I don't have a spine tester, but I can tell you that for the most part they are remarkably stiff for their weight. There is a a lot of variance from place to place though. When I was using a lot of this stuff, I would only harvest it in two places. East St. Louis Il and salt lake city. This was because the phragmites in both places was near identical in properties. I have cut some in other places like Sioux city IA a few places in Michigan and Wisconsin and the material was to spindly. Since this stuff is essentially a tapered shaft, you will find that it is not that finicky about the weight of the bows shooting it. I would guess this arrow would spine 55 - 60 lbs. However, with a heavy foreshaft and point to bring the weight of the arrow up, the spine would be around 40-45 lbs. But again, the taper of the shaft makes the arrow quite forgiving in regards to spine. Josh