Aerobic Training With Blood Flow Restriction for Endurance Athletes: Potential Benefits and Considerations of Implementation : The Journal of Strength & Conditioning Research

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Brief Review

Aerobic Training With Blood Flow Restriction for Endurance Athletes: Potential Benefits and Considerations of Implementation

Smith, Nathan D.W.1,2; Scott, Brendan R.2,3; Girard, Olivier4; Peiffer, Jeremiah J.2,3

Author Information
Journal of Strength and Conditioning Research: December 2022 - Volume 36 - Issue 12 - p 3541-3550
doi: 10.1519/JSC.0000000000004079
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Abstract

Introduction

An endurance athlete's maximal rate of oxygen consumption (V̇o2max), onset of blood lactate accumulation (OBLA), and economy of motion are all positively associated with exercise performance (62). These areas are therefore key targets for most endurance training programs, resulting in a substantial volume of scientific literature (114) showing the use of high-intensity interval training (HIIT) to be a time effective method (relating to intervention duration) for improving V̇o2max and OBLA across all levels of athletes from novice to elite (70,72). However, the magnitude and etiology of physiological and performance improvements after HIIT are protocol dependent (10,108). In addition, frequent and continued use of HIIT can lead to overtraining (115), and therefore, this modality rarely forms greater than 20% of an athletes overall training volume (124). An alternative or adjunct training modality that can simultaneously improve V̇o2max, OBLA, and economy of motion and produce similar, if not superior, adaptations to HIIT would be beneficial for endurance athletes.

Training with blood flow restriction (BFR) may provide such a modality (25). This involves placing inflatable cuffs proximally around the active limbs during exercise, at a pressure occluding venous return from the limb while only partially restricting arterial inflow (94). Three to four weeks of low-intensity aerobic exercise with BFR has improved V̇o2max, OBLA, and strength (indicating a benefit to economy of motion (128)) in nonendurance trained populations (3,25). Evidence also indicates that aerobic BFR training could improve these physiological characteristics in well-trained endurance athletes (43,85). Indeed, aerobic BFR interventions have been shown to improve V̇o2max of endurance athletes by up to 9.1% (baseline V̇o2max = 63.0 ml·kg−1·min−1) (43,85), yet no studies have examined OBLA or economy of motion in this population. As a result, specific recommendations for incorporating BFR into an athlete's training to improve endurance performance do not exist. Such recommendations are important because applying BFR elevates the physiological stress at similar absolute workloads (110), thus prescribing and monitoring the volume and intensity of aerobic BFR exercise are problematic, and essential considerations for endurance athletes.

Along with the generalized recommendations for nonendurance athletes (94), endurance athletes should make additional considerations when implementing aerobic BFR training; particularly for improving performance. Consideration for exercise prescription, cuff application, and session design, among others, are necessary as all the combinations of BFR and exercise protocols that exist are unlikely to cause sufficient physiological demands to stimulate adaptations in endurance athletes. As such, the greater number of studies, and thus prescriptions, on aerobic BFR training in nonendurance trained cohorts could be used to inform considerations for athletes. Therefore, this review discusses the potential effectiveness and methods of implementing aerobic BFR training to improve endurance performance. First, the physiological limitations of V̇o2max, OBLA, and economy of motion will be summarized. The acute responses to applying BFR during exercise and chronic responses to aerobic BFR training on these physiological qualities will then be highlighted. Practical methods of implementing aerobic BFR exercise to improve endurance performance will finally be outlined, which includes cuff application, session design, and considerations for monitoring training loads. As a result, this review provides both an overview of the current literature and a prospective view of the potential benefits and considerations of implementation for endurance athletes, which could guide future research and applied practitioners. For the context of this review, the term “exercise” will be used when referring to acute responses, and “training” will be used to refer to the effects of a longer-term training program.

Physiological Qualities for Endurance Performance

Well-developed V̇o2max, OBLA, and economy of motion enable endurance athletes to sustain high power outputs or speeds over extended periods of time (i.e., during competition) (62). Although the power/speed associated with V̇o2max can only be maintained for a few minutes (12), a high V̇o2max allows for a greater range of sustainable work rates. The power/speed at OBLA is determined by one's ability to convert oxygen into mechanical energy (i.e., economy of motion) (88) and represents the highest sustainable fraction of V̇o2max which an individual can maintain for extended periods (61). The V̇o2max, OBLA, and economy of motion have independently (20,23,42,50,51) and collectively (22,126) demonstrated high positive correlations to endurance performance. It is acknowledged that endurance performance is also influenced by a range of internal (e.g., carbohydrate availability (11) and hydration status (34)) and external (e.g., environmental conditions (96), teamwork, and tactics (81)) factors. These additional considerations are important for podium finishes but are not directly related to training methods and so will not be discussed.

