Introduction The effect of intracortical facilititation (ICF) is independent from hemispheric dominance and handedness (Baumer et al., 2007). Short term training selectively increases excitability over the contralateral primary motor cortex (M1c) of the trained hand (e.g. Lotze et al., 2003). M1-excitability measurements after long-term training (several weeks) are rare but – to our knowledge – are even absent for ICF-parameters. Objectives To evaluate effects of ICF during a two week comprehensive period of training of the left hand in strongly right handed healthy participants. Materials and ethods Thirteen strongly right-handed healthy participants (23 ± 3.5 years; 6 women) underwent a two-week arm ability training (AAT) for their left, nondominant hand for one hour per day (Platz, 2004). Performance increase was expressed as percentual changes over time and averaged over all eight AAT-tasks. Focal TMS was delivered to the optimal scalp position for activation of the musculus abductor policis brevis (APB) of each hand using a figure-of-eight coil. Resting motor threshold (rMT) and intracortical facilitation (ICF; test stimulus intensity 120% rMT; conditioning stimulus 80% rMT; interstimulus interval 10 ms) were used as measures of corticomotor excitability. Results Training resulted in considerable improvement of the trained left (30 ± 3.5%) hand, but – to a lesser extent – also the non-trained right hand (18 ± 5.4%) performance. RMT was not altered over time. For ICF we found a strong time effect in ANOVA; but post hoc t-tests showed only a relevant decrease over time for the untrained side (T(12) = 3.33; p = 0.006; Fig. 1). Conclusion Our study showed no significant changes in ICF for the trained hand side, but a decrease for the non trained hand’s motor cortex. It has been demonstrated that the interaction between an increase of unilateral movement and 10 h unilateral immobilization of the other hand is critical for excitability effects over M1c (Avanzino et al., 2011). We argue that interhemispheric changes induced by unilateral limb training of the non-dominant hand might be multifactorial; here we observed an increased interhemispheric balance in ICF after training the less experienced, and presumably in spontaneous behavior less intensively used hand. Download : Download high-res image (410KB) Download : Download full-size image