Without question, seat design is a major part of the problem, especially on long flights. It would be impossible for everyone on board to get up and walk around. There just isn't enough aisle room, especially on crowded flights. And even if there were aisle room, just imagine the injuries from an unexpected encounter with air turbulence.
Coach seats should have an inside width of at least 24 inches and plenty of leg room. At least a pax would be able to shift positions until he or she could get up to walk and stretch.