The Bernstein Center Freiburg

Informal Bernstein Seminar
Loïc Damm
Université Paris Descartes - CNRS
Laboratoire de neurobiologie des réseaux sensorimoteurs
Université Paris, France

Thursday, March 19th, 2009
11:00 h sharp
Library, first floor
BCCN Building
Hansastr. 9a
Arm impedance is a critical factor underlying stable interactions with the environment. This work focuses on the ability of the motor system to actively control mechanical impedance of the limb. The first experiment examined how humans produce forces with the hand on rigid objects in different directions. Biomechanical limb properties interplay with the force production process. The CNS relies on passive properties of the limb to ensure contact and increases impedance in the direction of instability. Second, we aimed at identifying the physiological basis of stiffness adaptation during free and constrained movements. When the hand moves freely through space, co-contraction ensures limb stiffness: this mechanism maintains the hand on the desired path. When constrained by a rigid surface, lower activity of antagonist muscles makes the limb more compliant: this mechanism allows the environment to guide the hand. H-reflex amplitude is also higher for constrained motion and appears to depend more on the net force exerted by the limb than on the desired level of compliance. The third experiment investigated the modulation of spinal excitability before catching a falling object. Spinal excitability increases before the onset of muscular activity. We interpret this result as a possible tuning of the limb dynamics in preparation for contact with the object. As seen in the preceding experiment, spinal loop gains are probably adjusted to regulate the influence of afferences on the ongoing movement. Our experimental work supports the hypothesis of an active regulation of the impedance during movement. The results are discussed in the context of motor control theories.
The talk is open to the public. Guests are cordially invited!