Ultrasound Muscle Thickness Is a Poor Index of Criterion Magnetic Resonance Imaging Measures of Resistance Training-Induced Muscle Growth

Introduction: The purpose of this study was to compare group level resistance training (RT)-induced changes in ultrasound measured muscle thickness (ΔMT US ) with criterion magnetic resonance imaging (MRI) measures of muscle growth (i.e. changes in maximum anatomical cross-sectional area [ΔACSA MRI ] and volume [ΔVOL MRI ]), and examine the relationship between ΔMT US and both ΔACSA MRI and ΔVOL MRI , for the quadriceps femoris (QF) and its constituent muscles. Materials and Methods: Healthy young males ( n =39) completed 15 weeks of lower-body RT (knee extension, knee flexion, and leg press; x3 sessions/wk). Pre and post RT participants undertook dominant thigh musculoskeletal scans. Ultrasound and MRI scans were manually segmented to quantify pre to post ΔMT US , ΔACSA MRI , and ΔVOL MRI of constituent muscles and the overall QF. Results: ΔMT US significantly underestimated increases in ΔACSA MRI (0.001≤ p ≤0.002) and ΔVOL MRI (0.001≤ p ≤0.012) for the overall QF and two out of four constituent muscles (vastus lateralis and vastus medialis), with a mean offset/underestimate of more than half the change in criterion MRI measures, and an average individual error that was typically more than the change in criterion MRI measures. Although vastus intermedius and rectus femoris ΔMT US did not differ compared to ΔACSA MRI , and ΔVOL MRI (0.057 ≤ p ≤ 0.113). There were no significant bivariate correlations between ΔMT US and either ΔACSA MRI or ΔVOL MRI for the overall QF (-0.152 ≤ r ≤ -0.015, 0.393≤ adjusted p ≤1.431) or any constituent muscles of the QF (-0.197 ≤ r ≤ 0.422, 0.140≤ adjusted p ≤1.002) in absolute or relative terms. Conclusions: Overall, ΔMT US was not an accurate or valid index of RT-induced muscle growth and it is recommended to exercise substantial caution in the use and interpretation of MT US in the context of RT adaptation.