The purpose of this research sub-study is to determine the changes in heart function during the bedrest period using ultrasound and MRI. This topic is important for medical care of astronauts in space. It can enable physicians on the ground to monitor exercise protocols that are used to prevent deconditioning-loss of strength during long space flights.

Your participation in this study would include ultrasound examinations with bicycle exercise before, during, and after the 12-week bedrest period. MRI exams would be performed before and after the bedrest period.

Each echo exam may last up to 1 hour. This time will be required to perform an ultrasound examination before and during supine (lying down) bicycle exercise. The exercise period will be approximately 10-15 minutes. The bicycle exercise will be a symptom-limited test. This means that the test will be stopped if you experience any discomfort. An ultrasound examination of your heart will be done to assess heart function. You will be asked to lie on your left side on an examination table while a technician takes pictures of your heart with a small probe that is gently pressed against your chest after applying a gel. The ultrasound data will be processed to evaluate myocardial strain, a value that may be useful in describing heart function.

Each MRI may last up to 1 hour. An MRI obtains body pictures created by using magnetic energy rather than x-ray energy. To have the scan, you will lie on a table that slides into the scanner, which is like a large tube. An MRI examination of your heart will be done to assess heart function. You will be asked to lie still and follow simple breathing instructions during the procedure. The MRI data will be processed to evaluate the volume of blood being pumped by your heart, a value that may be useful in describing heart function.