Zac Efron's in the Hospital After Tearing His ACL While 'Shredding the Gnar'

You know what they say: Sometimes you shred the gnar, and sometimes the gnar shreds you.

A photo posted Friday on Zac Efron's Instagram showed the actor giving a thumbs up from a hospital bed-a portion of his left leg bandaged and in a brace. In the caption, he explained that he opted for surgery after tearing his ACL "shredding the gnar"-aka, surfing.

"I tore my ACL shredding the gnar but all is good," Efron wrote. "I opted for surgery so I can come back stronger than ever. Thank you to Dr. Kvitne and his team at Kerlan-Jobe Surgery Center. Ill keep you updated as I heal and progress! Thanks for all the love and positivity!"

So wait, what exactly did Efron tear?

The ACL, or anterior cruciate ligament, is located inside the knee, and it helps connect the femur bone to the tibia. (To learn the causes and treatments for ACL tears-as well as what you can do to prevent it from happening in the first place-check out our guide to keeping your joints healthy .)

ACL tears are common in young, athletic guys like Efron. When he's not surfing, the 31-year-old actor is doing kettlebell workouts and bench presses , rock climbing , and cycling like his Greatest Showman costar Hugh Jackman . Sometimes he also pulls himself up waterfalls, because, uh, why not?

View this post on Instagram OSA A post shared by Zac Efron (@zacefron) on Nov 11, 2018 at 11:41am PST

Efron seems to be optimistic about his recovery. We guess he'll have to be careful filming Baywatch 2, which is apparently in the works-somehow .