 It causes the spine to stretch, back problems. It causes cardiovascular decline. It causes bone density loss. There are a lot of issues actually in the muscles. There's a lot of issues like bed rest. It's not a good thing to sit in a bed and never move. And basically weightlessness duplicates that situation. And they've done a lot of studies on six-month to year-long missions to try to figure out if exercise could solve some of the problems.