The purpose of the study was to examine the effect of prolonged vibration on the force fluctuations during a force-matching task performed at low-force levels. Fourteen young healthy men performed a submaximal force-matching task of isometric plantar flexion before and after Achilles tendon vibration (n = 8, vibration subjects) or lying without vibration (n = 6, control subjects) for 30 min. The target forces were 2.5-10% of the previbration maximal voluntary contraction force. The standard deviation of force decreased by a mean of 29 +/- 20% across target forces after vibration, whereas it did not decrease significantly in control subjects (-5 +/- 12%). This change was significantly greater compared with control subjects (P < 0.01 for both). Power spectral density of the force was predominantly composed of signals of low-frequency bandwidth (<or =5 Hz) with few higher frequency components. In vibration subjects, there was a significant decrease in power in the frequency range < or =2 Hz after vibration. The decrease in power at this frequency range was linearly related to the decrease in the force fluctuations (r = 0.96, P < 0.001). The results indicate that prolonged Achilles tendon vibration reduces the fluctuations in plantar flexion force in the frequency range < or =2 Hz during low-level contractions. It suggests that Ia afferent inputs contribute to the low-frequency force fluctuations in plantar flexion.