In human, the physiology of gait and balance is not clearly established. By using functional imaging and electrophysiological techniques, various brain regions from the cortex to the midbrain area, including the cerebellum, have been identified as involved in such control. The specific role of these structures in both the capacity to go forward (locomotion) and stand upright (balance), but also in the different phases of the gait initiation process, are not known, however. In this study,the investigators aimed to assess the specific role of both the supplementary motor area (SMA) and the cerebellum in postural control during the initiation of gait. For this purpose, the investigators plan to study the gait initiation in 20 healthy subjects before and after functional inactivation (using inhibitory repetitive transcranial magnetic stimulation, rTMS) of the cerebellum or SMA. Biomechanical, kinematic and electromyographic parameters of the gait initiation will be recorded using a force platform, reflective markers with infrared cameras (VICON system) and lower limbs surface EMG electrodes.

Inclusion Criteria:

• 18-70 years
• agree to participate to the study with signature of the informed consent
• heathy insurance
• normal clinical exam

Exclusion Criteria:

• previous medical history of neurological, rheumatological, orthopedic or psychiatric disorders
• contra-indication to MRI or TMS
• drug treatment that modifies the nervous central system excitability (antidepressant, antiepileptic, neuroleptic)
• chronic alcoholism