Super Bowl 50: A troubled year for American football

The Carolina Panthers will take on the Denver Broncos for Super Bowl 50 - a historic milestone that an estimated 115 million fans are expected to tune in for. It promises to be quite a celebration, but it comes at the end of a troubled 12 months for the National Football League.

From the rumbling fallout from the so-called Deflategate debacle to the ongoing questions about the long-term health effect of head trauma, it has been a tough 2015 for the sport's administrators.

BBC News asked four sports ethicists and historians to weigh in on the moral standing of the NFL on the eve of its 50th championship.

1. Concussions and player health

This year, the issue of head trauma and the possible link to serious cognitive issues in former players hit the big screen in the film, Concussion.

The film told the story of the doctor who originally linked a progressive degenerative brain disease called chronic traumatic encephalopathy with former football players who took repeated blows to the head, and the NFL's efforts to downplay the risks of concussions.

In 2015, the doctors at Boston University who specialise in examining the brains of deceased former players diagnosed 23 cases of CTE. They have found the disease in 90 out of 94 players' brains, including, most recent, legendary Oakland Raiders quarterback Ken Stabler, who had "moderately severe" CTE.

Meanwhile, concussion diagnoses went up by a third in the 2015 season, as the league tried various measures to make the sport more safe.

But what long time sports ethicist Jay Coakley takes greatest issue with is to do with kids playing tackle football. The NFL invested in a training programme for coaches called Heads Up Football which certifies them in proper tackling technique and educates them on spotting concussions. Coakley finds ads for the programme that tug at parents' heartstrings troubling.

"What we've seen in 2015 is a concerted effort on the part of the NFL to push that positive image in ways that I think push moral boundaries a little bit, in terms of communicating to people, 'We've made changes and now it's safe for your boys to go out there and play tackle football'. I think that's really a stretch," he says.

Former Raiders quarterback Ken Stabler, shown here in 1983, was diagnosed postmortem with CTE

A spokesman for the NFL referred BBC News to comments made by NFL commissioner Roger Goodell to the BBC last week.

"We are trying to make sure people have the facts so when they make the decision of which sports their children play they have the facts about the risk and what the rewards are. That's what all parents do, that's what I do," he said.

"We want kids to participate in sports, there are risks to not participating and sitting on your couch. It's called obesity and it's one of the biggest epidemics we have in our world. We want them to play and be active."

2. Domestic violence

This issue came back into the spotlight this week with police in Dallas investigating Cleveland Browns quarterback Johnny Manziel after a complaint by his ex-girlfriend.

The horrific elevator knock-out incident between Ray Rice and his wife dominated the debate over how the NFL handles domestic violence in 2014.

Goodell suspended Rice for two games before public outrage crescendoed after the release of the tape from inside the elevator. Rice was subsequently suspended indefinitely.

Since then the NFL has revamped its policies surrounding domestic violence, created a baseline suspension of six games and hired a former sex crimes prosecutor to perform internal investigations beyond that of local law enforcement. Player arrests dropped 38% in 2015.

The NFL was criticised in 2014 for having Janay Rice appear with her husband to address an incident where he knocked her unconscious

But there were other controversies this year, before the Manziel episode.

For example, Deadspin obtained disturbing photographs taken by police of a former girlfriend of Dallas Cowboys defensive end Greg Hardy. The photos shows extensive bruising all over the woman's body after she told police Hardy attacked her and threatened to kill her. Hardy was suspended for 10 games, but the punishment was later reduced to four games after arbitration.

"He still played this year which I thought was outrageous," says professor Shawn Klein, author of the blog The Sports Ethicist. "I think they probably need to do more education and awareness with the players and team personnel about how to spot signs of domestic violence or potential precursors so there can be an intervention before something happens."

3. Doping

In a bombshell documentary, Al-Jazeera linked Broncos quarterback Peyton Manning and other players to the use of performance enhancing drugs, including human growth hormone. Manning categorically denied any wrongdoing, saying the report is a "fabrication", and one of the main sources in the film later recanted.

However, Al-Jazeera stood by the story. Sports historian and Arizona State lecturer Victoria Jackson says that in the interest of transparency, the NFL should sign the World Anti-Doping Agency's code, which has been adopted by 600 other world sports organisations, and standardises which substances and processes are illegal for athletes. The NFL does test for HGH, but Jackson says an outside group needs to be responsible for oversight.

Denver Broncos quarterback Peyton Manning denies wrongdoing

"The NFL needs to defer to an independent agency," she says. "The league has too much invested in Manning. Doping has real health effects and the NFL Players Association should be pushing for the NFL to sign on to WADA."

However, the NFL Players Association has gone on the record challenging WADA for its own transparency issues over how they test players for human growth hormone.

"We rejected WADA participation into our drug policies precisely because they failed to be transparent with us over these very issues," a representative told Reuters.

4. Deflategate

Then came Deflategate, when New England Patriots staff were accused of intentionally under-inflating footballs so that they would travel further during the 2014 Super Bowl. Quarterback Tom Brady was suspended for four games - before the punishment was nullified by a judge - and the team was fined $1m after an NFL investigation found that Brady likely knew about tampering with the footballs.

However, Stephen Mosher, a professor of sport management and media, was more scandalised by the amount of time, money and attention poured into Deflategate.

"It was great public messaging for the NFL - preposterously stupid," he says. "It was talked about as if it was the greatest moral sin ever committed."

Patriots quarterback Tom Brady leaves a court after contesting his Deflategate punishment

After the NFL released the results of its own investigation, other scientists took it upon themselves to re-create the tests used to prove that the Patriots had cheated - including a little girl who used it as the topic of a science fair project. The Atlantic called Deflategate a "sports scandal that's full of hot air" for many reasons, but Jackson says more than just football fans' time was wasted.

"I think when a story takes over like that, my guess is if you're someone who's fearful of other stories you sit back and say, 'Oh, perfect! Everyone is spinning their wheels on a story that doesn't matter,'" she says. "But there are these bigger stories that deserve attention."

The NFL is appealing against the judge's ruling on Brady's punishment and Goodell has stood by the results of their own investigation.