Crewmembers continue to experience muscle atrophy after space flight, particularly from long-duration missions on the International Space Station (ISS), which results in higher risk for injury upon their return to Earth. More robust exercise equipment is arriving on the ISS and new exercise prescriptions are planned to counteract the muscle losses. With these new exercise countermeasures; however, there currently are no methods to monitor muscle status in flight to assist in determining the efficacy of these countermeasures. Given that medical imaging with ultrasound is available on the ISS, it is feasible that ultrasound scanning could be performed before, during, and after space flight to assess muscle size changes. This ground-based pilot study evaluated and validated a technique of using ultrasound to measure leg muscle cross sectional area (CSA) and volume. These ultrasound measures were also compared with a magnetic resonance image (MRI) of the leg to determine the accuracy and repeatability of the ultrasound muscle measurements. This pilot study is part of the larger Integrated Resistive and Aerobic Training Study (SPRINT) conducted on the ISS. Ultrasound will ultimately be used to assess the effectiveness of the SPRINT exercise prescription on ISS crewmembers using the Advanced Resistive Exercise Device (ARED) and the second generation treadmill (T2) during their missions. This ultrasound technique could also be applicable for evaluating future exercise countermeasures both on ISS and Constellation missions.
Subjects passed an Air Force Class III-equivalent physical examination and were cleared for exercise by the Human Test Subject Facility (HTSF) and by the HTSF Medical Monitor prior to entry into the study. Subjects completed a health history questionnaire each day prior to testing, which was reviewed by the test operators to ensure that their health status had not changed to the extent that testing was contraindicated. Potential issues were discussed with the Medical Monitor before testing commenced. Any subject with metal implants of any sort was excluded, since a MRI scan was required for participation.
Ten subjects were recruited for participation in this pilot study. Subjects were asked to participate in three separate data collection sessions, each with a duration of approximately 90 minutes or less. Two of the three sessions involved ultrasound scanning of the subject’s right leg muscles, calf (gastrocnemius, soleus) and thigh (vastus lateralis, rectus femoris). Panoramic ultrasound images were acquired by two sonographers using a customized template placed on the thigh and calf and analyzed by the same two sonographers (MyLab Gold, Biosound). MRI images of the leg were acquired while the subjects lay supine in a 1.5T scanner (Signa Horizon LX, General Electric) and analyzed by three trained investigators. The average of the two ultrasounds and three MRI values were used for validity analysis.
A high inter-experimenter reliability was found using the ultrasound template. A significant correlation was found between ultrasound and MRI CSA measures. There were significant differences between absolute values of MRI and ultrasound for all muscles. The present results indicate that utilizing a customized ultrasound template along with panoramic imaging provides reliable measures of leg muscle CSA, and thus could be used to provide in-flight measures of changes in muscle CSA.