Although physical activity has litt

Although physical activity has little in1uence on the proportion of type I fibers in a muscle, more substantial interventions can have an effect. • Space flight, for example, exposes muscles to a sustained decrease in gravity, reducing the proportion of type I fibers in leg muscles. • A few weeks of continuous electrical stimulation at a low frequency causes a marked increase in the proportion of type I fibers and a substantial decrease in fiber size. • Similarly, surgically changing the nerve that innervates a muscle alters the pattern of activation; eventually the muscle exhibits properties similar to those of the muscle that was originally innervated by the transplanted nerve.• Connecting a nerve that originally innervated a rapidly contracting leg muscle to a slowly contracting leg muscle, for example, will cause the slower muscle to become more like a faster muscle