The effect of constitutive inactivation of the gene encoding myostatin on the gain in muscle performance during postnatal growth hsa not been well characterized. Methods. We analyzed 2 murine myostatin knockout (KO) models: i) the Lee model (KOLee) and ii) the Grobet model (KOGrobet), and measured the contraction of tibialis anterior muscle in situ. Results. Absolute maximal isometric force was increased in 6-month old KOLee and KOGrobet mice, as compared to wild-type mice. Similarly, absolute maximal power was increased in 6-month old KOLee mice. In contrast, specific maximal force (relative maximal force per unit of muscle mass was decreased in all 6-month old male and female KO mice, except in 6-month old female KOGrobet mice, whereas specific maximal power was reduced only in male KOLee mice. Discussion. Genetic inactivation of myostatin increases maximal force and power, but in return it reduces muscle quality, particularly in male mice.