It has been suggested that a suppression of maximal voluntary contraction (MVC) induced by prolonged vibration is due to an attenuation of Ia afferent activity. The purpose of the present study was to test the hypothesis that aftereffects following prolonged vibration on muscle activity during MVC differ among plantar flexor synergists due to a supposed difference in muscle fiber composition. The plantar flexion MVC torque and surface electromyogram (EMG) of the medial head of gastrocnemius (MG), the lateral head of gastrocnemius (LG), and the soleus (SOL) were recorded in 13 subjects before and after prolonged vibration applied to the Achilles tendon at 100 Hz for 30min. The maximal H-reflexes and M-waves were also determined from the three muscles and the ratio between H-reflexes and M-waves (H/Mmax) was calculated before and after the vibration. The MVC torque was decreased by 16.6±3.7 % after the vibration (P < 0.05; ANOVA). The H/Mmax also decreased for all three muscles, indicating that Ia afferent activity was successfully attenuated by the vibration in all plantar flexors. However, a reduction of EMG during MVC was observed only in MG (12.7±4.0 %) and LG (11.4±3.9 %) (P < 0.05; ANOVA), not in SOL (3.4±3.0 %). These results demonstrated that prolonged vibration-induced MVC suppression was attributable mainly to the reduction of muscle activity in MG and LG, both of which have a larger proportion of fast-twitch muscle fibers than SOL. This finding suggests that Ia afferent activity that reinforces the recruitment of high-threshold motor units is necessary to enhance force exertion during MVC.