ISS042E236075 (02/10/2015) --- Astronauts in space must exercise regularly to keep muscles from deteriorating. The busy schedule aboard the International Space Station has these regular periods worked in as NASA astronaut Terry Virts shows in this Tweet he sent out on Feb. 10, 2015 with the comment: "Periodic Fitness Evaluation- riding the bike with a heart rate monitor, EKG, and blood pressure machine hooked up".