I first did this around four months ago when all of the high rep swing threads were being posted. I pushed myself beyond my limits with swings. I should have stopped when I first began feeling pain, but stubbornly determined to finish my goal for the workout. At the time, my tailbone was really sore, and my lower back in general. I went to a chiropractic clinic that is very involved in sports. The chiropractor didn't think anything serious happened. Did some soft tissue work and adjustments. The chiropractor gave me clearance to workout. I haven't struggled continuously with it since then, but notice that my swing endurance is limited more by muscle fatigue than by my lungs. Even though I am doing much better, it might take a while to fully recover. I was doing some stuff around the house. I moved a boat while on its trailer by lifting the trailer tongue with my hands and pushing it around rather than put it on the hitch on the back of my car. It didn't hurt while I was actually doing it, but it began to hurt shortly afterward. It is already feeling much better today.