Effects of electrode size on basic excitatory responses and on selected stimulus parameters.

Recent data have implicated the size of surface electrodes as an important factor affecting peripheral nerve excitation. Therefore, we studied the effects of electrode size on the basic excitatory responses and on stimulus characteristics. Four different sizes of self-adhesive surface electrodes were applied over the medial and lateral gastrocnemius muscle of 20 healthy subjects. The excitatory levels were sensory threshold, motor threshold, pain threshold, and maximally tolerated painful stimulation. Stimulus parameters included a symmetric biphasic waveform, 200 microseconds 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 charge as well as isometric plantar flexion force. Repeated measure analysis 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 ratios of voltage/current decreased nonlinearly, while the ratios of charge/voltage increased nonlinearly. The comfort of stimulation for the same amount of plantar flexion force improved significantly as electrode size became larger. We concluded that electrode size affects the stimulus parameters, comfort, and force generation associated with electrically induced excitatory responses. Electrode size should be considered an integral part of the attempt to improve subject response to transcutaneous electrical stimulation.