EFFECTS OF ELECTRODE SIZE ON BASIC EXCITATORY RESPONSES AND ON SELECTED STIMULUS PARAMETERS

Recent data have implicated the size of surface electrodes as an impor tant factor affecting peripheral nerve excitation. Therefore, we studi ed the effects of electrode size on the bask excitatory responses and on stimulus characteristics. Four different sizes of self-adhesive sur face electrodes were applied over the medial and lateral gastrocnemius muscle of 20 healthy subjects. The excitatory levels were sensory thr eshold, motor threshold, pain threshold, and maximally tolerated painf ul stimulation. Stimulus parameters included a symmetric biphasic wave form, 200 musec phase duration, and a pulse repetition rate of 50 pps. Amplitude was increased until the appropriate excitatory response was achieved. At this amplitude level, the computerized recording system collected data of stimulus peak current, peak voltage, and phase charg e as well as isometric plantar flexion force. Repeated measure analysi s of variance and Newman-Keuls post hoc tests revealed that increasing electrode size significantly decreased voltage but increased current and phase charge magnitudes. With increasing electrode area, the ratio s of voltage/current decreased nonlinearly, while the ratios of charge /voltage increased nonlinearly. The comfort of stimulation for the sam e amount of plantar flexion force improved significantly as electrode size became larger. We concluded that electrode size affects the stimu lus parameters, comfort, and force generation associated with electric ally induced excitatory responses. Electrode size should be considered an integral part of the attempt to improve subject response to transc utaneous electrical stimulation.