Assessing Corticospinal Pathways Using Transcranial Magnetic Stimulation

Begin with a seated participant, with electrodes positioned on the leg muscles, the tibialis anterior, and the soleus muscles.

Fix a tracker to monitor the head position.

Place the transcranial magnetic stimulation or TMS coil over the brain’s motor cortex region that controls leg muscles.

Apply a magnetic stimulus to one hemisphere of the brain.

The signal travels from the brain through the corticospinal tract within the spinal cord to the contralateral leg, producing a muscle response.

Determine the lowest TMS intensity that evokes a consistent muscle response.

Then, apply a higher TMS intensity while the participant remains relaxed, and record the baseline corticomotor response or CMR.

Next, repeat the TMS delivery at the same intensity while the participant pulls their foot upward, and record the CMR.

Compared to rest, an increased CMR during activity allows assessment of the corticospinal pathways controlling the leg muscles.

Begin tonic voluntary activation testing by first determining the maximum voluntary isometric contraction of each muscle bilaterally. For each motion, instruct the subject to maximally contract the contralateral examined muscle four times. Next, verify the position of the motion capture camera by placing the subject tracker, pointer, and the coil tracker in its capture volume space.

Then, perform the subject image registration by placing the tip of the pointer on the four anatomical landmarks. Now, determine the hot spot of both muscles bilaterally. First, find the suprathreshold intensity by applying a single stimulus over the centered spot next to the interhemispheric fissure.

Next, start at low intensity and gradually increase the TMS intensity by five percent increments until reaching an intensity that elicits a motor-evoked potential with a peak to peak amplitude greater than 50 microvolts in all contralateral examined muscles or three consecutive stimuli and repeat for each muscle. Apply one TMS pulse on each spot of the grid. Then, transfer the amplitude values of each spot for all contralateral muscles in a spreadsheet and sort amplitude from high to low.

Identify the hot spot of the contralateral tibialis anterior and soleus muscles as a location in the grid with the largest amplitude and the shortest latency. Select the grid spot in the neuronavigation system that corresponds to one of the muscle's hot spots. Next, set the initial intensity and step size at 45 and six percent maximum stimulator output.

Then, use an adaptive threshold hunting method for resting motor threshold determination. Do this twice for each muscle and use the average for the sunsequent corticomotor response assessment. Now, to assess bilateral corticomotor response during rest, select the grid spot in the neuronavigation system that corresponds to the examined muscle's hot spot.

Prior to each stimulus, instruct the subject to stay still and relax the examined muscles bilaterally and monitor the activity of all muscles using a real time visual feedback. Apply ten single TMS pulses at 120%of resting motor threshold of the examined muscle. If any muscle is active before or after TMS, discard that trial and apply an additional single pulse.

Repeat this until 10 waveforms for each contralateral examined muscle at rest have been collected. Next, assess the corticomotor response during tonic voluntary activation bilaterally. Select the same grid spots in the neuronavigation system that were used during resting conditions.

Ask subjects to contract the examined muscle at approximately 15%maximum muscle activity value and apply 10 single TMS pulses at 120%of resting motor threshold. Ask them to keep the displayed smooth moving line of the examined muscle within the two horizontal cursors and to sustain that contraction at that level for a few seconds. When tibialis anterior is the examined muscle, ask subjects to pull slightly up against the boot straps for the leg contralateral to stimulated hemisphere.

When soleus is the examined muscle, ask subjects to push slightly down against the boot on the contralateral leg. Monitor the muscle activity of the active and resting muscles using real time visual feedback. If the examined muscle's activity is below or above the predetermined range, or if any other muscle is activated, discard the stimulus and apply an additional single pulse.

Collect 10 trials while the examined muscle is activated at the predetermined range.