Measuring muscle quality: associations between echo intensity and normalized strength and power

Objective. Muscle quality has received considerable attention within the literature due to its influence on health and performance. However, it has been assessed by various measures (e.g. ultrasound imaging, normalized strength), contributing to a lack of a consensus definition. The purpose of this study was to investigate the association between common muscle quality measures: vastus lateralis echo intensity (EI) and peak torque (PT) and power (PP) normalized to thigh lean mass (TLM) and thigh fat mass (TFM).Approach. Participants' (n = 39; age = 32.7 ± 8.2 years; %BF = 24.1% ±8.1%) whole body and leg composition was assessed via dual-energy x-ray absorptiometry and custom thigh analyses. Subcutaneous fat-corrected muscle EI was assessed via ultrasonography. Maximal PT and PP of the dominant leg extensors were examined on a calibrated dynamometer and were normalized to both TLM and TFM, respectively. Pearson product-moment correlations were used to examine the associations between EI and PT/TLM, PT/TFM, PP/TLM, and PP/TFM.Main results. Greater muscle EI was significantly related with lower PT/TFM and PP/TFM (r = -0.74 to -0.70,P < 0.001), but unrelated to PT/TLM or PP/TLM (P ≥ 0.205).Significance. These findings suggest that ultrasound imaging (muscle EI) and normalized strength and power may not be used interchangeably to define muscle quality, and muscle EI may be more sensitive to the adiposity of the limb than TLM. Future research should consider using separate and consistent definitions when referring to imaging-derived or normalized strength and power values of muscle quality.