Physiological Limitations for Maximal Oxygen Uptake

An individual's V̇o2max is limited by central (oxygen delivery) and peripheral (oxygen extraction) factors (7) with the impact of each dependent on the athlete's initial fitness level (83). For instance, the V̇o2max of individuals with lower cardiovascular fitness (V̇o2max < 55 ml·kg−1·min−1) is limited by both central and peripheral components (86,100,102), whereas those with higher fitness (>55 ml·kg−1·min−1) have near-maximal extraction capabilities (31) and are therefore more impacted by changes to oxygen delivery. Enhancing oxygen delivery can be achieved through increases in either cardiac output (30) or oxygen-carrying capacity of the blood (104). Cardiac output is increased primarily through improved stroke volume, caused by increases in left ventricular hypertrophy (47,71), as maximal heart rates are similar between age-matched sedentary and trained individuals (26,31). Importantly, training-induced left ventricular hypertrophy does not cause the deleterious effects on left ventricular function associated with hypertension-induced left ventricular hypertrophy (a risk factor for several heart conditions) (79); echocardiography measurements have revealed that these 2 types of left ventricular hypertrophy are caused by different structural and physiological adaptations (79). In addition, V̇o2max is correlated with blood volume (r = 0.65), yet a stronger relationship is observed with hemoglobin mass (r = 0.95) (80), an essential component of the oxygen-carrying capacity of the blood (91). Endurance athletes aiming to increase their V̇o2max should therefore focus on training strategies which increase stroke volume or the oxygen carrying capacity of the blood.

Physiological Limitations for the Onset of Blood Lactate Accumulation

The fraction of V̇o2max at which OBLA occurs is dependent on the exercising muscle's ability to extract (102) and use oxygen to replenish adenosine triphosphate (59), as well as lactate uptake and utilization by other tissues (4,57). Both oxygen extraction and metabolic waste removal can be improved by increases in capillary density, as a result of greater blood residence times and reductions in diffusion distance (28). Indeed, the number of capillaries per square millimeter is associated with aerobic fitness (6,44) and demonstrates strong and moderate correlations with OBLA (r = 0.74) (22) and endurance performance (r = 0.50), respectively (22,84). However, within endurance cyclists, a stronger correlation has been noted between the capillary to fiber ratio (r = 0.88) when compared with the number of capillaries per millimeter squared (r = 0.50) (84). As such, it is possible that muscle fiber size may limit capillary density and subsequently OBLA.

Examination of cross-sectional and longitudinal data collection points indicates that greater skeletal muscle buffering capacity, in addition to capillary density, can increase OBLA and improve endurance performance (133). Intracellular and extracellular buffer systems act to prevent intracellular H+ accumulation, thereby attenuating metabolic acidosis, which can inhibit muscle contractile (i.e., cross-bridge cycling) (33) and metabolic (i.e., glycolysis) (32) processes. Increased buffering capacity can limit the development of fatigue and increase the rate of oxidative phosphorylation at any given workload (46), resulting in greater power/speeds associated with OBLA.

Physiological Limitations for Economy of Motion

The power/speed at any fraction of V̇o2max is dictated by the oxygen cost of exercise (i.e., economy of motion), which is positively correlated with the percentage of type I muscle fibers (24). Yet specifically, it is the contribution of type I fibers to force production that limits economy of motion (45,128). In trained cyclists (131), runners (125), and cross country skiers (45), greater muscular strength decreases the oxygen cost of locomotion at given absolute intensities irrespective of fiber type distribution (45,106,128). As such, improvements in economy of motion can be achieved through increases in muscle cross-sectional area (131) and the efficacy of excitation-contraction coupling (67). Indeed, preserving muscle excitability can prevent impairments in excitation-contraction coupling and metabolic flux rates that would otherwise increase the oxygen cost of a given workload (i.e., reduced economy of motion) during prolonged exercise (1,37). Both Na+-K+-ATPase and Ca2+-ATPase regulate the activity of ion pumps (36) that maintain high transmembrane gradients (1,37). As such, increasing Na+-K+-ATPase or Ca2+-ATPase pump concentrations could reduce the oxygen consumption for a given speed/power, thus improving economy of motion and allowing a greater fraction of V̇o2max to be sustained during endurance exercise (35).

