Effect of transcranial magnetic stimulation protocol on recruitment curve parameters

Introduction: The construction of transcranial magnetic stimulation (TMS) recruitment curves (RCs) involves stimulation over the target muscle representation in the primary motor cortex, with stimulus intensities beginning below an individual’s active motor threshold (AMT), and increasing in even increments until a plateau is observed in the amplitude of the motor evoked potential [MEP]. A sigmoidal equation is then fitted to the data, which provides four key parameters including minimum, slope, stimulus intensity at which MEP amplitude is 50% of maximum (V50), and plateau value (maximum). However, there are variations in protocols used to collect data for RCs within existing literature, thus the purpose of this study was to compare two of these common protocols. Methods: Nine participants were involved in a cross over study with two conditions one week apart. RCs were constructed for the extensor carpi radialis muscle of the right forearm using two different protocols. Protocol 1 involved increasing the stimulus intensity in intervals of 5% of the maximum stimulator output (MSO) from a level of 10% of MSO below AMT until there was no further increase in MEP amplitude (plateau). Protocol 2 involved increasing the stimulus intensity from a level of 90% of AMT in increments of 5% of the individuals’ AMT until responses plateaued. Five stimuli were delivered at all intensities in each protocol. Results: This study demonstrated a significant difference between testing protocols for slope (p = 0.002; ES = 1.877) and V50 (p < 0.001; ES = 8.490). However, there were no differences in the maximum (p = 0.251; ES = 0.11) or minimum value (p = 0.071; ES = 0.72) of the curves between the two protocols. Discussion: These findings show that variations in testing protocols result in different values for slope and V50 of the RC. Specifically, protocol 2 appears to be more sensitive in detecting slope and V50. Slope provides ameasure of the neurophysiological strength of intracortical and corticospinal connections, and the V50 is an indication of threshold characteristics of corticospinal cells and spinal alpha motor-neurons. Therefore, protocol 2 offers more meaningful results, which may be an important consideration in studies with multiple conditions, groups or time-points. This protocol also negates issues arising from variations in AMT between participants within a group or condition since stimulus intensities are determined relative to each individual’s AMT, thus results are more comparable and RCs can be constructed to represent group data.