The head of the World Anti-Doping Agency criticized the NFL Players Association for opposing testing of its players for human growth hormone (HGH).

David Howman said it appears the union’s objection to testing is not based on science, according to the Los Angeles Times. “You’re better [off] to expose yourself to a test than decline it, because the impression you leave is that you have something to hide,” he said.

The new NFL labor contract allows HGH testing, but only if the players union approves it, the article states. The union says it wants more scientific information before it will agree to testing.

The union has said it has concerns about whether the established ratio for a positive HGH test is appropriate for professional football players. The union said it wants information from doping officials about what the thresholds are for a positive test, and will not allow testing to start until it reviews that data.