Traditional Training to Improve Endurance Performance

Defining Traditional Endurance Training

The V̇o2max, OBLA, and economy of motion can all be improved through traditional training (i.e., without BFR), yet improvements are more difficult to elicit in trained individuals. Low-intensity to moderate-intensity training typically accounts for at least 80% of an endurance athletes' total training volume (114). These intensities can produce large increases in V̇o2max (83,97,121) and OBLA (8,56) in nonathletic populations; although, these findings are not consistent with endurance trained athletes (68,70,78). However, the remaining 20% of an athletes' training volume is commonly performed as HIIT (114), which consistently improves V̇o2max (69,107,116) and OBLA (68,107,116) in this population. The magnitude of these improvements depends on the duration, intensity, and work:rest ratio of the HIIT intervals, as well as the total duration of the training program (104,105,107). Indeed, shorter intervals (≤2.5 minutes and work:rest ratios from 2:1 to 1:9) at intensities at or greater than V̇o2max can improve V̇o2max (69,107) and OBLA (68,70,107) (up to 8.7 and 16%, respectively) within 4 weeks. Longer (>4 minutes; 2:1 work:rest ratio) intervals at 88–100% maximal heart rate can also provide similar benefits to V̇o2max (3.7%) and OBLA (10%) although these changes may only be observed with programs extending beyond 12 weeks (104,105).

In contrast with the benefits on V̇o2max and OBLA, HIIT does not improve economy of motion in endurance athletes (48,49,111,128). The use of resistance training, however, has been shown to improve this physiological quality in non–resistance-trained endurance cyclists (128), runners (125), and cross-country skiers (45). For instance, 3 sessions of half-squats to failure using a Smith Machine (4 sets at 4 repetition maximum) per week improved half-squat maximal strength by ∼33% and running economy at 70% V̇o2max by ∼5% (125). Resistance training also reportedly increases Na+-K+-ATPase (35) and Ca2+-ATPase activity (38) both of which may contribute to improved economy of motion during prolonged exercise (36). Although resistance training seems to provide a greater benefit to economy of motion in non–resistance-trained endurance athletes (128,131), the training demands of these athletes are likely to limit the ability to incorporate both HIIT and resistance training within a single training block. Therefore, an alternative training method which would allow endurance athletes to receive the benefits of both HIIT and resistance training is appealing.

Blood Flow Restriction Training to Improve Endurance Performance

For BFR training to increase endurance performance, it must improve the physiological limitations for V̇o2max, OBLA, economy of motion, or a combination of these capacities. Low-intensity aerobic BFR training interventions have improved V̇o2max and OBLA in recreationally active individuals (25), with increases in maximal strength (3) and Na+-K+-ATPase activity (15) indicating a potential benefit for economy of motion. These improvements caused by the BFR stimulus are influenced by the cuff pressure and mode (intermittent or continuous) of application. Cuff pressures have been prescribed as absolute values (often expressed in mm Hg) in early research (2), yet recent studies use a percentage of the minimum cuff pressure needed to occlude an individual's blood flow (i.e., arterial occlusion pressure [AOP]) (94) to avoid the risks associated with complete arterial occlusion. The cuff pressure can be applied in conjunction with the start and end of exercise intervals (i.e., intermittently) or inflated throughout continuous or interval exercise (i.e., continuously) (19). The exact combination of exercise and BFR stimuli will therefore influence the acute physiological responses to aerobic BFR exercise and chronic adaptations after aerobic BFR training (5).

The primary mechanisms underpinning BFR-induced training adaptations have largely been examined in relation to resistance exercise (95,98,112). Hypoxia and metabolite accumulation caused by the cuffs are the most plausible drivers (i.e., primary mechanisms) of the observed aerobic and resistance BFR training adaptations (2). The underlying adaptive pathways (i.e., secondary mechanisms) likely differ between the training modalities (16). During aerobic-based exercise, hypoxia shifts the primary energy-producing pathway from aerobic to anaerobic metabolism (82), subsequently accelerating anaerobic metabolite production. As such, BFR decreases arterial inflow to the exercising limb that reduces oxygen delivery to the musculature (58). This in turn increases the production of metabolites, which accumulate locally because of venous occlusion (127). Metabolic stress during resistance training with BFR is considered an important upstream driver of adaptation (95). The result is an increase in type II fiber recruitment and cell swelling that may contribute to the activation of signaling cascades associated with the mechanistic target of rapamycin pathway (95) as well as satellite cell activation and proliferation (90). In addition, cycles of local hypoxia and reperfusion during BFR exercise increase the oxidative stress compared with non-BFR activities (17). Enhanced oxidative stress from a single session of moderate-intensity BFR running has been associated with increased angiogenic signaling proteins (16). Considering these BFR-mediated physiological responses, it is possible that many of the physiological changes associated with BFR could provide benefits to V̇o2max, OBLA, and economy of motion, which will be highlighted in the following sections.

