Zac Efron skied down a mountain ''on one leg'' because he didn't realise how badly he was hurt when he tore his cruciate ligament.

The 31-year-old star underwent surgery earlier this year after suffering an anterior cruciate ligament (ACL) tear, but he has revealed he didn't recognise the extent of the damage after he was hurt in a skiing accident.

He told Ellen DeGeneres: ''I stood up on [my leg] and I tried to put my other ski on and [the injured leg] just gave out.

''They offered me [a sled], but it was gonna take too long and I didn't know I'd torn my ACL. I just thought it hurt, so I skied down the mountain on one leg.''

In February, Zac revealed he had suffered the injury whilst ''shredding the gnar'', a term used when someone excels in extreme sports such as snowboarding, skateboarding, surfing, or skiing.

He wrote: ''I tore my ACL shredding the gnar but all is good. 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. I'll keep you updated as I heal and progress! Thanks for all the love and positivity! (sic)''

The actor - who can be seen on screens as Ted Bundy in 'Extremely Wicked, Shockingly Evil, and Vile' - is known for his love of the outdoors, and once came face to face with a grizzly bear whilst on a hiking trip in Montana with his brother Dylan.

He said: ''Out of nowhere, [we were like] 'Okay, that's a bear. Everybody stop. And we all stood still. Somebody silently whispered 'But it's charging at us.' It took like two big canters at us.

''[Luckily it] disappeared behind some trees. It was the scariest part of the trip! But it was cool to see a bear that close. It was maybe 15 or 20 feet away. It was charging at us!''