The physiological, morphological and functional responses to resistance exercise with blood flow restriction: an examination of protocol type and exercise load

Abstract

Resistance training is performed by a wide range of populations, to achieve a diverse range of outcomes in skeletal muscle. Predominantly, such outcomes include the development and/or maintenance of skeletal muscle mass and strength but may also include muscular power or endurance. Blood flow restricted (BFR) resistance exercise is recognized as a novel strategy to achieve such outcomes in skeletal muscle, with much lower-loads (20-50% of one-repetition maximum) than those traditionally employed. The capacity to reduce external loading has made BFR exercise an attractive training modality for individuals who may not tolerate high levels of mechanical stress, such as frail older persons or clinical populations. Moreover, temporarily limiting training loads and/or volume has also generated interest in BFR exercise in athletic cohorts. Despite the promise held by this modality of exercise, there is large heterogeneity in the methods (e.g., BFR methodology and exercise prescription) adopted by practitioners, which often leads to variable training outcomes. Consequently, there is a need to understand each of the many variables that comprise the BFR exercise stimulus, such as those relating to the cuff application (e.g., pressure, restriction duration etc.) and the exercise prescribed (intensity/loading, volume etc). A review of the literature (Chapter Two) identified that one particular variable receiving little attention is whether blood flow must be restricted during the inter-set recovery periods (continuous BFR), or if this is only necessary during the working periods (intermittent BFR). A greater understanding of such variables will permit the configuration of optimal and evidence-based BFR protocols, meaning that this mode of exercise may be tailored to suit the goals and health considerations of a wide spectrum of individuals.As such, this thesis encompasses a series of independent, yet related studies that examine a wide range of acute and chronic responses to different low-load BFR resistance exercise protocols, with the ultimate aim of contributing towards the current evidence-based and best-practice guidelines for BFR exercise. Study One compared chronic training adaptations between continuous and intermittent BFR protocols. The lack of differences between protocols in several morphological and functional outcomes suggested that the additional metabolic stress imposed on skeletal muscle with a continuous BFR protocol may not be necessary. However, because the study involved untrained participants, the findings may not translate to well-trained and athletic cohorts.Study Two compared differences between continuous and intermittent BFR for a wide range of acute outcomes, including neuromuscular, cardiovascular, metabolic and biochemical responses. The inclusion of trained participants, and a traditional high-load resistance exercise group permitted more ecologically valid comparisons between BFR protocols and the current gold-standard method. It was observed that the maintenance of the BFR stimulus during rest periods significantly altered the intramuscular oxygen kinetics, and as a consequence, was capable of inducing the same magnitude of peripheral fatigue in skeletal muscle as much higher-load exercise. In addition, these differences occurred at no additional cardiovascular demand, thus indicating that this exercise modality is safe compared with traditional approaches. Together, these data suggested that a continuous BFR application may be favourable for presenting a maximal adaptive stimulus for trained individuals. However, an intermittent BFR application induces less fatigue, and may be preferable if subsequent performance takes precedence over adaptation.The final study of this thesis, Study Three, included a comprehensive comparison of low-load resistance exercise continuous BFR, and traditional high-load resistance exercise. Novel aspects of this study involved the use of trained participants, the high-quality assessments of muscle morphology (magnetic resonance imaging and immunohistochemical analysis) and the inclusion of both acute and chronic components. The key findings were that, in trained individuals, the exclusive use of low-loads with BFR did not negatively influence muscular strength or power and produced similar skeletal muscle hypertrophy to traditional high-load training. Acutely, these adaptations were achieved by activation of key intracellular signalling pathways, including the mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinases (ERKs), which were similar to high-load resistance exercise. Together, these findings validate low-load continuous BFR as an effective surrogate to high-load resistance training in trained individuals.This thesis examined the acute and chronic responses to different resistance protocols differing in loading conditions and BFR protocols. The studies presented herein address a prominent gap in the literature on BFR methodology, educating scientists and practitioners on the effects of maintaining vs releasing the BFR stimulus during inter-set rest periods. Moreover, the validation of low-load BFR exercise in trained individuals for stimulating acute anabolic processes within muscle that translate to favourable chronic adaptations has important implications for both clinical and athletic cohorts.