How is the rest of her working? Is she eating and drinking? Is she bright, alert, and responsive to you? Older Labs get a myositis(muscle inflammation) in their rear legs that causes wasting of the muscles. Is that present? That responds sometimes to high doses of Vitamin E. Ask your vet if your dog has myositis.
If it is just the joints (arthritis or tendonitis) that prevent her from getting up, there is an injectible medicine called Adequan that may help. It makes the joints have more fluid and it rehydrates damaged cartilage. If your vet thinks that is the cause, ask about that medication.
Continue the exercising to keep the blood going in the legs.
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Dr. Louis Gotthelf
Animal Hospital of Montgomery
Pet Skin and Ear Clinic