The neutral body posture (NBP) is the posture the human body naturally assumes in microgravity. Adopting any other posture while floating requires muscular effort. In the 1980s, NASA developed the (MSIS), a set of guidelines based on anthropometry and biomechanics, which included a definition of an average typical NBP created from measurements of crew members in the microgravity environment onboard Skylab. Still photographs taken on Skylab of crew members showed that in microgravity, the body assumed a distinguishable posture with the arms raised, the shoulder abducted, the knees flexed with noticeable hip flexion, and the foot plantar flexed.