EDEN PRAIRIE, Minn. -- Minnesota Vikings coach Mike Zimmer said he still wasn't sure if tackle Carter Bykowski would miss the rest of the season after surgery to repair a pectoral tear. No matter how long Bykowski is out, though, his injury is the latest in a concerning trend for the Vikings.

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Bykowski, who was hurt in last Saturday's preseason game against the Oakland Raiders, is the fifth Vikings player to sustain a pectoral injury since last September, and the third offensive lineman to be hurt in a game. Brandon Fusco missed 13 games with a pectoral tear last season, and Phil Loadholt was coming back from one before he tore his Achilles tendon Saturday night.

Defensive lineman Brian Robison and cornerback Josh Robinson also had pectoral injuries during the team's offseason program.

After Robinson got hurt in June, the Vikings started researching why they'd had so many issues with the injury in such a short time. Strength coach Evan Marcus, who came to Minnesota when Zimmer was hired in 2014, had changed the Vikings' strength training philosophy to emphasize free weights and power lifts over weight machines. And while Zimmer said the Vikings' research led them to emphasize better warm-ups and rotator cuff strength, it didn't sound last month as though the coach planned to change the overall plan.

"I changed the weight program because I want our football team to have a certain type of mentality," Zimmer said on July 26. "I want them to have a certain type of aggressiveness. I believe that's a lot about mindset as well. When you're down there and you hear those weights banging and guys working and sweating their rear ends off together, I think it's important that we do it that way. Maybe I'm just old school, but I believe that."

A quick survey of ESPN's NFL Nation writers Thursday showed teams are roughly split between free weights and machines, but several players have said free weights contributed to significant pectoral injuries. Giants tackle Will Beatty tore his pectoral muscle on a bench press in May, and the Redskins' Junior Galette said he won't bench again after he tore his pectoral muscle while benching 365 pounds in June when he was with the New Orleans Saints.

Running back Jerick McKinnon needed back surgery after getting injured while lifting weights last November, but it's worth noting that four of the Vikings' five pectoral injuries -- Fusco, Loadholt, Robinson and now Bykowski -- happened outside the weight room. On Thursday, Zimmer sounded like he was still looking for more information about what can increase the risk of a torn pectoral muscle.

"It's a combination of things, really," he said. "It's dehydrated muscles, it's getting in the position, sometimes it's over-strengthening, sometimes it's fatigue. Still, we're calling around to every expert that we can find and keep digging."