Objectives: Analysis of the computed recruitment order of an ensemble of ventral and dorsal root ®bers should enlighten the relation between the position of a bipolar electrode and the observed order of muscle twitches. Material and methods: Thresholds of selected spinal root ®bers are investigated in a two step procedure. First the electric ®eld generated by the electrodes is computed with the Finite Element Method. In the second step the calculated voltage pro®le along each target neuron is used as input data for a cable model. For every electrode position the electrical excitability is analyzed for 12 large diameter ventral and dorsal root ®bers of the second and fourth lumbar and ®rst sacral segment. The predictions of the neural responses of any target ®ber are based on the activating function concept and on the more accurate computer simulations of the electrical behavior of all nodes and internodes in the vicinity of the electrode.

Objectives: The purpose of this study was to evaluate the eect of spinal cord stimulation (SCS) on severe spasticity of the lower limbs in patients with traumatic spinal cord injury (SCI) under close scrutiny of the site and parameters of stimulation. Materials and methods: Eight SCI patients (four women, four men) were included in the study. Levels of spasticity before and during stimulation were compared according to a clinical rating scale and by surface electrode polyelectromyography (pEMG) during passive¯exion and extension of the knee, supplemented by a pendulum test with the stimulating device switched either on or o over an appropriate period. Results: Both the clinical and the experimental parameters clearly demonstrated that SCS, when correctly handled, is a highly eective approach to controlling spasticity in spinal cord injury subjects. The success of this type of treatment hinges on four factors: (1) the epidural electrode must be located over the upper lumbar cord segment (L1, L2, L3); (2) the train frequency of stimulation must be in the range of 50 ± 100 Hz, the amplitude within 2 ± 7 V and the stimulus width of 210 ms; (3) the stimulus parameters must be optimized by clinically assessing the eect of arbitrary combinations of the four contacts of the quadripolar electrode; and (4) amplitudes of stimulation must be adjusted to dierent body positions. Conclusions: Severe muscle hypertonia aecting the lower extremities of patients with chronic spinal cord injuries can be eectively suppressed via stimulation of the upper lumbar cord segment. Spinal Cord (2000) 38, 524 ± 531

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