Clonus in spastic subjects: A stable limit cycle?

Clonus properties at the ankle were examined in 4 spastic subjects to determine whether this oscillatory behavior has the properties of a limit cycle, and whether it is driven by peripheral sensory input or by a spinal generator of some type. Floquet Theory and Poincare sections were used to assess the presence of limit cycle behavior and the kinematic stability of clonus, while external perturbations were used to determine whether clonus kinematic, dynamic, and muscle activation characteristics were dependent on external conditions, a property of a system driven from the periphery. We found that during clonus, cycle-to-cycle variability in the Poincare sections was quite small, such that the Floquet multipliers were always less than unity. In addition, the steady-state periodic orbit was not dependent on the initial position of the ankle. Furthermore, by applying dorsiflexion torques of various magnitudes to the subject's ankle, we observed a direct correlation between the size of the applied load and the frequency of both ankle movement and EMG burst frequency in the soleus and gastrocnemius muscles. The frequency band for the various loads tested ranged from 3.8 Hz. to as much as 5.8 Hz. The timing of the EMG responses with respect to the onset of muscle lengthening in each clonus cycle was consistent with that of a delayed afferent response coming from the muscle spindles. The experimental findings in this study suggest that clonus, an involuntary reflex oscillation commonly exhibited in spastic patients, is a stable limit cycle that is mediated by peripheral reflex pathways.