From WENN.com

The Blue Bloods star was walking around the grounds of his Ventura, California home when he spotted a bobcat on his property, hopped up on nearby rock to get a closer look and lost his balance.

Walking with a limp as he appeared on U.S. chat show The Talk on Thursday (03Jan13), he explained, ""I was at the ranch, and as an any good rancher, I was doing my thing, manly stuff, and I saw a bobcat. I had my binoculars, as any good rancher would, and I'm looking at it and I stepped on this stationary rock, which wasn't stationary, and I'm rolling down the hill.

""It's one of those where you have a lot of time to think and go, 'That was really stupid.' And you wonder what's gonna happen when you get to the bottom. And I sprained my ankle.""