Blood flow restriction: the metabolite/volume threshold theory

Acute vascular and cardiovascular responses to blood flow-restricted exercise

Blood flow-restricted resistance exercise improves muscle strength; however, the cardiovascular response is not well understood.

PURPOSE

This investigation measured local vascular responses, tissue oxygen saturation (StO2), and cardiovascular responses during supine unilateral leg press and heel raise exercise in four conditions: high load with no occlusion cuff, low load with no occlusion cuff, and low load with occlusion cuff pressure set at 1.3 times resting diastolic blood pressure (BFRDBP) or at 1.3 times resting systolic blood pressure (BFRSBP).

METHODS

Subjects (N = 13) (men/women, 5/8, 31.8 ± 12.5 yr, 68.3 ± 12.1 kg, mean ± SD) performed three sets of leg press and heel raise to fatigue with 90-s rest. Artery diameter, velocity time integral, and stroke volume were measured using two-dimensional and Doppler ultrasound at rest and immediately after exercise. HR was monitored using a three-lead ECG. Finger blood pressure was acquired by photoplethysmography. Vastus lateralis StO2 was measured using near-infrared spectroscopy. A repeated-measures ANOVA was used to analyze exercise work and StO2. Multilevel modeling was used to evaluate the effect of exercise condition on vascular and cardiovascular variables. Statistical significance was set a priori at P < 0.05.

RESULTS

Artery diameter did not change from baseline during any of the exercise conditions. Blood flow increased after exercise in each condition except BFRSBP. StO2 decreased during exercise and recovered to baseline levels during rest only in low load with no occlusion cuff and high load with no occlusion cuff. HR, stroke volume, and cardiac output (Q˙) responses to exercise were blunted in blood flow-restricted exercise. Blood pressure was elevated during rest intervals in blood flow-restricted exercise.

CONCLUSIONS

Our results demonstrate that cuff pressure alters the hemodynamic responses to resistance exercise. These findings warrant further evaluations in individuals presenting cardiovascular risk factors.

Practical blood flow restriction training increases acute determinants of hypertrophy without increasing indices of muscle damage

Vascular blood flow restriction (BFR) training stimulates muscle hypertrophy by increasing muscle activation and muscle swelling. Previous studies used expensive pneumatic cuffs, which may not be practical for regular use. The aim was to investigate the acute effects of low-intensity practical BFR (LI-pBFR) on muscle activation, muscle swelling, and damage. Twelve trained male participants completed a 30-, 15-, 15-, 15-repetition scheme at 30% of their leg press 1-repetition maximum under control and LI-BFR conditions. Under the LI-BFR trial, knee wraps were applied to the thighs at a pressure that resulted in venous, not arterial, occlusion. In the control trial, wraps were applied with zero pressure. Ultrasound-determined muscle thickness was recorded at baseline; 0 minutes post with wraps; 0, 5, and 10 minutes post without wraps. Muscle activation was recorded during warm-ups and on the final set of 15 repetitions. Indices of muscle damage (soreness, power, and muscle swelling) were also recorded. There was a condition by time effect for muscle thickness (p < 0.0001, effect size [ES] = 0.5), in which muscle thickness increased in the LI-pBFR condition 0 minutes post with wraps and through 5 minutes post without wraps. No changes occurred in the control. There was a condition by time effect for muscle activation (p < 0.05, ES = 0.2). The LI-pBFR had greater activation than the control did. There were no condition by time effects on indices of muscle damage. Our data indicate that practical BFR significantly increases muscle activation and muscle thickness without increasing indices of damage.

Safety and possible effects of low-intensity resistance training associated with partial blood flow restriction in polymyositis and dermatomyositis

Introduction

Our aim was to evaluate the safety and efficacy of a low-intensity resistance training program combined with partial blow flow restriction (BFR training) in a cohort of patients with polymyositis (PM) and dermatomyositis (DM).

