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Rodríguez LP, López-Rego J, Calbet JAL, Valero R, Varela E, Ponce J: Effects of training status on fibers of the musculus vastus lateralis in professional road cyclists. Am J Phys Med Rehabil 2002;81:651–660.To evaluate possible changes occurring in muscle fibers related to the training status of professional road cyclists.A comparative study was performed on two groups of male road cyclists: ten 21-yr-old cyclists with a history of 3 yr of sport competition (RC21) and ten 25-yr-old cyclists with a history of 7 yr of competition (RC25). The control group was formed by two subgroups of five nontrained, sedentary volunteers who were matched for age with the study subjects (NT21 and NT25). Biopsies of the vastus lateralis muscle were obtained to determine the fiber variables: percentage, cross-sectional area, mitochondrial volume, and capillary density.Control group variables were within the normal range. According to their training status, cyclists showed an increased percentage of type I and IIC fibers (RC25 > RC21 > NT) and decreased percentage of type IIA (RC25 < RC21 < NT) and IIB fibers (RC25 = RC21 < NT), an increased cross-sectional area of all fiber types after 3 yr of training (RC25 = RC21 > NT) except IIB fibers (RC25 > RC21), an increased mitochondrial volume in all fiber types (RC25 > RC21 > NT) except type IIA fibers (RC25 > RC21 = NT21), and an increased capillary density (RC25 > RC21 > NT).Findings indicate a progressive increase in the muscle fiber types that are mostly implicated in endurance sports, accompanied by phenotypic changes in the fiber population of lesser participation.