It is well-established that unilateral motor practice can lead to increased performance in the opposite non-trained hand. Here, we test the hypothesis that progressively increasing task difficulty during long-term skill training with the dominant right hand increase performance and corticomotor excitability of the left non-trained hand. Subjects practiced a visuomotor tracking task engaging right digit V for 6 weeks with either progressively increasing task difficulty (PT) or no progression (NPT). Corticospinal excitability(CSE) was evaluated from the resting motor threshold(rMT) and recruitment curve parameters following application of transcranial magnetic stimulation (TMS) to the ipsilateral primary motor cortex(iM1) hotspot of the left abductor digiti minimi muscle(ADM). PT led to significant improvements in left hand motor performance immediately after 6 weeks of training (63±18%,P<0.001) and 8 days later (76±14%,P<0.001). Additionally, PT led to better task performance compared to NPT (19±15%,P=0.024 and 27±15%,P=0.016). Following the initial training session, CSE increased across all subjects. After 6 weeks of training and 8 days later, only PT was accompanied by increased CSE evidenced by a left and upward shift in the recruitment curves e.g. indicated by increased MEPmax (P=0.012). Eight days after training similar effects were observed, but 14 months later motor performance and CSE were similar between groups. We suggest that progressively adjusting demands for timing and accuracy to individual proficiency promotes motor skill learning and drives the iM1-CSE resulting in enhanced performance of the non-trained hand. The results underline the importance of increasing task difficulty progressively and individually in skill learning and rehabilitation training. This article is protected by copyright. All rights reserved.