Barry, I put a safety pin on the outside of each leg.
Whether or not the fabric constricts blood flow depends on what size you wear and how big your legs are. I up-sized one size, so if you are wearing your normal size I guess it might be a problem. I kinda like the idea that Stephenson's use for quick convertible pants (converta pants….scroll down page)
http://www.warmlite.com/vb_shirt.htm