Exercise results in mitochondrial biogenesis and enhanced fat metabolism. An action potential causes Ca2+ release from the sarcoplasmic reticulum (SR), the muscle-cell equivalent of the endoplasmic reticulum. The Ca2+, in addition to instigating muscle contraction, activates nuclear transcription factors that stimulate the expression of specific genes. The products of these genes, in conjunction with the products of mitochondrial genes, are responsible for mitochondrial biogenesis. Fatty acids activate a different set of genes that increase the fatty acid oxidation capability of mitochondria.
[Information from D. A. Hood, J Appl. Physiol. 90:1137–1157, 2001, Fig. 2.]