A photo of Andrew Luck, shirtless and showing off six-pack abs, recently went viral. It reveals he’s not into beer and fast food. What matters most is the strength of his throwing shoulder, and video evidence is emerging to show Luck is back.

It will take much more than a brief appearance in one preseason game to be sure, but Luck seems ready to let it rip and regain his status as one of the NFL’s elite quarterbacks. If that’s the case, the Colts could rebound and be relevant again.

Luck’s year away from action was shrouded in mystery over his career-threatening right-shoulder surgery and slow rehab. The Colts slipped to 4-12 and were hopeless without him.

“I think the Colts could really make a comeback with Luck,” South Point sports book director Chris Andrews said.

Each NFL season is full of surprise teams, the overachievers and underachievers that finish nowhere near preseason expectations. It makes the league a good test for the Bounce Theory. A bounce is generally considered a regression to form, for better or worse.

Indianapolis dumped Chuck Pagano as coach and hired Frank Reich, who was the Eagles’ offensive coordinator. The offensive line was fortified in the offseason, among other minor changes. It all starts with the quarterback and coach. In 2014, when Luck passed for a career-high 40 touchdowns, the Colts finished 11-5 and won the AFC South. A steady decline followed before the Colts crashed and burned last year.

“The Colts should bounce up,” Westgate sports book manager Ed Salmons said. “Reich is a huge upgrade from Pagano. The last few years, Luck would just get knocked on his ass the whole game. I want to see more of Luck, but I like the Colts.”

The Westgate opened Indianapolis’ season win total at 6½ in late April. With Luck showing positive signs, the number has been bumped to 7½.

A look at other bounce candidates, for better or worse:

Bills

This one is obvious. The Bills’ expected regression from 9-7 is reflected in the betting market, where their win total has been adjusted to 6 (Under -140). Buffalo finished with a minus-57 point differential and was a fortunate playoff participant last year. Tyrod Taylor, the quarterback who saved the season, scrambled off to Cleveland. At some point in September or October, the Bills will probably start rookie Josh Allen, who has intriguing arm strength, but is not ready to win many games in the NFL.

Browns

From 0-16, there’s nowhere to go but up. An inside look at the Browns, via HBO’s “Hard Knocks,” shows the team’s coordinators — Todd Haley and Gregg Williams — are far more competent than head coach Hue Jackson. Cleveland still has plenty of problems, but its win total is six, and Salmons has a bold prediction:

“Cleveland is going to bounce the other way,” Salmons said. “The mistakes that killed this team last year are not going to happen with Tyrod Taylor. He knows how to play the game and he can run. I like Haley. Anything to get Hue Jackson away from the offense is a good thing. The offensive line looks pretty good, and I really like their defense. I think the Browns will end up winning eight or nine games.”

Rams

Sean McVay, 32, is the youngest NFL head coach. He led the Rams to an 11-5 mark and wiped away the embarrassments of the Jeff Fisher era, getting instant improvement out of quarterback Jared Goff. The Rams are now a star-studded team tagged with a win total of 10. But Salmons said the home playoff loss to the Falcons might be a hint of trouble.

“The Rams are definitely a bounce team. It fits the pattern,” Salmons said. “The first year with McVay, they kind of caught the league by surprise. The big thing last year was they ran a system that no one really knew what was coming.”

Giants

The Giants are not the 11-win team of 2016 or the three-win mess of a year ago. The truth likely lies somewhere in the middle, hence a win total of seven (Over -140).

“If Eli Manning can play, there’s no excuse for the Giants not to bounce up,” Salmons said. “The question is, can he play?”

49ers

Is Jimmy Garoppolo the real deal? The 49ers bounced within the 2017 season, starting 0-9 and finishing 5-0 after trading for Tom Brady’s backup. Expectations are higher, the schedule is tougher (including road trips to Minnesota and Green Bay in the first six weeks), and Jimmy G is not sneaking up on anyone this time. The betting public is loving the 49ers and their win total of 8½, and that’s one reason to be leery.

“I don’t buy San Francisco,” Salmons said. “This year is not their year. I think the Niners are due to bounce down.”