If you’ve ever been envious of that guy at the gym running sprints with a weighted vest, don’t crown him king of hardcore quite yet.

Athletes who sprint-train with a weighted vest don’t necessarily improve their performance any more than guys who do the same training without the extra weight, a new study in the Journal of Strength and Conditioning Research found.

In the experiment, researchers at Spain’s University of Vigo asked soccer players to do sprint training twice a week either with or without a weighted vest. After six weeks, everyone shed the accessory and went through a series of performance tests.

But first: Why train with a weight vest in the first place?

In theory, adding resistance during sprint training should increase neural activation, strengthen your hip extensors, and recruit more muscle fibers, all without substantially changing your running form, which can deliver better sprint performance, explains study author Ezequiel Rey, Ph.D., professor of education and sports science at the University of Vigo in Spain.

Rey found that weight vests did confer some advantages. Specifically, athletes who trained with a weighted vest for six weeks saw more improvement in their 30-meter sprint times, but those who trained without the vests more greatly improved their 10-meter sprints.

But when it came to countermovement jumps (think: the jump in a burpee) and running repeated sprints, those who had trained with a weighted vest didn't see any more gains than those who had trained without one. And removing the nuances of distance, all athletes ran faster after six weeks of training.

Previous research on weighted sprint training is conflicting. Some studies show resisted towing can help improve reactive strength, jump, and sprint performance, while others support Rey's research that resistance in training doesn't do much for speed. Rey points out that the positioning of the load could come into play here, since most of the studies show no additional benefit to weight on your chest, while weighted tows involve the weight on the ground, at a lower center of gravity.

The other factor: How heavy is that vest? A 2016 study published in Sports Medicine found that 10–20% of an athlete’s body mass—the weight Rey used in his study—wasn’t any better for improving sprint performance than unresisted training.

"It's possible that higher loads may be needed for resisted sprint training, but you have to be careful because too much resistance can alter running kinematics," he tells Men’s Fitness.

In short: The research is pretty inconclusive on how well resisted training actually improves an athlete’s speed. So unless your goal is specifically to improve velocity over acceleration (which would pretty much only apply to elite athletes), skip the vest. It doesn’t help you jump higher or tackle sprint intervals better than training without the weight, but it definitely makes sprint training a heckuva lot more painful.