Lifting rear leg after running

my 12 yr border collie ice is starting to lift her rear leg after running around after the ball , not all the time but the other day & tonight she did it , i stop running her then she stops limping , so i was wandering if this could be the start of maybe a cruciate ligament problem or her knee is weakening in her old age ?? she also stops limping & lifting it when i rub her knee!!

Could be or maybe a luxating patella ( kneecap). Unusual at her age, but stranger things happen

It could be a touch of arthritis, or possibly lactic acid [ among other things of course, but those two come straight to mind for what you describe including the ease with which you can allevite it for her ].
I`d pop her to the vet to get her checked out as there are so many possibilities - some which might suggest rubbing / massage are not the thing to do even though it does help ]. Without a diagnosis you could be missing out on a simple course of meds if she needs them or even just an adjustment to her exercise regime.
Once you have a diagnosis, if it is just a mild joint or muscle thing the vet can help sort it out and we could perhps point you in the direction of suitable leg wraps which you can check out with the vet as well so that she can still enjoy her play but with a bit of support to help avoid pain - but it depends so much on finding the cause and her having any vet treatment if needed first :smt001

This advice from Patch is the best you could be given, we could suggest things to do that may disguise the problem. When you have a reason from the vet for the problem I'm sure many of us here with older dogs could give you suggestions but they depend on the reason for this to be happening in the first place.

thanx for that guys, i think i will get vet to check her leg then , see whats going on with it.