Blood Flow Restriction Exercise to Improve Maximal Oxygen Uptake

Although the use of BFR during aerobic training has not been widely examined in well-trained endurance populations, the use of BFR has been shown to increase V̇o2max in well-trained male athletes (V̇o2max > 61.2 ml·kg−1·min−1) (43,85,129). For example, after a 4 week low-intensity (<2 mmol·L−1) BFR rowing program (2 × 10 minutes efforts, 3 times per week), V̇o2max and the power at V̇o2max increased by 9.1 and 15.3%, respectively, a finding not demonstrated when the same training was completed without BFR (43). In addition, applying ∼130 mm Hg for half the recovery between 30-second maximal sprints (recovery period = 4.5 minutes) increased endurance cyclists' V̇o2max by 4.5% (85,129). Although the results of studies applying BFR during recovery versus exercise periods are not comparable, the 4.5% increase in V̇o2max in these cyclists alongside the 9.1% improvement in elite rowers (43) demonstrates BFR training can improve V̇o2max in athletes already possessing highly developed aerobic capacities.

Evidence of the mechanisms responsible for the improvements in V̇o2max after aerobic-based BFR training in trained athletes is lacking. However, plausible mechanisms can be discussed by extrapolating data from the extensive literature in untrained populations (92). The application of BFR during aerobic exercise is associated with an acute reduction (∼17%) in V̇o2max (110), the consequence of venous pooling leading to a decrease in stroke volume (92). To maintain the necessary cardiac output for a given workload under BFR heart rate is increased (92), thereby resulting in greater cardiovascular stress (130) which is a key component to increasing V̇o2max in both trained an untrained populations (2,93). Indeed, an elevated heart rate increases the mechanical stress on the heart (2,58) that improves stroke volume over multiple sessions (93) and subsequently V̇o2max during non-BFR exercise (2). The direct cause of BFR-induced improvements in stroke volume are unknown (85,93), yet greater mean arterial pressures during BFR exercise (130) and the elevated cardiac stress (2) may indicate left ventricular hypertrophy (65,123).

Alongside changes in cardiac function, 6 weeks of moderate-intensity aerobic BFR training has increased resting femoral artery diameter postexercise in recreationally active men, indicating greater oxygen delivery during exercise (13). This finding has not been demonstrated under non-BFR conditions despite femoral artery diameter being positively correlated with V̇o2max (r2 = 0.83) (99). The likely mechanism for the greater femoral artery diameter is ischemia-reperfusion injury on deflation of the BFR cuffs (101). Indeed, vascular compliance is reduced at cuff deflation (101), indicating the occurrence of endothelial damage and the subsequent activation of neutrophils and platelets, which produce reactive oxygen species (113). The resulting oxidative stress acutely reduces the bioavailability of vasodilators such as nitric oxide (113); however, the scavenger capacity of reactive oxygen species has been improved by an aerobic BFR training intervention (14,15). This chronic adaptation results in greater bioavailability of vasodilators that reduces microvascular resistance, and thereby increasing peripheral blood flow, during non-BFR exercise (13,40).

Not all aerobic BFR training interventions demonstrate improvement in V̇o2max, an outcome most likely associated with the prescribed intensity of the sessions (64). Indeed, when prescribing exercise intensity relative to V̇o2max under BFR conditions, the associated cardiovascular stress is less then when undertaking BFR exercise set to nonoccluded V̇o2max values (110). The method of prescribing BFR exercise intensity is therefore an important consideration when prescribing BFR exercise to improve V̇o2max to ensure a sufficient relative exercise intensity to stimulate adaptation.

Blood Flow Restriction Exercise to Improve the Onset of Blood Lactate Accumulation

Preliminary data indicate aerobic BFR training can provide a greater benefit to OBLA when compared with those to V̇o2max. Applying BFR during an incremental walking test transiently reduced the oxygen uptake corresponding to OBLA by 15%, although the percentage of V̇o2max at OBLA was not different compared with a non-BFR condition because of a concomitant reduction in V̇o2max (110). This finding indicates BFR increases relative exercise intensity at any given absolute workload (130), which is important for improving OBLA (27,78). Indeed, intermittent BFR during 4 weeks of interval cycling (work:rest ratio 2:1) at ∼66 W has increased the power output at OBLA by 16%, compared with 6% from work matched low-intensity and 25% from high-intensity (at ∼236 W) non-BFR training (25). No differences were noted between conditions, possibly a consequence of low statistical power (β = 0.58), although calculating the pretraining to post-training between-group effect sizes reveals BFR training produced a moderate benefit compared with low-intensity training (d = 0.34), with a small benefit of high-intensity training over BFR (d = 0.18). Therefore, using higher exercise intensities with BFR could produce a similar or greater improvement to OBLA than traditional HIIT, as improvements are dependent on relative intensity (78).

