Hind leg pops up quickly at the walk..?

I have a 8 year old haflinger mare who gets slim excercise and today I noticed as she walked in the pasture one of her hind legs like snaps or pops up dramatically. Maybe her hock is locking? I'm not sure, so if anyone knows what's happening please share! :)

Thanks!
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It sounds very similar to stringhalt.

Does it look something like this?

It does! Not that bad though thank goodness! Thank you I'll google that and get more info on it :)
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