Mammalian skeletal muscle fibers display a great adaptive potential. This potential results from the ability of muscle fibers to adjust their molecular, functional, and metabolic properties in response to altered functional demands, such as changes in neuromuscular activity or mechanical loading. Adaptive changes in the expression of myofibrillar and other protein isoforms result in fiber type transitions. These transitions occur in a sequential order and encompass a spectrum of pure and hybrid fibers. Depending on the quality, intensity, and duration of the alterations in functional demand, muscle fibers may undergo functional transitions in the direction of slow or fast, as well as metabolic transitions in the direction of aerobic-oxidative or glycotytic. The maximum range of possible transitions in either direction depends on the fiber phenotype and is determined by its initial location in the fiber spectrum. Key words: Ca-sequestering proteins, energy metabolism, fiber type transition, myofibrillar protein isofonns, myosin, neuromuscular activity