There are a few possibilities in this case. There is a condition in Labs known as Muscular Dystrophy. This is usually a progressive condition and they show progressive weakness as well as atrophy on the limbs. It should be mentioned to your vet and the only reliable way of diagnosis is through a muscle biopsy.
That being said, I would recommend an exam with your vet to determine as well if there are any problems such as Patellar luxation or a stretched or torn anterior cruciate ligament. Again, your vet should easily ascertain these.
As a homeopath, I see these possibilities as the result of ongoing low-grade chronic disease and I refer you to a book “Homeopathy Beyond Flat Earth Medicine” by Timothy Dooley MD. I think the problem may be treatable with homeopathy. A list of practitioners is available through www.theavh.org.
I would suggest you also increase any vitaminC preferably Calcium ascorbate, and begin a glucosamine product available at better pet health stores.
©Michael Goldberg DVM
Michael Goldberg DVM
Hudson Place Veterinary Clinic
1300 West 73 Ave.
Vancouver, B.C. V6P 3E7