PHILADELPHIA — The Pittsburgh Penguins were in the late stages of practice on Friday at the Flyers Training Center when an errant puck hit Sidney Crosby in the back of his head.

This could have been great cause for alarm, but instead, Crosby and everyone else started laughing. Justin Brazeau skated over to the Penguins captain and rubbed the back of his noggin for good measure.

Nothing could have better illustrated where the Penguins are in their first-round NHL playoff series with the Philadelphia Flyers — or the team’s mindset.

Down 3-1 after dropping the first three games, little has gone right for the Penguins against their most bitter rival, and advancing will require an improbable and historic comeback. Yet they’re still standing, still smiling and still carrying an unwavering, quiet confidence as they head back to Pittsburgh for Monday’s Game 5.

It was reminiscent of a scene from earlier this season.

“Yeah,” head coach Dan Muse said. “And I think we’ve also seen other moments like that throughout the year.”

The Penguins were bruised and battled when they reached the holiday break. They held a full team workout on Dec. 27, one day before resuming the unofficial second half of the season. Suddenly, the bad energy from losing nine of 10 games entering that stretch had evaporated. Following that workout, Crosby explained that everything was fine, that the Penguins were “a very good team.”

They went 17-3-4 in their next 24 games.

Many players have pinpointed that moment as a turning point. Muse noted that his team has the ability to dust itself off and remain positive, even amid profound adversity, such as attempting to come back from down 3-0.

The Penguins, according to their coach, have been doing this all season, both behind the scenes and in full view.

“Maybe we’re coming off a day we don’t like,” Muse said. “The group has come in and had the right energy, set the tone. It’s 100 percent credit to them. Yes (Friday’s practice was like that practice in December), but it’s not the only time we’ve seen it. And honestly, it wasn’t a big surprise.”

The locker room following that practice at the Flyers’ practice rink confirmed what onlookers had noticed: The mood was unusually lively for a team facing such long odds.

Why?

“It’s 100 percent on the leadership on the team,” Muse said.

Indeed, it was Crosby who was laughing, even as pucks clanged off his helmet. Evgeni Malkin was good-naturedly jabbing the media for asking about his future in Pittsburgh. Kris Letang, on his 39th birthday, had a smile on his face for most of the session.

There was a palpable positive energy throughout the workout, and the big names seemed to be leading the way.

“I think everybody on the coaching staff, we all felt it right away,” Muse said. “Even going into the practice, the feel in the room and definitely the practice, it kind of set the tone there. We needed to have a good day, especially with the two days in between. They made sure that it happened.”

Never in Penguins history has the team rallied from being down 3-0. They’ve come back twice from 2-0 deficits in the Crosby era — both in 2009, against the Washington Capitals and Detroit Red Wings — and roared back from 3-1 deficits in 1992 and 1995, both times against Washington.

Of course, it could also be argued that never in Penguins history has a group with such low expectations accomplished so much.

One way or the other, the Penguins still believe in themselves. They also realize that if they can win Monday night, the pressure will shift back onto the Flyers.

“You’ve got to win four games,” Noel Acciari said. “We’ve been a resilient team all year. We’ve had our ups and downs. These past few games were down. But we couldn’t get too down on ourselves. Just take it one game at a time, one period at a time and kind of play our game. I don’t think we’ve played our game the first three. We showed more in Game 4 that we got back to that. So, build off that for Game 5.”

The Penguins looked down for the count after their December swoon, in which they lost game after game in embarrassing fashion. They were out of playoff position at that point, which was where preseason expectations suggested they’d remain. There were three games in which the Penguins blew three-goal leads, and another in which they allowed a short-handed goal that tied the game with 0.1 seconds remaining in the third period before losing in overtime.

Such a stretch likely would have killed most teams. Being down 3-0 in a series kills most teams, too. Actually, that’s an understatement. Teams down 3-0 come back to win around 2 percent of the time, to be precise. It’s happened just four times in NHL history.

The Penguins were hardly at their best in Game 4, and they will need to elevate their game to have a chance. There was a growing sense Sunday morning, before the team’s plane took off for Pittsburgh, that they felt they were figuring it out.

“I think yesterday was a good step in terms of getting to our game,” Muse said. “We aren’t fully there yet. We want to always be going back to looking at things we can do better. We look at things we like from a game and carry it with us. There was more in terms of us playing our game yesterday, but at the same time, I think we can be better. We need to continue to elevate, have more of those positives, and make sure that we’re tidying up in some areas as well.”

Their in-game performance needs to be better. Maybe it will be; maybe it won’t. But the team attitude appears to be in a healthy place.

NOTES — The Penguins did not practice Sunday and will hold a morning skate Monday before Game 5. … Muse seemed pleased with Ilya Solovyov, who made his Stanley Cup playoff debut in Game 4. Though he played less than eight minutes, Solovyov was a solid contributor and seemed to please the coaching staff. “It’s his first game back in a little while,” Muse said. “He got those games at the back end of the regular season. There was good in terms of things we’re looking for, playing a heavy game, making a good first pass.” … Anthony Mantha has never scored a goal in 18 career Stanley Cup playoff games. Muse commented that the Penguins are going to look for different ways to get him “into those scoring areas.”