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The sign by the couch. It read:

This Mercury couch was custom-made for Donald "Deke" Slayton to help him cope with the high-acceleration "g-forces" of riding missiles into space. The couch prevents fainting by elevating the feet, keeping blood from leaving the head.

This is a training couch, not meant for flight. Slayton's flight couch stayed earthbound because he was grounded by a minor heart defect. Slayton became Chief Astronaut and finally reached space on the Apollo-Soyuz mission in 1975.

G forces were a problem not only during launch, but especially during reentry: On the flight of Freedom 7, for example, Shepard experienced 6 Gs during launch and nearly 12 Gs on reentry.