cruciate ligament resisted?

[MarcoPoloCX]

When my dog goes for a strenuous run.
E.g. Prolonged sprint after a fox, say, with lots of jumping and twisting, he sometimes end up with a limp on his hind right leg. He doesn't put weight on it.

I go home and stretch the leg straight and bend all the way up and feel around the knee but it doesn't seem to cause any pain. And next day he's totally fine again. No limping and back to running.

Same thing can happen the next time he goes for a strenuous run.

Limp one day fine the next. No pain with leg being folded or stretched.

Is this likely to be weakened cruciate ligament? Or muscle related?

You don't mention his age, Marco etc, but if he's geting on a bit it may just be age related. My old mutt's still able to chase after the scent of a hare on the common, but if he does too much of it, he'll be limping a bit the following day. As a puppy, he apparently had 2 'synthetic' back knee joints (courtesy of the insurance - happily) and it amazes me that they're still working well after 8 years or so given the amount of the twisting and jumping you mention yours does. Unless the joint is swollen and/or hot, I wouldn't worry too much.