PURPOSE:Muscle weakness is common in patients requiring extracorporeal membrane oxygenation (ECMO), but early identification is challenging. This study aimed to 1) quantify the change in quadriceps size and quality (echogenicity) from baseline to day 10 using ultrasound in patients requiring ECMO, 2) determine the relationship between ultrasound measures, muscle strength and highest mobility level. MATERIALS AND METHODS:Prospective cohort study involving ultrasound measurement of quadriceps at baseline, days 10 and 20. Muscle strength and highest mobility level were assessed at days 10 and 20 using the Medical Research Council sum-score (MRC), hand-held dynamometry (HHD) and the ICU mobility scale (IMS). RESULTS:25 patients (age 49 ± 14 years, 44% male) received ECMO. There was a significant reduction (-19%, p < .001) in rectus femoris cross-sectional area by day 10. Echogenicity did not change over time. There was a negative correlation between echogenicity and MRC at day 10 (r = -0.66) and HHD at day 20 (r = -0.81). At day 20, there was a moderate correlation between total muscle thickness and IMS (rho = 0.59) and MRC (rho = 0.56). CONCLUSIONS:In patients requiring ECMO there was marked wasting of the quadriceps over the first 10 days. Ultrasound measures were related to muscle strength and highest mobility level.