Determination of pathological clonus characteristics using a haptic ankle manipulator

Clonus is a self sustained oscillatory movement of the extremities often seen after lesions of the central nervous system. There is still controversy about the origin of clonus whether it is i) an autonomous pacemaker-like oscillator, ii) a reflexive mediated feedback property or iii) a combination of both. With the use of a haptic robot we were able to initiate clonus in patients (n=7). We would conclude the underlying mechanism to be autonomous when the frequency of movement would not change with additional external damping and inertia. Results however showed distinct changes in frequency of movement, varying the peripheral conditions. Furthermore, when the clonus was suddenly stopped by a controlled increase of a huge external damper, clear bursting was observed in the triceps EMG while there was no rotation of the ankle. In conclusion, i) clonus is initiated by spinal reflexes (probably la or lb afferents), ii) also during clonus the same spinal reflexes are likely to contribute to the maintenance of ankle rotation and iii) the after bursting directs to self sustained firing of neural circuitry. These conclusions implicate the existence of a reflexive controlled oscillator. Future research will be performed using detailed model simulations to test different gains between reflexive feedback and neural oscillations, e.g. from the alpha - motorneurons.