: high intensity (HI), low-intensity with BFR (LI+BFR), high intensity and low intensity combined + BFR (COMB), and low intensity (LI). Torque, muscle activation and local muscle endurance were evaluated before the test and at the 2nd, 4th and 6th weeks after exercise. All groups had increased torque, muscle activation and local muscle endurance (p<0.05) after the intervention, but the effect size and magnitude were greater in the HI, LI+BFR and COMB groups. In conclusion, the groups with BFR (the LI+BFR and COMB) produced magnitudes of muscle activation, torque and muscle endurance similar to those of the HI group.

Equipment

The BFR technique involves applying a tourniquet cuff to a limb. The cuff is manually tightened or pneumatically inflated to a pressure that occludes venous flow yet allows arterial inflow during rehabilitative exercise.[2] Some clinicians may use rudimentary techniques to achieve occlusion, such as surgical tubing or elastic straps wrapped tightly around the proximal portion of the exercising limb. This method may or may not completely occlude all arterial and venous blood flow, and there is no way of knowing what occlusive pressure the vessels experience. Further, the relatively thin diameter of the tubing or straps may cause highly localized stresses and ineffective transmission of pressure, potentially causing direct damage to the soft tissues and structures underneath the application site. Review of BFR rehabilitation literature[13] shows that inconsistencies exist in methodology, equipment and in levels of restriction pressure used. Current non-personalized methodologies of setting BFR pressure may occlude rather than restrict blood flow, increasing the risk of injury during rehabilitation. Furthermore, these non-personalized methods of setting pressure do not provide a consistent stimulus within and across patients, reducing the efficacy of the BFR rehabilitation and inhibiting the meaningful comparison of a full range of BFR studies. James A McEwen et al suggest the use of surgical-grade tourniquet technology with automatic Limb Occlusion Pressure (LOP) measurement capability. These are adapted to incorporate and deliver optimal protocols, for safe and effective application of BFR to consistently achieve optimal patient outcomes in rehabilitation[13]

BFR cuff width

The cuffs developed for BFR are wider than those traditionally used. The most frequently used cuff width is 10 to 12 cm, although cuffs greater than 15 cm may be more desirable. Additionally, cuffs are now tapered to conform with the natural proximal-to-distal narrowing of the thigh or upper arm and are limb-circumference specific, which allows them to be fitted to various limb circumferences. Together, these advances enhance the transmission of pressure, and may appropriately mitigate complications caused by finely localized stresses from the cuff itself and reduce the pressure required to reach the same level of occlusion achieved with narrower cuffs.[14]

BFR cuff material

Many of the narrow cuffs used are made of elastic material whereas the wider cuffs are made of nylon. The difference in the material may result in differences in the ability to restrict blood flow and some of this difference may be due to differences in initial pressure[15]. The initial pressure is the pressure applied to the limb by the elastic cuffs prior to actual inflation. Research suggests that arterial occlusion pressure was significantly greater when using the elastic cuff as opposed to the nylon cuff. [16]

BFR cuff pressure

Blood flow restriction cuff pressure prescription methods have included a standard pressure for all patients, such as 180 mmHg; a pressure relative to the patient's systolic blood pressure, such as 1.2- or 1.5-fold greater than systolic blood pressure; or a pressure relative to the patient's thigh circumference. A pressure specific to each individual would be the safest prescription method, because the same pressure may not necessarily occlude the same amount of blood flow for all individuals under the same conditions.[14]

Devices and procedures specifically designed for BFR have been developed to individualize the pressure for each patient using plethysmography or Doppler ultrasound to detect blood flow. At rest, the cuff pressure is slowly increased until the flow of blood is no longer detected, a term frequently referred to as total limb occlusion pressure (LOP) or arterial occlusion pressure (AOP). A submaximal percentage (eg, 40%–80%) of that pressure is prescribed for the exercise session. This pressure prescription method ensures that patients are not exercising with a pressure that is too high relative to their systolic blood pressure and accounts for the volume and density of soft tissue underneath the cuff. Further, this method allows for appropriate progression of the pressure, similar to the way a clinician would progress resistive load, repetitions performed per set, and total number of sets performed for a resistance exercise.[14]

