Have a feeling Nugget maybe right...I reckon it's the cylinder. We have the same kind of carriers in PICU for doing ward transfers. You can't sling em over your shoulder (well us manly beasts can't anyway because the strap is so small) so you have to carry them. They swing about causing no end of damage. Either that or it's going upstairs and using you leg to "help" the bag up and the rolls of tape you keep in the pocket of your combats keeps pushing into your leg. cheers