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.