Fatigue and the design of the respiratory system.

One source of muscle fatigue may be the failure to provide the required oxygen by any step in the oxygen transport cascade or a lack of the necessary machinery to utilize that oxygen. We favor abandoning the concept of a single rate-limiting step for the concept of tuned resistors, each contributing to the overall resistance to oxygen flow. However, because some of these steps have considerably less phenotypic plasticity than others, these are the component parts of the respiratory system that must be built with adequate "reserve" to accommodate adaptive increases in the other steps (Lindstedt et al., 1988; Weibel et al., 1992; Lindstedt et al., 1994). These structures will usually appear to be over built except in those rare individual animals at the species-specific limit of VO2 in which these less malleable structures may be limiting.