Methods

In total, 13 patients with PM and DM completed a 12-week twice a week low-intensity (that is, 30% one-repetition-maximum (1RM)) resistance exercise training program combined with partial blood flow restriction (BFR). Assessments of muscle strength, physical function, quadriceps cross sectional (CSA) area, health-related quality of life, and clinical and laboratory parameters were assessed at baseline and after the intervention.

Results

The BFR training program was effective in increasing the maximal dynamic strength in both the leg-press (19.6%, P <0.001) and knee-extension exercises (25.2% P <0.001), as well as in the timed-stands (15.1%, P <0.001) and timed-up-and-go test (−4.5%, P =0.002). Quadriceps CSA was also significantly increased after the intervention (4.57%, P =0.01). Similarly, all of the components of the Short Form-36 Health Survey, the Health Assessment Questionnaire scores, and the patient- and physician reported Visual Analogue Scale were significantly improved after training (P <0.05). Importantly, no clinical evidence or any other self-reported adverse event were found. Laboratory parameters (creatine kinase and aldolase) were also unchanged (P >0.05) after the intervention.

Conclusions

We demonstrated that a 12-week supervised low-intensity resistance training program associated with partial blood flow restriction may be safe and effective in improving muscle strength and function as well as muscle mass and health-related quality of life in patients with PM and DM.

Trial registration

Clinicaltrials.gov NCT01501019. Registered November 29, 2011.

Is there a minimum intensity threshold for resistance training-induced hypertrophic adaptations?

In humans, regimented resistance training has been shown to promote substantial increases in skeletal muscle mass. With respect to traditional resistance training methods, the prevailing opinion is that an intensity of greater than ~60 % of 1 repetition maximum (RM) is necessary to elicit significant increases in muscular size. It has been surmised that this is the minimum threshold required to activate the complete spectrum of fiber types, particularly those associated with the largest motor units. There is emerging evidence, however, that low-intensity resistance training performed with blood flow restriction (BFR) can promote marked increases in muscle hypertrophy, in many cases equal to that of traditional high-intensity exercise. The anabolic effects of such occlusion-based training have been attributed to increased levels of metabolic stress that mediate hypertrophy at least in part by enhancing recruitment of high-threshold motor units. Recently, several researchers have put forth the theory that low-intensity exercise (≤50 % 1RM) performed without BFR can promote increases in muscle size equal, or perhaps even superior, to that at higher intensities, provided training is carried out to volitional muscular failure. Proponents of the theory postulate that fatiguing contractions at light loads is simply a milder form of BFR and thus ultimately results in maximal muscle fiber recruitment. Current research indicates that low-load exercise can indeed promote increases in muscle growth in untrained subjects, and that these gains may be functionally, metabolically, and/or aesthetically meaningful. However, whether hypertrophic adaptations can equal that achieved with higher intensity resistance exercise (≤60 % 1RM) remains to be determined. Furthermore, it is not clear as to what, if any, hypertrophic effects are seen with low-intensity exercise in well-trained subjects as experimental studies on the topic in this population are lacking. Practical implications of these findings are discussed.

Blood flow restriction pressure recommendations: the hormesis hypothesis

Blood flow restriction (BFR) alone or in combination with exercise has been shown to result in favorable effects on skeletal muscle form and function. The pressure applied should be high enough to occlude venous return from the muscle but low enough to maintain arterial inflow into the muscle. The optimal pressure for beneficial effects on skeletal muscle are currently unknown; however, preliminary data from our laboratory suggests that there may be a point where greater pressure may not augment the response (e.g. metabolic accumulation, cell swelling) but may actually result in decrements (e.g. muscle activation). This led us to wonder if BFR elicits somewhat of a hormesis effect. The purpose of this manuscript is to discuss whether pressure may be modulated to maximize skeletal muscle adaptation with resistance training in combination with BFR. Furthermore, the potential safety issues that could arise from increasing pressure too high are also briefly reviewed. We hypothesize that with BFR there is likely a moderate (∼ 50% estimated arterial occlusion pressure) pressure that maximizes the anabolic response to skeletal muscle without producing the potential negative consequences of higher pressures. Thus, BFR may follow the hormesis theory to some degree, in that a low/moderate dose of BFR produces beneficial effects while higher pressures (at or near arterial occlusion) may decrease the benefits of exercise and increase the health risk. This hypothesis requires long term studies investigating chronic training adaptations to differential pressures. In addition, how differences in load interact with differences in pressure should also be investigated.

