In its historic definition, the term homeostasis refers to the constancy of the internal milieu in the face of external perturbations; the latter in principle may be caused by extracellular factors, or in the case we shall consider, by change in intracellular biological function. Of all the tissues in the vertebrate body, skeletal muscle displays the special quality of being able to routinely sustain very large changes in work and metabolic rates. Compared to the 1.5-to 2-fold differences in metabolic rates between resting and activated states, which is common to many tissues (liver and brain, to mention two), skeletal muscles in most animals must be able to sustain up to, or even more than, 100-fold changes in ATP turnover rates. Amongst vertebrate endotherms, the highest muscle metabolic rate (in the range of 600·µmol·ATP·g -1 ·min -1 ) appears to be that of hummingbird breast muscle during hovering flight, which is a rate over 500 times muscle RMR (Suarez, 1992;Suarez et al., 1990Suarez et al., , 1991. During muscle ischemia, hypoxemia or hypoxia, the metabolism of muscle, like that of many other tissues under conditions of oxygen lack, may need to sustain a suppression of metabolism even below resting rates (Hochachka and Guppy, 1987), thus extending even further the enormous range between the lowest and highest sustainable ATP turnover rates of this remarkable tissue.Current popular interpretations of such large scale differences in steady state energy turnover are regulated assume cybernetic feedback control circuitry. The standard