Contrast-enhanced Ultrasound (Night before and at t = 2, t = 7) All subjects will undergo imaging of the vastus lateralis muscle using an HDI-5000 and a P4-2 transducer. Imaging will be performed in a transaxial plane over the lateral portion of the vastus lateralis muscle group using power Doppler imaging after the continuous infusion of Definity® microbubbles. A mechanical index of 1.3 will be used. Depth, focus, and gain will be optimized and held constant throughout each study. Intermittent imaging will be performed using an internal timer. Data will be recorded on an SVHS videotape. A suspension of octafluoropropane gas-filled albumin microbubbles (Definity®, Lantheus Medical) will be infused intravenously at a rate of 3.0 mL/min for 8 min. Once systemic microbubble concentrations reach steady-state (~1.5 min), background images will be obtained at a frame rate of 1 second. Intermittent imaging will then be performed at a pulsing interval (PI) ranging from 1-25 seconds, thus allowing progressively greater replenishment of the ultrasound beam elevation between destructive pulses. At least three images will be acquired at each PI. Videotape images will be digitized to an offline image analysis system. Several background frames will be averaged and digitally subtracted from similarly averaged frames at each PI. Background-subtracted video intensity (VI) at each PI will be measured from a region of interest (ROI) placed around the vastus lateralis muscle. PI vs. VI data will be fitted to the function: y = A (1 - e-βt), where y is video intensity at PI t, A is plateau video intensity (an index of microvascular blood volume) and β is the rate constant, which provides a measure of microvascular flow velocity. The investigators have used this technique extensively in past studies.