No Evidence of Hysteresis in Quadriceps or Hamstring Active Motor Evoked Potentials

BackgroundThe excitability of the corticospinal tract (i.e., corticospinal excitability) is a valuable tool for assessing neurophysiology and the effectiveness of interventions in individuals with and without neurological and/or orthopaedic injuries. Corticospinal excitability is often measured with an input-output recruitment curve, which is produced by stimulating the motor cortex via transcranial magnetic stimulation (TMS) at several intensities and measuring the changes in the evoked responses. However, it is currently unclear if hysteresis in motor evoked potentials (MEPs) (i.e., changes in MEP amplitude due to the order of stimulus intensities) affects the resulting measure of excitability, particularly in lower extremity muscles.ObjectiveTo evaluate whether the order of stimulus intensity (ascending, descending, randomized) affects input-output recruitment curves measured in the lower extremity muscles.MethodsRecruitment curves were produced in neurologically intact individuals by stimulating the primary motor cortex at 70% to 140% of active motor threshold in 10% increments. We examined three stimulus intensity ordering paradigms: ascending (70$\to$140), descending (140$\to$70), and randomized. We measured MEPs of the quadriceps and the antagonistic hamstring muscles using surface electromyography in addition to quadriceps motor evoked torque. We computed the area under the recruitment curve (AUC) of the raw and normalized motor evoked responses and used classical and Bayesian inference methods to comprehensively evaluate hysteresis in MEPs.ResultsClassical hypothesis testing revealed no significant main effects of stimulus order. Bayesian analyses also confirmed that the null model was more favored than the main effects model.ConclusionsCorticospinal excitability of the quadriceps and antagonistic hamstring muscles were not influenced by stimulus intensity order. Any of the three approaches (ascending, descending, randomized) may be used when measuring recruitment curves for the quadriceps and hamstring muscles.