MATHIEU-COSTELLO, O., and R.T. HEPPLE. Muscle structural capacity for oxygen flux from capillary to fiber mitochondria. Exerc. Sport Sci. Rev., Vol. 30, No. 2, pp. 80–84, 2002. The concept of major functional resistance to O2 flux at the capillary-fiber interface implies that muscle structural capacity for O2 flux from capillary to fiber mitochondria needs to be assessed in terms of capillary surface per fiber surface. Morphological data support this notion and show the importance of assessing the size of the capillary-fiber interface relative to muscle fiber O2 demand.