Saying that he mustve wrenched it pretty good, Secret Agent Mark Keller informed colleagues yesterday that his back has been a bit hinky ever since that one time he crashed through a stained-glass skylight and landed in a fountain during a black-tie event last month. If I have to rappel down an all-glass skyscraper, its really not that bad, maybe a few knots, but whenever I barrel roll out of a burning car before it careens off a cliff, the backll really flare up these days, Keller told reporters, adding that hanging from a moving helicopter is actually good for his back because it keeps everything good and stretched out. The thing is, it usually doesnt hurt while Im sliding down a steep rooftop while trying to dodge gunfire, its the next morning when I really feel it. I probably just have to take it easy the next time I jump out of a plane at 20,000 feet and parachute onto a surfacing submarine. Keller added that he would maintain his daily routine of running and jumping from the tops of train cars, as he insists that the movement prevents his back from stiffening up. 