The Effect of Increasing Blood Flow Restriction Pressure When the Contractions Are Already Occlusive.

CONTEXT: Blood flow restricted exercise involves the use of external pressure to enhance fatigue and augment exercise adaptations. The mechanisms by which blood flow restricted exercise limits muscular endurance are not well understood. OBJECTIVE: To determine how increasing blood flow restriction pressure impacts local muscular endurance, discomfort, and force steadiness when the contractions are already occlusive. DESIGN: Within-participant, repeated-measures crossover design. SETTING: University laboratory. PATIENTS: A total of 22 individuals (13 males and 9 females). INTERVENTION: Individuals performed a contraction at 30% of maximal isometric elbow flexion force for as long as possible. One arm completed the contraction with 100% of arterial occlusion pressure applied, while the other arm had 150% of arterial occlusion pressure applied. At the end of the protocol, individuals were asked to rate their perceived discomfort. MAIN OUTCOME MEASURES: Time to task failure, discomfort, and force steadiness. RESULTS: Individuals had a longer time to task failure when performing the 100% arterial occlusion condition compared with the 150% arterial occlusion pressure condition (time to task failure = 82.4 vs 70.8 s; Bayes factors = 5.77). There were no differences in discomfort between the 100% and 150% conditions (median discomfort = 5.5 vs 6; Bayes factors = 0.375) nor were there differences in force steadiness (SD of force output 3.16 vs 3.31 N; Bayes factors = 0.282). CONCLUSION: The results of the present study suggest that, even when contractions are already occlusive, increasing the restriction pressure reduces local muscle endurance but does not impact discomfort or force steadiness. This provides an indication that mechanisms other than the direct alteration of blood flow are contributing to the increased fatigue with added restrictive pressure. Future studies are needed to examine neural mechanisms that may explain this finding.

The impact of acute and chronic resistance exercise on muscle stiffness: a systematic review and meta-analysis.

PURPOSE: Ultrasound is commonly used to measure changes in skeletal muscle morphology in response to both acute and chronic resistance exercise, but little is known on how muscle stiffness changes via ultrasound elastography, which was the purpose of this systematic review and meta-analysis. METHODS: The online data bases of Pubmed, Scopus, and Web of Science were each searched up until February 2020 and the data were analyzed using a random effects model. RESULTS: A total of eight studies (four acute and four chronic) met the inclusion criteria for the quantitative analysis. Following a single bout of exercise, muscle stiffness was increased within the first hour [ES: 1.52 (95% CI 0.14, 2.91); p = 0.031], but was no longer elevated when measured 2 days post-exercise [ES: 0.76 (95% CI - 0.32, 1.83); p = 0.16] or ≥ 7 days post-exercise [ES: 0.20 (95% CI - 0.53, 0.94); p = 0.58]. There was no impact of long-term resistance training on changes in muscle stiffness [ES: - 0.04 (95% CI - 0.24, 0.15); p = 0.653]. CONCLUSION: The primary findings from this meta-analysis indicate that muscle stiffness increases acutely following a single bout of resistance exercise, but does not change long-term with chronic resistance training when measured via ultrasound shear elastography. Given the small number of studies included in this review, future studies may wish to examine changes in muscle stiffness in response to both acute and chronic resistance exercise.