The aim of this study was to investigate if, and via what mechanisms, resistance training of the plantar flexor muscles affects voluntary activation during maximal voluntary eccentric and concentric muscle actions. Twenty healthy subjects were randomized into a resistance training group (N = 9) or a passive control group (N = 11). Training consisted of 15 sessions of unilateral mainly eccentric plantar flexor exercise over a 5-week period. During pre- and post-training testing, dynamic plantar flexor strength was measured and voluntary activation was calculated using the twitch interpolation technique. The Soleus H-reflex was used to assess motoneurone excitability and presynaptic inhibition of Ia-afferents whereas the Soleus V-wave to test for both changes in presynaptic inhibition of Ia-afferents and changes in supraspinal inputs to the motoneurone pool. H-reflexes, V-waves, supramaximal M-waves and twitches were evoked as the foot was moved at 5 degrees (.)s(-1) through an angle of 90 degrees during passive ankle rotations (passive H and M) and during maximal voluntary concentric and eccentric plantar flexion (MVC H, M and V-wave). Training induced significant improvements in plantar flexor strength and voluntary activation during both concentric and eccentric maximal voluntary actions. Soleus passive and MVC H:M ratios remained unchanged after training, whereas the Soleus V:M ratio was increased during both concentric and eccentric contractions after training. No change was seen in the control group for any of the parameters. The enhanced voluntary strength could be attributed partly to an increase in voluntary activation induced by eccentric training. Since the passive and MVC H:M ratios remained unchanged, the increase in activation is probably not due to decreased presynaptic inhibition. The increased V:M ratio for both action types indicate, that increased voluntary drive from supraspinal centers and/or modulation in afferents other than Ia:s, may have contributed to such an increase in voluntary activation.