Type 2 diabetes mellitus is associated with an accelerated muscle loss during aging, decreased muscle function, and increased disability. To better understand the mechanisms causing this muscle deterioration in type 2 diabetes, we assessed muscle weight, exercise capacity, and biochemistry in db/ db and TallyHo mice at prediabetic and overtly diabetic ages. Maximum running speeds and muscle weights were already reduced in prediabetic db/ db mice when compared with lean controls and more severely reduced in the overtly diabetic db/ db mice. In contrast to db/ db mice, TallyHo muscle size dramatically increased and maximum running speed was maintained during the progression from prediabetes to overt diabetes. Analysis of mechanisms that may contribute to decreased muscle weight in db/ db mice demonstrated that insulin-dependent phosphorylation of enzymes that promote protein synthesis was severely blunted in db/ db muscle. In addition, prediabetic (6-wk-old) and diabetic (12-wk-old) db/ db muscle exhibited an increase in a marker of proteasomal protein degradation, the level of polyubiquitinated proteins. Chronic treadmill training of db/ db mice improved glucose tolerance and exercise capacity, reduced markers of protein degradation, but only mildly increased muscle weight. The differences in muscle phenotype between these models of type 2 diabetes suggest that insulin resistance and chronic hyperglycemia alone are insufficient to rapidly decrease muscle size and function and that the effects of diabetes on muscle growth and function are animal model-dependent.