Ultrasound predicts skeletal muscle fat infiltration in healthy middle‐aged and young adults: Validation against MRI

This study developed an ultrasound prediction equation for measuring fat fraction in lower limb muscles of healthy middle‐aged and young participants and applied it in a randomised controlled trial (RCT) to assess the relationship between fat fraction and strength in exercising and non‐exercising females. Twenty‐eight participants were recruited into <40 years (10m, 5f, age 28.9 ± 5.3 years, body mass index (BMI) 23.6 ± 2.3 kg m−2) and >50 years (8m, 5f, age 60.4 ± 5.7 years, BMI 24.8 ± 3.6 kg m−2) groups. T1‐weighted magnetic resonance imaging (MRI)‐Dixon fat fraction in gastrocnemius medialis (GM) and vastus lateralis (VL) and 1H‐magnetic resonance spectroscopy for intramyocellular (IMCL) and extramyocellular (EMCL) lipid content in GM was used to validate ultrasound measures of muscle, subcutaneous fat thickness (SFT) and echointensity to generate a multivariable model of fat infiltration. For the RCT, 72 females aged 40–60 years were recruited to either a low impact resistance exercise (4–5 sessions/week for 12 weeks) or a control group, and muscle strength was tested using isokinetic dynamometry. Multiple linear regression models for ultrasound measurement of fat fraction were validated for GM (r2 = 0.71) and VL (r2 = 0.91). SFT was the dominant variable in the older group in GM (77%) and VL (92%), but of near equal proportion, with echointensity, in GM (52%) and VL (51%) in the younger group. Strength was negatively correlated with fat fraction (r = −0.44). Strength increased in the exercising group, but fat fraction remained unchanged. A multivariable model using ultrasound measurements of SFT, muscle thickness, echointensity and age demonstrates that ultrasound is capable of predicting lower limb muscle fat infiltration.