Improvements to OBLA could occur through greater muscle oxygen extraction and metabolite removal caused by angiogenesis, although the method of applying BFR will determine the contributing underlying mechanism. Both continuous and intermittent BFR cause local hypoxia and shear stress, which can independently activate angiogenic growth factors (29,52), yet the primary contributing pathway will likely differ. Continuous BFR reduces oxygen tension and increases metabolic stress (39,66), stimulating capillarization through nitric oxide independent pathways (73). However, the multiple periods of ischemia and reperfusion caused by intermittent BFR (54) would primarily stimulate angiogenesis through enhanced shear stress. Additional evidence is required to establish if continuous and intermittent BFR cause similar magnitudes of improvement in OBLA, as this could influence how cuff inflation/deflation strategies are periodized. In addition to the mode of applying the BFR pressure, during resistance training (89), intermittent BFR with 80%, but not 40%, of AOP increases angiogenic signaling. Both the cuff pressure and how BFR is cycled on and off therefore seem to be important for inducing angiogenesis, one mechanism that could lead to improvements to OBLA.

Increased intracellular buffering capacity (13) presents another likely mechanism for BFR-induced improvements to OBLA (85). Four weeks of 30-second sprint interval training (4.5-minute recovery periods) with BFR for half the duration of each recovery interval increased endurance cyclists' critical power by 3.3% without the occurrence of angiogenesis or mitochondrial biogenesis (85), indicating metabolite removal did not cause the improvement. The likely mechanism is greater intracellular buffering capacity as it is not dependent on metabolite removal (63). This theory is supported by a 16% reduction, compared with pretraining values, in the net thigh lactate release at 90% of peak power occurring in conjunction with BFR-induced increase in femoral artery diameter and oxygen delivery (13). Thigh oxygen uptake at 90% peak power output, calculated using Doppler flow and arteriovenous blood sampling, and vastus lateralis mitochondrial protein content did not change (13). Therefore, improved intracellular buffering capacity is the most likely mechanism for the reduced net lactate (15) and could be an additional mechanism (to angiogenesis) by which BFR can improve OBLA during nonrestricted exercise.

Blood Flow Restriction Exercise to Improve Economy of Motion

In contrast to V̇o2max and OBLA (25), the impact of BFR training on economy of motion has not been examined. However, BFR-induced improvements in maximal strength (3) and Na+-K+-ATPase activity (15) indicate a potential benefit to economy of motion as these qualities reduce the oxygen cost of locomotion during prolonged submaximal exercise (67,128). In recreationally active individuals, 4 weeks of cycling (15 minutes, 40% V̇o2max) with continuous BFR resulted in a 7.7% increase in maximal knee-extensor force production (i.e., strength) that the authors attributed to the 5.1% increase in quadriceps cross-sectional area (2). Walking with BFR has also been shown to improve maximal strength in recreationally active individuals (3), but not resistance-trained basketballers (93), indicating that BFR strength gains may be moderated by the training status. This means that the impact of BFR exercise on economy of motion is also likely dependent on the training history of an athlete (125).

Separately to strength gains, aerobic interval training with intermittent BFR (work:rest ratio 2:1) has been shown to cause fiber type specific increases to the abundance of different Na+-K+-ATPase isoforms (15). This increase likely contributed to the net reduction in leg K+ release during unilateral knee extensions at 90% of knee-extensor incremental peak power and resulted in a 12% greater increase in time to exhaustion compared with the non-BFR condition (15). The increase in time to exhaustion after BFR training may indicate improved economy of motion, although this was not measured (15). In addition, training induced increases in Na+-K+-ATPase isoform FYXD-1 are unrelated to the magnitude of muscle hypoxia and lactate accumulation, key outcomes of BFR training, and are instead associated with BFR-induced oxidative stress and adenosine monophosphate-activated protein kinase signaling (16). This indicates oxidative stress associated with intermittent BFR (101) may cause superior improvements in Na+-K+-ATPase isoforms compared with continuous BFR. However, further research is required to establish if increases in Na+-K+-ATPase isoforms from aerobic BFR training interventions can improve economy of motion as this may influence aerobic BFR session structure, particularly the inflation and deflation of BFR cuff.

Prescription of Aerobic Exercise With Blood Flow Restriction for Endurance Athletes

Although aerobic BFR training interventions can improve V̇o2max, OBLA, and potentially economy of motion (Figure 1), there are no specific recommendations for implementing this training strategy when targeting these outcomes in endurance athletes. A recent position statement highlights general guidance for implementing aerobic BFR training, with a focus on minimizing the risk of harm or serious injury (94). However, specific considerations for endurance athletes for improving key physiological attributes of endurance performance should be highlighted. The following section of this review will outline practical methods of safely applying BFR during aerobic exercise, including the determination of cuff pressure and exercise intensity. Additional considerations for implementing BFR into a periodized training program will also be covered, including the potential impact of BFR on monitoring training loads.

F1
Figure 1.:
Schematic representation of the acute responses (dark gray area) and chronic adaptations (light gray area) to aerobic exercise with BFR and potential benefit on performance for well-trained endurance athletes (white area). Lines between boxes that cross but do not connect are identified by breaks in one line, with black circles representing the continuation of the broken line. Black text = has been shown within aerobic BFR research. Gray text = not been shown but likely within aerobic BFR exercise. ↑ = increase; ↓ = decrease; ↔ = no change; BFR = blood flow restriction; mTOR = mammalian target of rapamycin; Na+-K+-ATPase = sodium-potassium adenosine triphosphatase; O2 = oxygen; OBLA = onset of blood lactate accumulation; ROS = reactive oxygen species;VO2max = maximal rate of oxygen uptake.

