The main objective of this thesis is to study the motor flexibility in complex movements when an unexpected event makes the initial motor plan inefficient. In this way, three kinematic and electromyographic studies and a fourth with functional magnetic resonance imaging were realized. (1)The main result of the first study clearly demonstrate that during complex movements express motor corrections in the upper and lower limbs, with latency responses of less than 100 ms, were revealed by contrasting electromyographic activities in perturbed and unperturbed trials. Such findings could indicate that visual on-going movement corrections may be accomplished via fast loops at the level of the upper and lower limbs and may not require cortical involvement. (2) When an unexpected target jump occurred, correction times were strongly correlated together for some pairs of muscles independently of their occurrences during the motor sequence and independently of the location of the muscles at the anatomical level. This second study suggests that the CNS re-programs a new motor synergy after the target jumps in order to correct the on going reaching movement. (3) When the target size is varied during the initial motor plan execution, the movement duration can increase independently of the variability of the final endpoint. These results suggests that when the speed-accuracy trade-off is unexpectedly modified, terminal feedbacks based on intermediate representations of the endpoint velocity are used to monitor and control the hand displacement. (4) Finally, when catching a falling ball and the possibility of prediction about the ball weight was manipulated, the last study of this thesis showed that both the right and left cerebellum is engaged in processing sensory–motor errors, and more particularly the lobules IV, V and VI. For classical loops involved in motor flexibility, sensory-motor errors are processed within the cerebellum. However, some shorter sub-cortical loops seem also to be involved for faster motor corrections. The coordination between these different loops needs to be explained more precisely.
