What's that tape Rory McIlroy is wearing at the 2018 PGA Championship?

All eyes were on Rory McIlroy in Thursday's opening round of the PGA Championship. Grouped with Tiger Woods and reigning champion Justin Thomas, the trio made up the morning's highest-profile group.

Turns out, more eyes may have been on McIlroy's arm than on his game.

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Rory had what appears to be a large band-aid on this arm. He's been known to wear KT tape before, as he did at last year's The Players Championship on his aching back.

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But this was different. We investigated what that big green S on that adhesive patch exactly was.

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A closer look at the bandage shows that it is from Staminapro, who label their product as a recovery patch. Upon further investigation, we find out that Staminapro is now pretty widely used by professional athletes. In this screenshot from Golf Channel, you can see that Tony Finau used the Staminapro patch on his dislocated ankle at this year's Masters.

Per the Staminapro website, their recovery patch is used because, "athletes push their bodies to the limit, which produces inflammation that causes muscle fatigue and soreness."

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Following the first round of play, one in which McIlroy shot a 70 to finish even, he confirmed that it was indeed a Staminapro patch. "These are the anti-inflammatory patches, so yeah," McIlroy said.

"I started feeling it the weekend of Firestone. Funny enough, it hurts the most with chipping because I sort of hold the angle a little bit. It's just that in there. The whole forearm has been tight. I hit a lot of balls last week at Firestone working on a few things, and it's just a little inflamed and a little tight, but it's fine."

Is Rory feeling a little fatigue in the steamy heat of St. Louis, or is he simply getting ahead of the game, preparing himself for the four-day grind with Team Europe Ryder Cup implications? Keep a close eye on McIlroy, and his arm, as the weekend progresses.