Excessive levels of glucocorticoids such as prednisone can promote tissue catabolism and atrophy (muscle weakness). Resistance training in animals and humans has been shown to be successful in attenuating or reversing the glucocorticoid-associated muscle atrophy. Endurance training is also capable of preventing glucocorticoid-mediated muscle atrophy. However, the cellular and molecular basis of atrophy prevention could be dictated by the type of exercise performed. Glucocorticoid-mediated atrophy is selective for fast-twitch muscle fibers and, particularly the white regions of fast-twitch muscle fibers.
[Robert Hickson Ph.D, & Jane Marone, M.D., Exercise and Inhibition of Glucocorticoid-Induced Muscle Atrophy, Exercise & Sport Sciences Reviews, 1/93, Vol. 21, Issue 1, PP:135-168]
In other words, prednisone therapy makes patients weak. Therefore patients are encouraged to exercise in order to minimize the muscle atrophy/wasting.