Muscle fibre activation and fatigue with low‐load blood flow restricted resistance exercise—An integrative physiology review

Abstract

Blood flow-restricted resistance exercise (BFRRE) has been shown to induce increases in muscle size and strength, and continues to generate interest from both clinical and basic research points of view. The low loads employed, typically 20%-50% of the one repetition maximum, make BFRRE an attractive training modality for individuals who may not tolerate high musculoskeletal forces (eg, selected clinical patient groups such as frail old adults and patients recovering from sports injury) and/or for highly trained athletes who have reached a plateau in muscle mass and strength. It has been proposed that achieving a high degree of muscle fibre recruitment is important for inducing muscle hypertrophy with BFRRE, and the available evidence suggest that fatiguing low-load exercise during ischemic conditions can recruit both slow (type I) and fast (type II) muscle fibres. Nevertheless, closer scrutiny reveals that type II fibre activation in BFRRE has to date largely been inferred using indirect methods such as electromyography and magnetic resonance spectroscopy, while only rarely addressed using more direct methods such as measurements of glycogen stores and phosphocreatine levels in muscle fibres. Hence, considerable uncertainity exists about the specific pattern of muscle fibre activation during BFRRE. Therefore, the purpose of this narrative review was (1) to summarize the evidence on muscle fibre recruitment during BFRRE as revealed by various methods employed for determining muscle fibre usage during exercise, and (2) to discuss reported findings in light of the specific advantages and limitations associated with these methods. K E Y W O R D Sfatigue, ischemia, motor units, muscle hypertrophy, occlusion 4 of 48 | WERNBOM aNd aaGaaRd 3.5.), which in turn can raise the recruitment threshold of the highest threshold units (discussed in Section 3.4.). As a result, the approximate upper limit for motor unit recruitment during more normal and physiologically relevant contraction durations (eg, 1-3 seconds) remains to be fully defined.

Concentric strength training at optimal or short muscle length improves strength equally but does not reduce fatigability of hamstring muscles

“…In light of this, it is interesting that similar non length‐specific strength gains were observed for both training lengths. This indicates that activation of the muscle with intended maximal effort during training is an important element in the stimulus for strength gains, regardless of the work/torque produced in the contractions, a finding which in some aspects is analogous to findings with blood flow restricted training, where significant strength gains can be achieved despite the load lifted is considerably lower than in conventional strength training (Wernbom and Aagaard, ).…”

Effect of blood‐flow restricted vs heavy‐load strength training on muscle strength: Systematic review and meta‐analysis

“…34,52,[66][67][68][69] Also, it should be recognized that myocellular muscle damage and even rhabdomyolysis may occur in response to unaccustomed or excessive BFR exercise. 68,[70][71][72] Thus, although the present data indicate that BFR training and heavy-load resistance training may be equally efficient in evoking gains in maximal muscle strength, and more long-term studies are required to validate BFR training as an equally safe, feasible, and efficient training regime.…”

“…However, these studies have largely been conducted in healthy subjects at young or old age . Also, it should be recognized that myocellular muscle damage and even rhabdomyolysis may occur in response to unaccustomed or excessive BFR exercise …”

A Systematic Review and Meta-Analysis on Neural Adaptations Following Blood Flow Restriction Training: What We Know and What We Don't Know

“…In this context it needs to be highlighted, however, that all the studies presented here and in the present review used bipolar surface EMG to measure muscle excitation. This can be problematic as changes in EMG cannot unambiguously be attributed to changes in motor unit (MU) recruitment and firing because additional influences such as muscle fiber potential, motor unit synchronization and fatigue can influence the EMG amplitude (Wernbom and Aagaard, 2020 ). With the course of technological advancements, Fatela et al ( 2019 ) non-invasively estimated MU recruitment and firing rates by means of high-density EMG and decomposition techniques.…”