To begin, use a mildly abrasive cream to clean the skin inside the forearm. Start from the wrist flexion crease, and extend to six centimeters proximal approximately. Extend cleaning to the area from the wrist midline to the radial side of the forearm.
Wipe away any residue using an alcohol prep pad. Next, apply the conducting gel to a reusable stimulating bar electrode. Use enough gel to cover the metal discs of the anodal and cathodal contact points.
Place the stimulating electrode on the Palmer side of the wrist with the cathode proximal to the anode. Place the cathode slightly medial and proximal to the radial styloid process. On the peripheral stimulator, set the stimulus type selector two monophasic, stimulus duration to 200 microseconds, and select an appropriate voltage and amperage, double checking any multiplication factors, While holding the stimulating electrode, deliver a single electrical stimulus by depressing the trigger switch on the constant current stimulator.
Then visually inspect the abductor pollicis brevis, or APB muscle, and the electromyography, or EMG display, for evidence of a muscle contraction. The muscle contraction, also known as the M wave, is elicited by direct activation of the motor axon by the electrical stimulus and should occur between six and nine milliseconds after the peripheral electrical stimulus artifact. If there is no evidence of a muscle contraction, then ask the participant if they felt a tingling sensation radiating toward the fingers or immediately underneath the electrode.
If the participant reports no sensation or is restricted to the skin immediately below the electrode, then increase the amperage in increments of 0.05 until the participant reports a tingling sensation radiating up to the fingers or thumb. If the participant reports a radiating sensation in a digit other than the thumb, reposition the electrode by moving it radially until the feeling radiates to the thumb. Once the optimal position of the stimulating electrode is determined, secure the electrode to the wrist using three pieces of tape.
Position the first piece over the middle of the electrode, and then use the second and third pieces to secure the top and bottom of the electrode. After securing the electrode, ask the participant to assume the desired limb orientation to be used during TMS stimulation. Check to make sure a thumb twitch is still elicited.
