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Abstract The effect of weight and force is one of the most important factors to organize and perform reaction time (RT) tasks. The elderly are involved in this variable when they gain weight. The present study was designed to examine the effect of limb weight gain on premotor and motor components of simple and discriminative reaction time in young and elderly men. Subjects (16 young and 16 elderly) were asked to perform forearm flexion in response to auditory (simple and discriminative) stimuli. The weight of the involved limb increased (1.2 kg) in 50% of trials and reaction time and muscle tension were measured by electromyogram apparatus in all trials. The results showed that the main effects of all three factors of limb weight gain, increasing number of stimuli and age on premotor component were significant. The effect of the increased number of stimuli from simple to discriminative was not significant in the motor component (P=0.77). However, the interactive effects between limb weight gain and age was significant (P=0.002). These results suggested that those variables that cause variation in limb inertia and generally motor components not only involve motor component but also influence a part of premotor processes simultaneously.