Determining Blood Flow Restriction Pressure for Endurance Exercise

The pressure for BFR should be set at a fraction of AOP and not arbitrary absolute values that may increase between-subject variability (76) and cause complete arterial occlusion (75,76), suboptimal training stimuli (18), or thrombosis (77). The physiological responses to a given absolute cuff pressure are also individual (55,74), thus inflating cuffs to 160 mm Hg (2) or wrapping elastic cuffs around the top of the thighs to a perceived tightness (43) will likely result in each athlete receiving different magnitudes of BFR and thus physiological adaptations. Accounting for these interindividual differences, and also cuff-width differences, a similar relative BFR stimulus can be achieved using a given percentage of AOP (76). An individual's AOP is influenced by limb circumference (75), blood pressure (74), cuff width (109) and orientation (122), and body posture (119). Most of the interindividual variance in leg AOPs is explained by the thigh circumference when using either large (13.5 cm; r2 = 0.49) or small (5 cm; r2 = 0.16) width cuffs (75). However, narrow width cuffs require greater absolute pressures to reach the AOP compared with wider cuffs (109). Similarly, upright body positions require greater AOPs compared with supine measurements because of orthostatic increases in diastolic blood pressure (119). In accordance with the above-mentioned BFR considerations, practical recommendations for determining AOP are provided below.

The recommended BFR pressures (40–80% of AOP) have been derived from individuals in a supine position (94) and therefore are not directly comparable with AOPs determined in seated or standing positions (53). Indeed, when using 11.5-cm cuffs, supine AOPs are 9.1 and 29.4% lower than when seated or standing, respectively (53). Although the AOP can be determined while standing or seated, the pressure used during exercise should still be between 40 and 80% of the supine AOP. Practically, this equates to ∼28–56% of standing AOP and 36–73% AOP for seated measurements using 11.5-cm wide cuffs (53). The corresponding values when using 5-cm wide cuffs have not been investigated, and as narrow cuffs require greater pressures to cause occlusion than wider cuffs (109), ranges cannot be provided.

When choosing the body position used for determining AOP, the cuff position, body orientations, and resting time before the measurement require consideration. For exercise using the lower limbs, the center of the cuff's bladder should be placed directly over the femoral artery to ensure an accurate AOP measurement and similar relative BFR between sessions (122). After positioning the cuffs athletes must rest in either the supine, standing, or seated body position for 5 minutes before the AOP measurement (53), with the supine and standing positions requiring the arms to be relaxed by the side of the body. However, to measure AOP on a bike, the heel should be placed on the pedal axle at the furthest point away from the cyclist (i.e., knee extension) while leaning toward the handlebars (53) because this position will replicate that used by cyclists while riding. When choosing either the supine, standing, or seated body position, one must consider which position the athlete can hold for the 5-minute resting and AOP measurement.

When using BFR during aerobic-based training a trade-off exists between cuff pressure, exercise performance (134), and the subsequent training adaptations (5). For instance, increasing cuff pressure from 45 to 60% of AOP resulted in a 37% decrease in total work completed during a cycling session consisting of 10-second repeated maximal sprints with 20-second recovery (13 total sprints compared with 7) (134). In addition, sprinting under 60% AOP resulted in a greater decline in isometric maximal voluntary torque from baseline (−47.5%) compared with 45% AOP (−24.6%) and non-BFR conditions (−8.6%) (134). These results indicate that greater cuff pressures will reduce acute exercise performance; however, this may not influence training adaptations. Indeed, similar increases in biceps brachii thickness (10%), strength (18%), and endurance (62%) have been observed after 8 weeks of resistance training (30% of 1 repetition maximum) using either 40% or 90% of AOP (21). Taken as a whole, endurance athletes using BFR should select a percentage of AOP that allows them to complete the prescribed volume of exercise, as both high and low cuff pressures are likely to lead to beneficial training adaptations (21). Further investigations examining the relationship between cuff pressure, exercise performance, and physiological adaptation are important to identify if BFR pressure and exercise volume influence the physiological adaptations after an intervention, which would have implications for sessional design.

