Age-, sex-, and region-specific differences in skeletal muscle size and quality

Abstract

Ultrasonography-derived cross-sectional area (CSA) and echo intensity (EI) are increasingly utilized by investigators to study muscle size and quality, respectively. We sought to examine age, sex, and region (proximal, middle, distal) differences in vastus lateralis and rectus femoris CSA and EI, and determine whether correction for subcutaneous fat thickness influences the magnitude of EI differences. Fifteen younger men (mean age=23 years), fifteen younger women (21 years), eleven older men (74 years), and fifteen older women (70 years) participated. Clear differences were observed among age, sex, and region for vastus lateralis CSA (p≤0.013, d=0.38-0.73), whereas rectus femoris CSA was only different between younger and older participants at the proximal region (p=0.017, d=0.65). Uncorrected EI was greatest at the distal region of both muscles (p<0.001, d=0.59-1.38), with only the younger men having significantly lower EI values than the other groups (p≤0.043, d=0.37-0.63). Subcutaneous fat correction resulted in a marked increase in the magnitude of sex-specific EI differences (p≤0.032, d≥0.42). Additionally, subcutaneous fat correction increased the uniformity of EI throughout the thigh. These findings highlight considerable region-specific differences in muscle size and quality among younger and older men and women, and highlight the need to correct for subcutaneous fat thickness when examining EI.
 Novelty Bullets
 • Rectus femoris CSA is similar between younger and older adults except at the most proximal site evaluated.
 • Age- and sex-specific differences in uncorrected EI are non-uniform across the thigh.
 • Correction for subcutaneous fat thickness substantially increased EI in women, resulting in greater sex differences.

Examination of the confounding effect of subcutaneous fat on muscle echo intensity utilizing exogenous fat

“…Echo intensity (EI) of skeletal muscle sonograms has been used as an estimate of muscle quality in individuals with various phenotypes (Blue et al 2018;Herda et al 2020;Mota and Stock 2017;Stock et al 2020), with greater EI scores indicating a greater accumulation of intramuscular connective tissue and/or fat (Pillen et al 2009;Young et al 2015). EI has been proved to be reliable (Caresio et al 2015;Lanferdini et al 2019), thereby encouraging its application in investigating training and aging adaptations (Fukumoto et al 2012a;Mota and Stock 2017).…”

“…Associations between subcutaneous fat thickness and EI estimates of muscle quality in humans are usually discussed from a physiological perspective (Herda et al 2020;Oranchuk et al 2020;Ryan et al 2016;Vakula et al 2019), addressing the damaging effect of obesity on muscle quality that has been confirmed in animal models (Biltz et al 2020;Collins et al 2018). However, the confounding effect that a fat layer may have on EI estimates of muscle quality has not been carefully investigated and is currently not properly taken into account in the literature (Arts et al 2010;Nijboer-Oosterveld et al 2011;Schless et al 2019;Stock et al 2020).…”

“…The confounding effect of subcutaneous fat thickness on EI estimates of muscle quality leads to an underestimation of the negative impact of obesity/overweight on muscle health (Oranchuk et al 2020;Ryan et al 2016) and may lead to erroneous conclusions regarding chronic exercise induced changes in muscle quality (Wong et al 2020) and differences in muscle quality between men and women or between muscles and muscle regions (Oranchuk et al 2020;Stock et al 2020). Until now, most studies that correct ultrasound-based estimates of muscle quality for the confounding effect of subcutaneous fat have counted on the equations developed by Young et al (2015).…”

“…Until now, most studies that correct ultrasound-based estimates of muscle quality for the confounding effect of subcutaneous fat have counted on the equations developed by Young et al (2015). Young et al (2015) proposed a correction factor for subcutaneous fat thickness on EI estimates of muscle quality that has been widely used (Blue et al 2018;Ryan et al 2016;Stock et al 2020). However, their experiment consisted on changing fat thickness by applying different levels of external pressure on the skin, which is expected to lead to changes in tissue parameters, such density and stiffness, that influence the speed of US wave propagation (Kot et al 2012;Winn et al 2016), producing changes in EI that may not represent the effect of subcutaneous fat thickness (Ishida et al 2016).…”

“…Given the popularity of EI in clinical settings, it is crucial to maximize its accuracy(Heymsfield et al 2015).Every 1 cm increase in AT may result in an underestimation of EI of more than 39 arbitrary units on a scale from 0 to 255. Considering that subcutaneous fat thickness varies substantially across individuals and across muscles and muscle regions(Oranchuk et al 2020;Stock et al 2020), estimates of muscle quality that are not corrected for this effect are biased. More importantly, muscle quality is often considered in people with obesity, undergoing weight gain/loss and nutritional interventions(Blue et al 2018;…”