The literature suggests that the pressure applied largely depends upon the width of the cuff applying the stimulus as well as the size of the limb to which the stimulus is applied. The pressure applied should be high enough to occlude venous return from the muscle but low enough to maintain arterial inflow into the muscle.[17] Loenneke JP et al in the study[18] found that an arterial occlusion of 40% to 50% causes increase in muscle activation [66% vs. 87% maximal voluntary contraction (30% 1RM)] but no further increase in strength was seen with a higher pressure. It would, therefore, appear that a pressure level that results in 40% to 50% arterial occlusion is optimal. However, Wilson et al. (2013) found that in practical blood flow restriction (where wraps are used to apply pressure), a perceived wrap tightness of 7 out of 10 results in complete venous, but not arterial, occlusion. This is consistent with the stated aim of blood flow restriction training to maintain arterial inflow while occluding venous return during exercise. Furthermore, this level of perceived wrap tightness has also been used by Lowery et al. (2014) in an investigation demonstrating the efficacy of blood flow restriction training in eliciting hypertrophy[19][20].

Clinical Application

Available scientific data has reported multiple benefits of low-intensity-Blood Flow Restriction (LI-BFR) in health outcomes and rehabilitation: reduce muscle atrophy, increases in muscular strength and hypertrophy improvements in elderly, vascular function and cardiovascular system. Thus, this training methodology results very useful for a large range of populations from athletes, recreationally training to clinical exercise[21].

Procedure

The tourniquet is placed on the proximal arm for upper limb and proximal thigh for the lower limb. Inflate the cuff to an appropriate level of safety and effectiveness. For lower extremity, the cuff is inflated to restrict 80% of the arterial blood flow and 100% of the venous blood flow and for the upper extremity, the cuff is inflated to restrict 50%of the arterial blood flow and 100% of the venous blood flow. With the cuff in place and appropriate pressure applied, the patient carries out standard upper extremity or lower extremity exercises. The intensity of the exercise is 20-30% of 1 RM in BFR training.

Exercise Prescription[7]

1. Training Frequency

In theory, strength training with BFR can be done daily and in some studies, Nielsen et al suggest that it has been done twice a day. Several studies looking at Endurance training and BFR has shown effects with 4 - 6 days of training.

2. Training Duration

The effect's size for training duration demonstrates that longer duration up to 10 weeks has the largest effect size. Early hypertrophy is observed with BFR and this may be from increased satellite fusion and resultant hypertrophy. It is common that the patient notices hypertrophy within the first 2 weeks of BFR training.

3. Rest Periods

The largest effect size is seen with rest periods of 30 seconds. It is important to keep the cuff inflated during the rest periods to capture the metabolites.

4. Tourniquet Cuff Pressure

The amount of pressure needed to occlude blood flow in the limb depends on the limb size, underlying soft tissue, cuff width and device used.

5. Limb Occlusion Pressure

It is the minimal amount of pressure needed to occlude arterial blood flow. 3rd Generation Tourniquet System features a built-in system to measure vascular flow, which allows personalised tourniquet pressure for each individual patient and eliminates the need to account for cuff width, limb size or blood pressure. A pressure of 80% occlusion for lower extremities and 50% for the upper extremities is recommended.

6. Training Intensity

A load of 15-30% 1RM has the largest size effect. A Higher load may have actually pumping effect to eliminate the metabolites and blunt the response. Lower intensities such as cycling, walking and isometrics have a lower response than 15-30% load.

7. Exercise Selection

BFR is typically a single joint exercise modality for strength training or low-level cardio exercise.

