Temporal summation in human peripheral axons when stimulated transcutaneously with a 10-kHz waveform

Transcutaneous spinal cord stimulation, as used for rehabilitation of impaired motor function after spinal cord injury, often involves a 10-kHz waveform modulated to produce repetitive bursts of stimulation. Kilohertz-frequency waveforms may facilitate the summation of subthreshold depolarisations, but the optimal burst duration for nerve stimulation has not been systematically investigated. In 11 adults, the ulnar nerve was stimulated transcutaneously with a 10-kHz waveform that contained 1, 2, 4, 6, 8 or 10 pulses, in random order. Compound muscle action potentials (CMAPs) and sensory nerve action potentials (SNAPs) were measured from motor threshold up to the maximal CMAP (M_max). The efficacy of each waveform was determined at M_max as CMAP amplitude divided by total phase charge. For CMAPs and SNAPs, increasing the number of pulses shifted the stimulus-response curves to the left for current and to the right for total charge. Accordingly, an increase in the number of pulses decreased the current but increased the total charge at sensory and motor thresholds and M_max. Efficacy decreased as the number of pulses increased. Onset latencies were delayed for waveforms with six or more pulses compared to a single pulse. These findings provide evidence of the summation of subthreshold depolarisations in sensory and motor axons in humans. However, the optimal number of pulses for summation remains unclear due to the opposing changes in current and total charge. It is clear, though, that more than six pulses is suboptimal, as there were no further decreases in threshold current while total charge continued to increase.