Stiffness control of the leg in perturbed gait and posture

This thesis investigates the adaptability of stiffness control in the leg during different conditions. Reflex stiffness was studied as occurs during the startle response (freezing). Unexpected auditory stimuli were presented in six phases of the step cycle during walking on a treadmill. EMG responses were observed in both flexors and extensors of the lower leg, often in coactivation. The amplitude of the reflex responses was dependent on the timing of the startle stimulus in the step cycle. The responses seem adapted to achieve extra stability dependent on the phase of the step cycle. However, even in the first trials, the changes in kinematics were small allowing a smooth progression of gait. The startle response was also studied in Parkinson patients. In these patients rigidity causes an increase in the intrinsic stiffness. Although the patients showed comparable latency, duration, and phase dependency, the habituaton rate was clearly decreased. This finding indicates that habituation of the startle response is a sensitive measure in these patients. Stiffness that is generated in anticipation to a resistance during a movement was studied with a new method using a trap door to induce ankle inversions during walking and jumping. During walking EMG activity in the lower leg showed the main response was observed after the end of the rotation of the trap door and was thereby too late to resist the induced stretch. Kinematic and EMG data of the whole body showed an avoidance of excessive stretch of the lateral structures of the lower leg by moving the body towards the side of the inversion, thereby decreasing the amount of ankle inversion. This was followed by a correcting response to maintain balance. To be able to study balance correcting response during quiet standing a new gravity-driven multidirectional platform was developed and teste