Abstract : When unexpected stiffness changes of the ground surface occur while walking, the central nervous system needs to apply appropriate control actions to assure dynamic stability. Many studies in the motor control field have investigated the mechanisms of such a postural control and have widely described how centre of mass trajectories, step patterns and muscle activity adapt to avoid loss of balance. However, considerably less attention has been given to the role of the ground reaction forces. The aim of this study was to examine how ground reaction forces adapt when stepping on a soft, compliant surface. Differently from the classical methods to record ground reaction forces based on the use of force platforms positioned at fix locations along the walking path, here we used instrumented shoes, each one equipped with a pair of 3D force torque and motion units under the sole. Preliminary results showed that when stepping over the soft ground participants actively modulated the ground reaction forces under the supporting foot in order to exploit the elastic and compliant properties of the surface to dampen the impact and to likely dissipate the mechanical energy accumulated during the " fall " onto the new compliant surface. Interestingly, this motor strategy emerged already in the first trial, when participants experienced the transition for the first time. We believe that the results presented in this study may be helpful for the development of new control policies to improve stability in humanoid robotic locomotion.
