Patients admitted to intensive care unit (ICU) are known to develop significant impairments in physical function. Patients with critical illness suffer up to 30% reductions in muscle size within the first ten days of admission to the ICU. Muscle strength testing, Medical Research Council-sum score, is current gold-standard to diagnosis ICU-acquired weakness and predicts risk of mortality and long-term physical function. Muscle power different from muscle strength in that it accounts for velocity of movement, is potentially a better independent predictor of function that has not been studied in this population. In addition, we hypothesize that muscle size and quality measured through ultrasound imaging has better applicability and prediction that strength testing. Therefore, we prospectively collected data surrounding these muscle parameters in patients admitted to the medicine ICU at University of Kentucky. Primary outcomes included physical function, muscle power with a novel assessment tool for the critically ill population, muscle strength, and muscle size and quality assess through ultrasound imaging. 36 patients admitted to ICU and 18 aged-matched controlled were enrolled. Patients had significantly lower scores on muscle power assessment at ICU discharge (33.6 ±19.0 W; t= 4.01, p < 0.001) and at hospital discharge (40.9 ±16.5 W; t= 4.81, p < 0.001) in comparison to controls (59.3± 14.7 W). Patients with better scores on muscle power assessment had significantly better scores on physical function measures (Six-minute walk test; rs = 0.548, p = 0.0001). Muscle size (cross-sectional area of rectus femoris muscle) and muscle power were strongly correlated (rs = 0.66, p < 0.0001). These data suggest that patients with critical illness have significantly reduced muscle power which directly related to deficits in physical function.