Maximum voluntary contraction (MVC) and cross-sectional area (CSA) of fast and slow twitch fibers are reduced in the lower limb muscles of elderly subjects. Isokinetic training at medium and high velocities has been widely used to improve muscle performance and force in young as well as elderly subjects. EMG and mechanomyogram (MMG) are compound signals in which the electrical and mechanical activities of recruited motor units (MUs) are summated. The aim of the present study was to verify the hypothesis that isokinetic training in the elderly induces changes in EMG and MMG parameters, compatible with a functional retrieval of fast twitch fiber MUs. In ten sedentary males (62-78 years), the surface EMG and MMG were recorded from the vastus lateralis muscle during isometric contractions at 20, 40, 60, 80 and 100% of the MVC, before and after 12 weeks of isokinetic training (six series of ten repetitions, each at an angular velocity of 2.09 rad s(-1) and 4.19 rad s(-1), two times a week). With training: (a) MVC and CSA increased by about 35+/-5% and 8+/-1%, respectively (P<0.05); (b) the ratio MVC/CSA increased significantly in all subjects by 25+/-5%; (c) the EMG root mean square and MMG spectral mean frequency increased significantly at the highest workloads. In conclusion, our data indicate that isokinetic training in the elderly improved muscle size and performance significantly. The EMG and MMG changes suggest that these results may be due to a retrieval of the fast twitch fiber MUs, contributing to muscle action.