Skeletal muscle is composed of different fibre types, which differ in contractile as well as in metabolic properties. The myosin molecule, which exists in several different isoforms, is of major importance in determining the contractile properties of the muscle cell. The plasticity of skeletal muscle is reflected in this tissue's adaptability to changes in the functional demand. In both rats and humans, a decrease in activity level will in most cases change the muscle fibre composition towards faster myosin isoforms and an increase in activity level (such as seen with exercise training) will induce an increase in slower myosin isoforms. The glucose transporter protein 4 (GLUT4), which is the major insulin regulatable glucose transporter in mammalian skeletal muscle, is found in larger amounts in slow muscle fibres compared with fast muscle fibres. An increase in activity level will increase the GLUT4 protein expression and a decrease in activity level will in most cases decrease GLUT4. Thus, there seems to be some kind of relationship between the muscle fibre type and GLUT4. However, the main factor regulating both the GLUT4 protein expression and the muscle fibre composition seems to be the activity level of the muscle fibre. Patients suffering from non-insulin-dependent diabetes mellitus (NIDDM) are insulin resistant in their skeletal muscles but are generally normal when it comes to skeletal muscle fibre composition and the GLUT4 protein expression. There is good evidence that exercise training beneficially impacts on insulin sensitivity in healthy individuals and in patients with type II diabetes. An increase in the GLUT4 protein expression in skeletal muscle may at least partly explain this effect of training.