Volume and Intensity for Endurance Exercise With Blood Flow Restriction

The recommended upper restriction time for BFR exercise using 40–80% supine AOP is 20 minutes, 2–3 times a week (94), which eliminates its use during long-duration aerobic-based training (114). Endurance athletes can incorporate BFR during exercise of shorter durations, such as interval training where high intensities can be maintained throughout a session without compromising the accumulated sessional workload. However, endurance cyclists have perceived BFR during repeated 30-second maximal sprints as being intolerable even at “moderate” cuff pressures (129), yet recreationally active individuals have successfully completed 7.5 ± 6.4 10-second sprints with 60% AOP (134). Sprint BFR training therefore requires further research before suitable prescription guidelines can be identified for athletic populations. Nevertheless, submaximal efforts of up to 2 minutes at 80% AOP have been used within the literature (130), indicating the usefulness of this technique across a range of possible aerobic-based interval scenarios that are likely tolerable at 40–80% AOP.

Prescribing the intensity of submaximal intervals with BFR can be problematic as BFR results in an increase in acute physiological and perceptual responses while simultaneously reducing the sustainable power or speed (i.e., performance) when compared with unrestricted exercise (64,110,130). Indeed, aerobic BFR exercise increases submaximal oxygen uptake (92), cardiovascular stress (92,101), ratings of perceived exertion (19), and the blood lactate concentration at absolute non–BFR-derived workloads (110). The elevated physiological demands at reduced sustainable mechanical outputs indicate that prescribing aerobic BFR exercise from any intensity metric obtained from non-BFR performance tests is problematic for endurance athletes, as the relative physiological intensities are not comparable.

Without using traditional non-BFR intensity metrics, prescribing BFR training intensity could be achieved using a self-regulated “highest sustainable intensity” approach (118). This involves producing the highest self-paced power or speed across multiple intervals, with the aim of producing the greatest accumulated sessional workload. Endurance athletes using such an approach can reliably regulate intensity across intervals (117), with self-paced interval training producing similar fitness and performance outcomes (105,116) compared with metric prescribed HIIT (132,133). This self-paced model may produce sufficient cardiovascular and metabolic demands to elicit training adaptations for aerobic BFR exercise with endurance athletes. The training stimulus could progressively increase throughout an intervention as an athlete's highest sustainable intensity improves. To date, anchoring BFR exercise intensity using a self-selected intensity approach has not been examined, and so, the limitations of this approach are unknown. Indeed, the earlier onset of metabolite accumulation (110) and impaired removal of metabolites (87) while under BFR may interfere with an athlete's ability to reliably self-regulate aerobic BFR exercise intensity within a session, preventing effective training adaptions, and therefore requires investigation.

Regardless of how exercise intensity is prescribed, it is an important consideration as the combination of high intensities and cuff pressures can reduce sessional exercise volume and minimize physiological adaptations. For example, progressively increasing both interval intensity (60–85% of the velocity associated with V̇o2max) and cuff pressure (160–240 mm Hg, intermittent application) across 4 weeks of aerobic BFR interval training (2-minute intervals with 1-minute recovery periods) required the number of sets during each session to be reduced from 10 to 5 (5). The same progressive increase in exercise intensity without changing cuff pressure (240 mm Hg) allowed subjects to complete 10 sets during each session (5). Furthermore, progressively increasing both cuff pressure and exercise intensity increased V̇o2max by a smaller amount (8.4%) compared with increasing exercise intensity alone (14.8%) (5), indicating no clear benefit from using greater cuff pressures. Higher exercise intensities should therefore be used with lower percentages of AOP to optimize physiological adaptation, with the most suitable pressure for athletes likely being the greatest percentage of AOP not requiring a reduction in sessional exercise volume.

Adding Aerobic Blood Flow Restriction Exercise Into Periodized Training Programs

The periodization of exercise training with BFR is not documented; thus, the impact of BFR on program design or training loads is yet to be clarified. It is therefore difficult to determine how to best incorporate this type of training into an endurance athlete's plan. Although applying BFR during intermittent or continuous exercise can increase essential physiological markers (e.g., oxygen uptake, cardiac stress, and lactate concentration) known to improve endurance performance (105,107), this technique is unlikely to match the improvements from traditional HIIT (19). As such, aerobic BFR training will never present a replacement for HIIT; yet, it could be used as an adjunct training stimulus to maintain or develop V̇o2max, OBLA, and/or economy of motion. Indeed, as moderate intensity aerobic BFR training will likely induce adaptations through different mechanisms to traditional HIIT, it is possible that periodically using aerobic BFR training instead of HIIT could lead to greater improvements in performance compared with only prescribing HIIT. Aerobic BFR training could also be used for specialization blocks; for example, the greater metabolic stress when BFR cuffs are applied during submaximal work (87) could be used to specifically target power improvements at OBLA. In addition, substituting higher intensity exercise with BFR for HIIT may improve muscle strength, and thus economy of motion, without adding additional training volume through resistance exercise, although further research on this point is required. Incorporating sessions of aerobic BFR exercise into a periodized training plan could therefore be a time efficient method (relating to weekly training time) for improving the physiological qualities associated with endurance performance compared with only relying on HIIT or resistance training.

