Ultrasonography (US) has emerged as a method of assessing muscle size and quality. These measurements may be confounded by fluid shifts resulting from a change in position. The purpose of this study was to investigate the time course of fluid shifts and its influence on muscle size and quality measurements. Thirty-five males were recruited for the study. Ultrasound images and segmental bioelectrical spectroscopy assessments of the right thigh occurred after transition to supine and sequentially every five minutes, to 30 minutes. Subcutaneous fat and corrected echo intensity (EI) for the young and older men showed a significant age × time interaction (P