Electrical stimulation induced spasticity reductions in stroke as evidenced by changes in elbow torque responses

We studied the effects of electrical stimulation of the skin on upper extremity spasticity in 9 hemiparetic stroke subjects. The effects were quantified by comparing reflex torque responses during ramp and hold angular perturbations of the elbow. Electrical stimulation was applied to skin over the biceps muscle of all nine subjects for a period of ten minutes at a 20 Hz frequency, pulse duration 0.1 ms, with an intensity level between sensory and motor thresholds. In a subset of subjects (n=2), the protocol was repeated using frequencies of 1 and 100 Hz. In seven subjects, peak torque responses were reduced for at least 30 minutes. Significant increases in mean threshold onset angle of the reflex torque without changes in reflex stiffness were measured. These long-term changes may reflect synaptic plasticity of spinal circuitry outside the stretch reflex loop. Comparable reductions in spasticity were observed when repeating the same protocol with different stimulation frequencies