Effect of respiratory training induced by CO2 breathing on maximal specific tetanic force in adult mdx and C57BL10 mice as shown by the force-frequency relationship. Values are expressed as meansGSEM. *, P!0.05. White circle, control C57BL10 mice (nZ8); dark circle, trained C57BL10 mice (nZ8); white triangle, control mdx mice (nZ10); dark triangle, trained mdx mice (nZ10).