Arnold Schwarzenegger shows us one side of the profound plasticity of the vertebrate muscular–skeletal system. Muscles become larger and bones appropriately remodeled when they experience increased work loads. Parenthetically, although Mr. Schwarzenegger has not been called upon to go topless in his recent films, his continuing apparent bulk under a sports jacket shows that use-specific hypertrophy is not confined to a narrow age range. I shall return to this point later. The anti-Arnold effect, or disuse-specific atrophy, also is well known. It is strikingly illustrated by the rapid loss of muscle and bone mass that occurs in zero gravity. Mammals, including humans, that spend a few days in earth orbit return with substantial reduction in muscle and bone mass (Bodine-Fowler et al. 1992; Cann & Adachi 1983; Morey & Baylink 1978). NASA acknowledges that muscular–skeletal atrophy is the most serious medical problem associated with space flight. Another well-known plastic response in vertebrates is the use-induced gain and disuse-induced loss of aerobic capacity. Here, changes in many organ and enzyme systems are involved (Close 1972; Laughlin et al. 1989; Nieman 1990; Bigard et al. 1991), and the effects are equally as dramatic as Mr. Schwarzenegger's pectoralis muscles. I and most other humans cannot run a single 5 minute mile, but some humans who have trained for marathons can run 26 of these in succession.

Phenotypic plasticity in systems that support strength and endurance is adaptive. The muscular–skeletal system has a way of detecting demands placed upon it, and responds appropriately, building as much, but no more, capability as is called for.