Fatigue of the vastus lateralis muscle was studied in healthy well-conditioned students, who differed considerable regarding their muscle fibre type distribution. Muscle force decline during repeated maximum voluntary knee extensions at a constant angular velocity (180 degree X s-1 or rad X s-1), using isokinetic equipment, was taken as the criterion for the degree of fatigue. In an attempt to study quantitative as well as qualitative changes in the EMG pattern, integrated EMG (IEMG) and the frequency of the mean power (MPF), computed from the power spectral density function (PSDF), were analysed. It was found that individuals with muscles made up of a high proportion of fast twitch (FT) muscle fibres demonstrated higher peak knee extension torque, and a greater susceptibility to fatigue than did individuals with muscles mainly composed of slow twitch (ST) muscle fibres. An IEMG decline (p less than 0.01) was demonstrated during 100 contractions in individuals rich in FT fibres. Only a slight, but not significant, reduction in IEMG occurred in individuals with high percentage of ST fibres. Concomitantly, MPF decreased (p less than 0.001) in individuals with a high percentage of FT fibres, while their opposites demonstrated only a slight decrease (non-significant). It is suggested that muscle conctraction failure might also be related to qualitative changes in the motor unit recruitment pattern, and that these changes occur more rapidly in muscles composed of a high proportion of FT muscle fibres than in muscles composed to a high proportion of ST fibres.