By HOWARD FENDRICH, AP Pro Football Writer

WASHINGTON (AP) — Nearly two full seasons since signing a labor deal that paved the way for HGH testing in the NFL, the league and the players' union still haven't agreed to implement anything.

The NFL Players Association won't concede the validity of a test that's used by Olympic sports and Major League Baseball, and the sides haven't been able to jointly pick a scientist to help resolve that impasse.

So now Congress steps in. The House Oversight and Government Reform Committee is holding a hearing Wednesday to take a look at the science behind tests for human growth hormone, a substance that is hard to detect and believed to be used by athletes for a variety of benefits, whether real or only perceived — such as increasing speed or improving vision.

"The players are claiming that the testing is questionable. What's bothering me about all of this is that the players made an agreement in 2011 ... that they would begin the human growth hormone testing, and it seems to me that they have thrown roadblocks and found excuses not to do it. And that concerns me. An agreement is an agreement," said Rep. Elijah Cummings of Maryland, the ranking committee Democrat, who noted that he expects there will be additional hearings.

"We also want to make sure that the players are treated fairly," Cummings said in a telephone interview. "We want to hear the science, so we can make some valid judgments as to the players' allegations that this may not be valid."