This study examines the effect of high-level skilled behaviour on motor cortex representations of upper extremity muscles of ten sportswomen. We used transcranial magnetic stimulation to map proximal medial deltoid and distal extensor carpi radialis muscle representations on both hemispheres during low-level voluntary contraction. We compared cortical representation areas between two groups of subjects and between hemispheres within subjects. The first group comprised five elite volleyball attackers and the second group five runners. Four stimuli were delivered on multiple scalp sites (1.5 cm apart) to induce motor-evoked potentials recorded by surface EMG. Maps were described in terms of excitable scalp positions and of motor-evoked potentials. We observed differences in map areas between the two groups. Volleyball players had larger cortical representations of the proximal medial deltoid muscle than runners. Furthermore, the volleyball players had larger map areas for dominant muscles compared with non-dominant muscles. There was no difference, however, in map area for either muscle between the dominant and non-dominant arm in the runner group. Our results show that heavy training in a specific skill induces an expansion of proximal muscle representation in the contralateral primary motor cortex. This enlarged map area for proximal muscle is accompanied by an increase in the overlapping of proximal and distal muscle representations. This could reflect the fact that motor learning of co-ordinated movement involves a common control of both muscles. This reorganization supports the hypothesis of a cortical plasticity driven by activity.