WASHINGTON -- A congressman accused the NFL Players Association of "trying to back out" of an agreement to start testing for human growth hormone in pro football.

Speaking at a House Oversight and Government Reform Committee hearing about the science behind the testing, Rep. Elijah Cummings of Maryland, the panel's ranking Democrat, noted Wednesday that nearly two full NFL seasons have passed since the league and the players' union signed a labor deal in August 2011 that set the stage for adding HGH to the sport's drug program.

The NFLPA 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 agree on a scientist to help resolve that impasse. HGH is a banned substance that is hard to detect and used by athletes for what are believed to be a variety of benefits, whether real or only perceived - such as increasing speed or improving vision.

"They say they need more time ... before doing what they agreed to do. To me, it seems obvious the Players Association is simply running out the clock," Cummings said in his opening statement. "Although they agreed to HGH testing, they are now trying to back out of the contract."

Cummings and committee chairman Darrell Issa, a California Republican, both said additional hearings are expected.

"It is our hope (to) move these parties closer together," Issa said.

Issa also said there could be a connection between head injuries in football and the use of HGH, "based in part on the strength of the players hitting each other."

The committee did not ask anyone from the league or union to testify Wednesday. Witnesses included Pro Football Hall of Fame member Dick Butkus, U.S. Anti-Doping Agency Chief Science Officer Larry Bowers, and National Institutes of Health Deputy Director Lawrence Tabak.

Bowers told the committee "there is a broad consensus among scientific experts who regularly work in the growth hormone field" that the test is reliable and valid, and that "the chances of an athlete who has not used synthetic growth hormone testing positive are comparable to the chance of that same athlete being struck by lightning during his or her lifetime."

He closed by saying: "I would like to point out that the only people who are still questioning the methodology and validity of the ... test are lawyers, not scientists."

Tabak said many studies vouch for the reliability of HGH testing, even though the naturally occurring hormone and the artificial form are tough to tell apart.

He also pointed out the "serious risks" to athletes who give themselves HGH.

Even once scientific issues are resolved, there will be other matters the league and union need to figure out, including who administers the test and what the appeals process will be. The latter could be of particular import in the aftermath of the decision in the New Orleans Saints' bounty case Tuesday, when NFL Commissioner Roger Goodell's suspensions of four players were tossed aside by former Commissioner Paul Tagliabue.

The collective bargaining agreement that ended the NFL lockout 16 months ago included a provision for HGH testing - but only once the NFLPA approved the process.

"First, I applaud the NFL and players for taking a bold and decisive position on HGH in their 10-year agreement. Now let's get on with it," Butkus told the committee. "The HGH testing process is proven to be reliable. It's time to send a clear message that performance-enhancing drugs have no place in sports, especially the NFL."