You may have heard tales of legendary NFL running back Herschel Walker lugging a tractor tire behind him during sprints to prep for the rigors of the gridiron. And while Walker knows a thing or two about fitness — at 49, he’s now a competitive mixed martial artist — a study published in the December 2010 issue of the Journal of Strength and Conditioning Research suggests dragging weight won’t necessarily give you Herschel-like speed. Researchers had 20 male college lacrosse players perform 13 sprint-training sessions over seven weeks. One group towed sleds equal to 10 percent of their body mass, one group wore weighted vests bearing 18.5 percent of their mass, and one group trained with no resistance at all. In the end, the first two groups did not gain a speed edge over the weight-free group. In fact, the weight-free guys improved their sprint performance slightly more than the others. That said, weighted modalities can make you stronger. And if you play football, working out with a sled just might increase your odds of dragging would-be tacklers into the end zone … and making Walker proud.