Spinal cord electrical stimulation has been used to decrease spasticity in spinal cord injured patients. With this therapeutic method, the polarity of the applied electrical field and the frequency of the stimulation is critical for a satisfactory therapeutic result. However, frequencies widely ranging from 20 Hz to 1,500 Hz have been used in clinical trials, and results were variably reported. So, this study was designed to determent the optimal frequencies for spinal cord stimulation.

In eight cats which were hemisected at the T11 level, spasticity was markedly present in sex cats. Immediately after surgery, all reflex and locomotor hindlimb activity was depressed ipsilaterally in the six cats. At the end of the first week postoperatively, muscle tone and deep tendon reflexes began to increase and the hindlimb partially regained waking capability and a marked degree of spasticity appeared by the fourth week postoperatively.

Cathodal stimulation was applied at the L2 level using currents of less than 1 mA at 50, 100, 250, and 500 Hz with a duration of 0.25 msec in these six cats. Electromyographic changes in hamstring and quadriceps muscles (during spasticity induced by dorsiflexion of the paw or by painful stimuli) were monitored. Spasticity was markedly suppressed at 100 Hz in four cats, and moderately in the other two cats. But it was variable, usually increasing at 250 Hz and 500 Hz.

According to the above results, 100 Hz is the most effective frequency for spinal cord stimulation to release spasticity in spinal cord injured cats.