Fig. 6. Simple qualitative model of tissue design limits for aerobic metabolism and performance levels as a function of body temperature. In a trade-off between the cellular space required by mitochondria, sarcoplasmic reticulum (SR) and myofilaments, the increased mitochondrial energy production at warm temperatures allows muscular power output at high body temperatures to be maximized. Design limits for aerobic metabolism are reached at lower performance levels in Antarctic fish. The model takes account of the higher mitochondrial densities seen in Antarctic fish than in mammal and bird muscles (see text).