Echo intensity as an indicator of skeletal muscle quality: applications, methodology, and future directions

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INVITED REVIEW

Echo intensity as an indicator of skeletal muscle quality: applications, methodology, and future directions Matt S. Stock1,2   · Brennan J. Thompson3,4 Received: 13 August 2020 / Accepted: 7 November 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract Purpose  This narrative review provides an overview of the current knowledge of B-mode ultrasound-derived echo intensity (EI) as an indicator of skeletal muscle quality. Method  PubMed and Google Scholar were used to search the literature. Advanced search functions were used to find original studies with the terms ‘echo intensity’ and/or ‘muscle quality’ in the title and/or abstract. Publications that conceptually described muscle quality but did not include measurement of EI were not a focus of the review. Result  Importantly, the foundational premise of EI remains unclear. While it is likely that EI reflects intramuscular adiposity, data suggesting that these measurements are influenced by fibrous tissue is limited to diseased muscle and animal models. EI appears to show particular promise in studying muscular aging. Studies have consistently reported an association between EI and muscle function, though not all chronic interventions have demonstrated improvements. Based on the existing literature, it is unclear if EI can be used as a marker of muscle glycogen following exercise and nutritional interventions, or if EI is influenced by hydration status. Inconsistent methodological approaches used across laboratories have made comparing EI studies challenging. Image depth, rest duration, participant positioning, probe tilt, and the decision to correct for subcutaneous adipose tissue thickness are all critical considerations when interpreting the literature and planning studies. Conclusion  While some areas show conflicting evidence, EI shows promise as a novel tool for studying muscle quality. Collaborative efforts focused on methodology are necessary to enhance the consistency and quality of the EI literature. Keywords  Muscle quality · Body composition · Resistance training · Aging · Ultrasound · MRI · CT · Obesity · Exercise Abbreviations EI Echo intensity MRI Magnetic resonance imaging

Communicated by Michael Lindinger. * Matt S. Stock [email protected] 1



School of Kinesiology and Physical Therapy, University of Central Florida, 12805 Pegasus Drive, HPA 1, Room 258, Orlando, FL 32816‑2205, USA

2



Neuromuscular Plasticity Laboratory, Institute of Exercise Physiology and Rehabilitation Science, University of Central Florida, Orlando, FL, USA

3

Department of Kinesiology and Health Science, Utah State University, Logan, UT, USA

4

Sorenson Legacy Foundation Center for Clinical Excellence, Movement Research Clinic, Utah State University, Logan, UT, USA



Background: what is echo intensity? Echo intensity (EI) is being increasingly implemented by investigators in the fields of exercise physiology and rehabilitation science as a measure of skeletal muscle composition, or quality. The foundational premise of EI is t