Atrophy

Atrophy is the wasting away of muscle.  This can be caused by a number of reasons including lack of exercise, poor nourishment, bad circulation, loss of nerve supply, injury, disease or a genetic problem.  Most of these causes can be reversed, or at least lessened, with exercise, which allows a good blood flow to the muscles and enables them to nourish and repair.  However it may be difficult to do this.  

So long as the dog has led a healthy active life atrophy may only be seen in older age.  This can be due to a reduced amount of exercise due to arthritis.  An older dog will also have decreased growth in the muscles due to a reduction of being able to process proteins.  Therefore just maintaining muscles can be difficult.    

Atrophy shows itself as limb weakness, paw dragging and the dog being unable to support its own body weight.  The dog may also appear thinner.