By Jonathan Stempel

(Reuters) - A U.S. appeals court on Thursday said the National Football League was within its rights to suspend Minnesota Vikings running back Adrian Peterson in a child assault case, handing the league a new legal victory in disciplining its star players.

Reversing a lower court ruling, the 8th U.S. Circuit Court of Appeals in St. Paul, Minnesota, said league-appointed arbitrator Harold Henderson acted within his authority in upholding sanctions imposed by Commissioner Roger Goodell.

Peterson missed six games after being suspended indefinitely and fined six weeks salary, following his no-contest plea to a misdemeanor charge for disciplining his 4-year-old son with a switch in May 2014.

In February 2015, U.S. District Judge David Doty in Minneapolis agreed with the NFL Players Association (NFLPA) that Henderson went too far in upholding the sanctions, which were based on the league's new personal conduct policy.

Doty said Henderson, appointed under the NFL collective bargaining agreement after the union challenged the sanctions, could not retroactively uphold a more severe punishment than permitted at the time of Peterson's misconduct.

The NFL reinstated Peterson for the 2015 season, and the only remaining issue was whether it could collect a fine from the seven-time All-Pro and 2012 most valuable player.

Writing for a three-judge appeals court panel, Circuit Judge Steven Colloton said federal courts had only limited authority to disturb arbitration rulings such as Peterson's.

He refused to second-guess Henderson's finding that Goodell had power under the collective bargaining agreement and new personal conduct policy to impose Peterson's punishment, if the commissioner thought lesser sanctions in prior cases had been inadequate.

"The arbitrator at least arguably acted within the scope of the issues submitted to him, so his decision must be upheld," Colloton wrote.

In a statement, the players union said: "While the NFLPA disagrees with the decision, we accept this loss."

The NFL, its lawyers and the Vikings did not immediately respond to requests for comment.

Thursday's decision came three weeks after New England Patriots quarterback Tom Brady abandoned his appeal of his own four-game suspension.

Goodell suspended the two-time MVP after finding that Brady had been "generally aware" of a plan to deflate footballs in a 2015 championship game, which the Patriots won.

Doty had ruled three months after a former federal judge overturned Goodell's indefinite suspension of former Baltimore Ravens running back Ray Rice for assaulting his then-fiancee.

Goodell had originally suspended Rice for just two games, prompting much criticism.

The case is National Football League Players Association v National Football League et al, 8th U.S. Circuit Court of Appeals, No. 15-1438.

(Reporting by Jonathan Stempel in New York; Additional reporting by Mica Rosenberg; Editing by Jeffrey Benkoe and Jonathan Oatis)