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If he is actually putting some weight on theleg, then it may just be a bruised leg or soft tissue injury like a sprain. For this I would recommend strict cage rest for a few days and then reevaluate the situation. If there is some improvement, you might continue crate rest for a few more days and then progress to leashed walks. A partial tear of an ACL can be reinjured and tear completely with too much exercise, so keeping the dog calm is recommended until your vet can evaluate the injury.
If he is not putting weight on the leg at all, then the injury is likely to be more serious. Dogs who will not put weight on a leg could have a dislocated hip, or knee injury such as an ACL tear. You can read about dislocated hips here:
And read about ACL injuries here:
If he is not putting weight on it, it probably is NOT a pulled muscle. I would have him seen by your vet as soon as possible.. If possible, contain him in a crate until you can get him seen. If it is a dislocated joint it is best addressed as quickly as possible. In addtion, buffered aspirin can be given to a dog with a dosage of up to 5 mg per pound every 12 hours. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. The aspirin may need to clear your dog’s system before other medications can be given, so keep that in mind if you decide to give aspirin and be sure and tell your vet when your dog is seen. Read side effects and precautions here.
If aspirin is used, your vet may not be able to prescribe other drugs until it clears from your dog's system which can take a week or more. I'd probably hold off on the aspirin if you are going to get your dog in pretty quickly.
In addition, you might want to get a doggie seat belt for your little guy to help avoid injuries while riding in the car. They are pretty effective.
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