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.