  • The standard repetition scheme used in BFR is a set of 30 repetitions followed by a 30-second rest followed by 3 more sets of 15 with 30 second rests in between (30/15/15/15). This gives us the target 75 repetitions. The first set of 30 can be seen as the priming load to begin the Cori cycle or the Lactic acid cycle. This first set is typically tolerated well by the patient and they often feel like it is too easy. The tourniquet is left inflated during the rest period, this is very important in order to trap metabolites.
  • The following 3 sets and rest periods will feel very difficult because of the subsequent lactate build up. The RPE is closely related to lactate accumulation. Also, the patients may feel their heart rate raise somewhat during the exercise. This is normal because of the reduced venous return, subsequent decreased stroke volume and increased HR to maintain cardiac output. If at anytime the patient becomes faint, dizzy has moderate to severe pain under the tourniquet cuff or begins to feel numbness or paresthesia in the limb stop the exercise session.
  • Once the patient finishes the exercise session the reperfusion of blood into the limb flushes out the lactate and the lactate “burn” in the limb generally goes away relatively quickly. They do often feel very fatigued in the limb and studies measuring force production immediately after BFR even at low loads have demonstrated significantly reduced force. Because of this high intensity exercises such as olympic lifts, plyometrics, agility work should not be done immediately after BFR. These same exercises should also not be done while using BFR. However, there will be times when the patient is unable to hit the target volume. Remember volume is key for strength and hypertrophy in BFR training.
  • Exercises:

Upper extremity: Upper body ergometer, isometrics, scapular rows, serratus punches, shoulder exercises, PNF patterns, bench press, push-up, elbow flexion, elbow extension, elbow supination, elbow pronation, wrist and all hand gripping exercises.

Lower extremity: Walking, cycling, isometrics, leg extension, hamstring curl, straight leg raises, terminal knee extension, hip range of motion exercises, leg press, squat, lunge, ankle and all Foot Exercises.

8. Exercise Progression

Below are guidelines to follow concerning exercise progression and difficulties with volume achievement: Load: 20-30% 1RM (Determined, Estimated). If the patient achieves:

75 Repetitions = Continue with training, re-assess 1RM within 1-3 sessions. Reestablish new 20-30% range as strength improves.

60-74 Repetitions = Continue with training, but extend rest period between sets 3 and 4 to 45 seconds. Until 75 repetitions is completed.

45-59 Repetitions = Continue with training, but extend rest period between all sets to 45- 60 seconds.

<44 Repetitions = Reduce the load by approximately 10% until 75 repetitions are achieved.

Forced to stop before 75 repetitions because of undue pain, soreness or general uncomfortable feeling underneath the tourniquet cuff = Reduce tourniquet pressure by 10mmHg at each training session until cuff tolerance is achieved. Ramp cuff pressure back up 10 mmHg to target limb occlusion pressure if patient can tolerate.

9. Rehabilitation Guidelines
  • In the very early phases of rehab after injury or surgery, the goal is to mitigate atrophy and promote a healing environment. During this phase simply performing BFR with low-level exercises such as isometrics with or without electrical-stimulation, mat exercises (SLR/4-way hip) or no exercise can achieve this. For example, researchers in the UK are beginning a study using BFR in the ICU to decrease muscle atrophy.
  • Once the patient moves into a more sub-acute stage, BFR can be used for slightly higher loads such as walking and cycling. This has been shown to increase strength, hypertrophy and endurance. As the patient can tolerate additional load, he can move into isotonic exercises with a 15-30% load. This will produce even more substantial gains.
  • As the patient transitions into the later phases of rehab, a typical transition to BFR and HIT training on alternating days can be made. This has demonstrated even larger gains than BFR alone. This also acts as a bridge for the patient to discharge to a home strengthening program consisting of just HIT training.
  • The initial training with BFR has made permanent anatomical changes through increased myocyte incorporation and thus improving the patient's muscle protein synthesis potential. BFR can also be used as a form of strength and endurance training to supplement the patient while they are working on a higher-level program such as plyometrics, running and agility into the late phases of rehab. In cases of setbacks due to reinjury, BFR is a great modality to start back up during these times to diminish the losses the patient might have during this setback. The patients really appreciate the fact that they are maintaining or increasing their strength while they are recovering instead of meds and RICE while they watch their gains atrophy daily.

Side Effects

Reported side effects while performing BFR exercises are fainting and dizziness, numbness, pain and discomfort, delayed onset muscle soreness[22].