Changes in muscle architecture induced by low load blood flow restricted training

In order to ascertain whether differing structural mechanisms could underlie blood flow restricted training (BFRT) and high intensity training (HIT), this study had two aims: (i) to gain an insight into the acute variations of muscle architecture following a single bout of two different volumes of BFRT, and (ii) to compare these variations with those observed after HIT. Thirty-five young men volunteered for the study and were randomly divided into three groups: BFRT low volume (BFRT LV), BFRT high volume (BFRT HV) and traditional high intensity resistance training (HIT). All subjects performed a bilateral leg extension exercise session with a load of 20% of one repetition maximum (1RM) in the BFRT groups, whereas the load of the HIT group was equivalent to an 85% of their 1RM. Before and immediately after the exercise bout, ultrasound images were taken from the rectus femoris (RF) and the vastus lateralis (VL). All groups increased their RF (p < 0.001) and VL (p < 0.001) muscle thickness, while the increases in pennation angle were larger in HIT as compared to BFRT LV (p = 0.013) and BFRT HV (p = 0.037). These results support the hypothesis that acute muscle cell swelling may be involved in the processes underlying BFRT induced muscle hypertrophy. Furthermore, our data indicate differing structural responses to exercise between BFRT and HIT.

Potential mechanisms for a role of metabolic stress in hypertrophic adaptations to resistance training

It is well established that regimented resistance training can promote increases in muscle hypertrophy. The prevailing body of research indicates that mechanical stress is the primary impetus for this adaptive response and studies show that mechanical stress alone can initiate anabolic signalling. Given the dominant role of mechanical stress in muscle growth, the question arises as to whether other factors may enhance the post-exercise hypertrophic response. Several researchers have proposed that exercise-induced metabolic stress may in fact confer such an anabolic effect and some have even suggested that metabolite accumulation may be more important than high force development in optimizing muscle growth. Metabolic stress pursuant to traditional resistance training manifests as a result of exercise that relies on anaerobic glycolysis for adenosine triphosphate production. This, in turn, causes the subsequent accumulation of metabolites, particularly lactate and H(+). Acute muscle hypoxia associated with such training methods may further heighten metabolic buildup. Therefore, the purpose of this paper will be to review the emerging body of research suggesting a role for exercise-induced metabolic stress in maximizing muscle development and present insights as to the potential mechanisms by which these hypertrophic adaptations may occur. These mechanisms include increased fibre recruitment, elevated systemic hormonal production, alterations in local myokines, heightened production of reactive oxygen species and cell swelling. Recommendations are provided for potential areas of future research on the subject.

Blood flow-restricted exercise in space

Prolonged exposure to microgravity results in chronic physiological adaptations including skeletal muscle atrophy, cardiovascular deconditioning, and bone demineralization. To attenuate the negative consequences of weightlessness during spaceflight missions, crewmembers perform moderate- to high-load resistance exercise in conjunction with aerobic (cycle and treadmill) exercise. Recent evidence from ground-based studies suggests that low-load blood flow-restricted (BFR) resistance exercise training can increase skeletal muscle size, strength, and endurance when performed in a variety of ambulatory populations. This training methodology couples a remarkably low exercise training load (approximately 20%-50% one repetition maximum (1RM)) with an inflated external cuff (width, ranging between approximately 30-90 mm; pressure, ranging between approximately 100-250 mmHg) that is placed around the exercising limb. BFR aerobic (walking and cycling) exercise training methods have also recently emerged in an attempt to enhance cardiovascular endurance and functional task performance while incorporating minimal exercise intensity. Although both forms of BFR exercise training have direct implications for individuals with sarcopenia and dynapenia, the application of BFR exercise training during exposure to microgravity to prevent deconditioning remains controversial. The aim of this review is to present an overview of BFR exercise training and discuss the potential usefulness of this method as an adjunct exercise countermeasure during prolonged spaceflight. The work will specifically emphasize ambulatory BFR exercise training adaptations, mechanisms, and safety and will provide directions for future research.

