Loss of Strength

Many natural changes due to aging -reduced lung capacity, less efficient digestion, circulation, and filtration, as well as less exercise - result in less nourishment reaching the muscles.

In some geriatric conditions, the body will actually turn to the muscles themselves as a source of needed nourishment. For all these reasons, the loss of muscle strength is an inevitable consequence of advancing years.

Treatment 
An easy-to-digest diet that contains good levels of trace elements, as well as good-quality protein and balanced essential fatty acids and fiber, is most important for elderly dogs.

So is regular physical exercise. Walks and trots several times daily keep muscles supple and active.