In addition to incorporating BFR into the normal training patterns of an athlete, using BFR with lower mechanical loads is well suited for rehabilitation and the off-season. Although low workloads with BFR are unlikely to improve V̇o2max or strength for endurance athletes, it could contribute to preventing or attenuating the detraining effect. For example, 1 weekly HIIT session during 8 weeks of reduced training volumes prevented a decrease in the percentage of V̇o2max at OBLA that occurred in a group that did not perform HIIT (103). Walking or cycling with BFR could also benefit injured athletes unable to tolerate high mechanical stress, by shortening the time between completing rehabilitation and returning to preinjury performance capacity (103). Both low and moderate-to-high intensity aerobic exercise with BFR can therefore benefit endurance athletes in a range of scenarios because of the heightened physiological demands at a low workloads (92).

Managing Blood Flow Restriction Exercise Training Loads

Implementing sessions of aerobic BFR exercise into an athlete's periodized training program involves managing internal (e.g., heart rate) and external (e.g., power output) training demands, which are important to assess adaptations (41) and to avoid overtraining (60). Importantly although, common training load metrics may not reflect the demands of a single aerobic BFR exercise session in a comparable way with unrestricted exercise, as BFR elevates physiological responses at reduced workloads (19). This will have consequences for understanding how the prescribed training (i.e., the external load) will impact on the response to exercise (i.e., the internal training load). Progressing the mechanical load without accounting for the elevated physiological stress could prevent athletes from completing training blocks or cause nonfunctional overreaching. For example, heart rate–based training load calculations, which are common in endurance sports, are unlikely to accurately reflect the overall physiological demands of exercise, as venous pooling caused by the cuffs elevates submaximal heart rate and is more pronounced as exercise intensity increases (92). Applying BFR during exercise also increases ratings of perceived exertion during aerobic exercise (19), which may be influenced by an increase in discomfort associated with the cuffs (120). Ensuring that athletes are able to accurately rate the exertion associated with exercise, while omitting any discomfort associated with the BFR cuffs themselves, would therefore be necessary for subjective training load analyses. Considering that the basis for most BFR training is that it alters the relationship between physical workload and physiological intensity, it is unlikely that managing the training load based on either internal or external metrics in isolation will quantify the true demands associated with aerobic BFR training interventions.

An alternative method to managing BFR exercise training loads could be integrated training load metrics, such as the ratio between internal and external load. Calculating the dose-response relationships between external and internal loads during aerobic BFR may provide a better reflection of the overall stress of a single BFR exercise session. For example, during a non-BFR periodized training plan, an increase in internal load at a given work rate can be considered an indicator of athlete fatigue or reductions in fitness, whereas a decrease in internal demands for a given physical dose represents an improvement in fitness or supercompensation (9). Practitioners should be aware although that the nature of BFR training will heighten the physiological responses for a given mechanical dose. As such, the internal: external load ratios for aerobic BFR exercise sessions would not be comparable with unrestricted exercise, and it may be more appropriate to monitor BFR and non-BFR training loads separately. However, no research has yet examined different approaches to measuring training load in BFR exercise, and how these metrics can be incorporated into the overall monitoring strategy for athletes. Future training studies could aim to calculate a variety of training loads for their specific mode, volume, and intensity of BFR exercise, so suitable approaches can be identified for a range of aerobic BFR training interventions.

Practical Applications

The physiological mechanisms by which low-intensity aerobic BFR training interventions improve some key physiological qualities of endurance performance indicate incorporating BFR exercise sessions could benefit well-trained endurance athletes. Aerobic BFR training interventions may cause meaningful improvements in V̇o2max and OBLA, and potentially economy of motion simultaneously, or alternatively a single quality could be targeted and improved by a greater magnitude than the others. However, practitioners looking to use BFR must consider the heightened physiological stress at lower workloads, which will both increase the relative exercise intensity and make it difficult to manage training loads. Owing to the limited aerobic BFR research in well-trained endurance athletes, the optimal parameters of aerobic BFR training (e.g., exercise intensity and cuff pressure) to maximize improvements in physiological qualities, and thus competitive endurance performance, need to be clarified with additional specific research in this athletic cohort. Until best practice approaches are identified, practitioners are encouraged to apply BFR during aerobic intervals at a cuff pressure between 40% and 80% of AOP that does not cause a large reduction in sessional exercise volume and monitor the associated training load independently from non-BFR training.

Acknowledgments

N. D. W. Smith was supported by an Australian Government Research Training Programme (RTP) scholarship. B. R. Scott is supported by a National Health and Medical Research Council Investigator Grant. The authors have no conflicts of interest to disclose. All authors have contributed substantially to the submitted work and have reviewed and agree with the submission of the article for review.

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Keywords:

Kaatsu; vascular occlusion; physiological determinants; aerobic exercise prescription; interval training adaptations; training load

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