Rehabilitation of an osteochondral fracture using blood flow restricted exercise: a case review

OBJECTIVES

In this case review we report on a bodybuilder who used a practical model of blood flow restriction (BFR) training to successfully rehabilitate himself following an injury to his right knee.

RESULTS

The patient originally thought he had torn his meniscus however repeat radiographs and magnetic resonance imaging (MRI) confirmed an osteochondral fracture. The patient initially sought out a low load alternative to help with the maintenance of skeletal muscle mass. However, following rehabilitation with low load BFR resistance training, radiographs indicated that the bone had begun to heal suggesting that this type of training may also benefit bone.

CONCLUSIONS

In conclusion, this case review provides evidence that practical BFR using knee wraps can serve as an effective stimulus during rehabilitation from a knee injury.

Blood flow restriction: rationale for improving bone

Low intensity exercise with blood flow restriction has been shown to increase muscle hypertrophy and strength similar to high intensity resistance exercise. Interestingly, low intensity resistance exercise to failure has shown the same muscle protein synthesis response as higher intensity exercise, questioning the need for blood flow restriction during low intensity exercise. The purpose of this manuscript is to discuss the mechanisms and potential benefits of blood flow restricted exercise on bone adaptation and provide rationale as to why low load resistance exercise to failure would be unlikely to produce these benefits. The studies completed thus far support the hypothesis that training with blood flow restriction may provide not only a novel modality to induce adaptation in muscle but also bone, which was previously thought to only occur with higher intensity/impact exercise. We hypothesize that the main mechanism behind the proposed favorable bone responses observed thus far is through increased intramedullary pressure and interstitial fluid flow within the bone caused by venous occlusion. Therefore, although similar muscular benefits may be observed from low intensity exercise performed to failure (e.g. strength, hypertrophy, and endurance), the response of bone might be different, highlighting the potential importance of the blood flow restriction stimulus.

Effects of cuff width on arterial occlusion: implications for blood flow restricted exercise

The purpose of this study was to determine the difference in cuff pressure which occludes arterial blood flow for two different types of cuffs which are commonly used in blood flow restriction (BFR) research. Another purpose of the study was to determine what factors (i.e., leg size, blood pressure, and limb composition) should be accounted for when prescribing the restriction cuff pressure for this technique. One hundred and sixteen (53 males, 63 females) subjects visited the laboratory for one session of testing. Mid-thigh muscle (mCSA) and fat (fCSA) cross-sectional area of the right thigh were assessed using peripheral quantitative computed tomography. Following the mid-thigh scan, measurements of leg circumference, ankle brachial index, and brachial blood pressure were obtained. Finally, in a randomized order, arterial occlusion pressure was determined using both narrow and wide restriction cuffs applied to the most proximal portion of each leg. Significant differences were observed between cuff type and arterial occlusion (narrow: 235 (42) mmHg vs. wide: 144 (17) mmHg; p = 0.001, Cohen's D = 2.52). Thigh circumference or mCSA/fCSA with ankle blood pressure, and diastolic blood pressure, explained the most variance in the cuff pressure required to occlude arterial flow. Wide BFR cuffs restrict arterial blood flow at a lower pressure than narrow BFR cuffs, suggesting that future studies account for the width of the cuff used. In addition, we have outlined models which indicate that restrictive cuff pressures should be largely based on thigh circumference and not on pressures previously used in the literature.