Contraindications

All patients should be assessed for the risks and contraindications to tourniquet use before BFR application. Patients possibly at risk of adverse reactions are those with poor circulatory systems, obesity, diabetes, arterial calcification, sickle cell trait, severe hypertension, or renal compromise[23]. Potential contraindications to consider are venous thromboembolism, peripheral vascular compromise, sickle cell anaemia, extremity infection, lymphadenectomy, cancer or tumor, extremity with dialysis access, acidosis, open fracture, increased intracranial pressure vascular grafts, or medications known to increase clotting risk[7].

Safety Implication

  1. Thrombus Formation

Although speculative, an initial safety concern regarding LL-BFR training included thrombus formation (i.e., blood clot). Research examining LL-BFR training with healthy individuals and older adults with heart disease found no change in blood markers for thrombin generation or intravascular clot formation. Furthermore, data from two surveys of nearly 13,000 individuals utilizing BFR training found that the incidence of deep venous thrombosis was <.06% and pulmonary embolism was <.01%.[2]  The systematic review[24] to examine the safety along with short- and long-term effects of BFR exercise on blood hemostasis in healthy individuals and patients demonstrate that short-term BFR exercise does not exacerbate the activation of the coagulation system nor enhance fibrinolytic activity in young healthy subjects. The findings posit that BFR would be relatively safe for adults considered young and healthy, those who are middle-aged with stable ischemic heart disease, and older healthy adults. But the review also suggests the need to verify the effects of BFR exercise on hemostasis and its safety of BFR exercise on hemostasis as there is limited evidence available.


[25]


2. Muscle Damage

Data from the aforementioned surveys found the incidence of excessive muscle damage (i.e., rhabdomyolysis) to be <0.01%. The amount of muscle damage associated with BFR training is conflicting; however, a comparison between maximal eccentric actions and LL-BFR training to exhaustion in untrained individuals revealed comparable amounts of exercise-induced muscle damage. However, performing LL-BFR training to exhaustion in clinical populations is not recommended, therefore, it appears the risk of LL-BFR training resulting in excessive muscle damage is minimal. In general, it is well established that unaccustomed exercise results in muscle damage and delayed onset muscle soreness (DOMS), especially if the exercise involves a large amount of eccentric actions. DOMS is normal after unaccustomed exercise, including after LL-BFR training, and should subside within 24–72 hours[2].

3. Central Cardiac Responses

Research studies suggest [26] that the cardiovascular system during exercise does not experience higher overload, which could be a risk factor for cardiac patients and physically inactive persons. This type of exercising could be considered safe.

4. Peripheral Vascular Changes

The effects of exercise on peripheral vascular changes are mixed. With ageing, the arterial compliance decreases and the resistance exercise may increase arterial stiffness in the elderly. Ozaki et al (Ozaki 2011) found significantly improved arterial compliance after 10 weeks of BFR walking in the elderly population. Clark et al (Clark 2011) found no change in arterial stiffness after 4 weeks of BFR training[7].

5. Tourniquets

By using the 3rd generation system the risk of tourniquet complication is very low, ranging from 0.04% to 0.8%. However, there is an inherent risk to tourniquet use. Some of the common complications[7] are:

  • Nerve injury: Mechanical compression and neural ischemia play an important role.[27] Nerve injury can range from mild transient loss of function to irreversible damage and paralysis.
  • Skin injury
  • Tourniquet pain
  • Chemical Burns
  • Respiratory, Cardiovascular, Cerebral circulatory and haematological effects caused by prolonged ischaemia
  • Temperature changes

Summary

BFR training can be viewed as an emerging clinical modality to achieve physiological adaptations for individuals who cannot safely tolerate high muscular tension exercise or those who cannot produce volitional muscle activity. However, continued research is needed to establish parameters for safe application prior to widespread clinical adoption[2]


[28]

References

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  4. Accessed fromhttps://www.sportsmed.org/AOSSMIMIS/members/downloads/SMU/2017Spring.pdf
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  27. JP Sharma, R Salhotra - Indian journal of orthopaedics, 2012 Tourniquets in orthopaedic surgery. Indian J Orthop.Jul-Aug 2012, v.46(4).
  28. Blood Flow Restriction Training American Physical Therapy Association Available at https://www.youtube.com/watch?v=FZWhPx5u9K0