Introduction Diabetes is associated with an accelerated loss of muscle mass and function. We compared the contractile properties of single muscle fibers in young rat soleus muscle of uncontrolled streptozotocin-induced diabetic animals (n=10) and non-diabetic controls (n=10). Methods Single fiber maximal force, shortening velocity, and power were assessed during maximal activation with calcium using the slack test 4 weeks after induction. Myosin heavy chain expression was determined using sodium dodecyl sulfate poly-acrylamide gel electrophoresis. Oxidized myosin levels were detected by analyzing protein carbonyls in muscle homogenates. All fibers expressed type I the myosin heavy chain isoform. Results Diabetic rats had higher blood glucose (537 vs. 175 mg/dl; P<0.001) and lower body weight (171 vs. 356 g; P<0.001) than controls. Muscle fibers from diabetic rats showed smaller cross-sectional area (1128 vs. 1812 μm(2) ), lower maximal force (258 vs. 492 μN), and reduced absolute power (182 vs. 388 μN FL/s) (all P<0.0001). No differences were seen in shortening velocity, specific force or specific power. Myosin carbonylation was higher (P<0.01) in diabetic rats. Discussion After 4 weeks of untreated diabetes, there are significant alterations in muscle at the level of isolated single fibers and myosin protein, although some contractile properties seem to be protected. This article is protected by copyright. All rights reserved.