Glucocorticoid-induced myopathy causes selective atrophy of type IIb (fast-twitch) fibers. Muscle weakness caused by glucocorticoids is almost always reversible upon cessation or dose reduction.
Her lack of myalgia and normal CK would argue against an inflammatory or necrotic myopathy. Ragged red fibers may be seen with mitochondrial myopathy, but her presentation does not suggest a mitochondrial myopathy. Her pattern of weakness is not consistent with inclusion body myositis in which rimmed vacuoles may be seen.