Comparing cortico-motor hotspot identification methods in the lower extremities post-stroke: MEP amplitude vs. latency

•TMS hotspots chosen using MEP amplitude as a criterion were more spatially co-located than when using MEP latency.•Intra and inter-session hotspot localization was poor regardless of the criterion used.•Double cone coil use may lend itself to the use of MEP amplitude as a hotspot criterion. Transcranial magnetic stimulation (TMS) is a technique used to probe and measure cortico-motor responses of the nervous system. However, lower extremity (LE) specific methodology has been slow to develop. In this retrospective analysis, we investigated what motor evoked potential metric, amplitude (MEPamp) or latency (MEPlat), best distinguished the motor-cortical target, i.e. hotspot, of the tibialis anterior and soleus post-stroke. Twenty-three participants with stroke were included in this investigation. Neuronavigation was used to map hotspots, derived via MEPamp and MEPlat, over a 3cm × 5cm grid. Distances between points with the greatest response within a session and between days were compared. Both criterion, amplitude and latency, provided poor identification of locations between trials within a session, and between multiple visits. Identified hotspots were similar only 15 % and 8% of the time between two assessments within the same session, for amplitude and latency respectively. However, MEPamp was more consistent in identifying hotspots, evidenced by locations being less spatially distant from each other (Amplitude: 1.4 cm (SD 0.10) Latency: 1.7 (SD 1.04), P = 0.008) within a session and between days (Amplitude: 1.3 cm (SD 0.95), Latency 1.9 cm (SD 1.14), P = 0.004). While more work is needed to develop LE specific methodology for TMS, especially as it applies to investigating gait impairments, MEPamp appears to be a more consistent criterion for hotspot identification when compared to MEPlat. It is recommended that future works continue to use MEPamp when identifying tibialis anterior and soleus hotspots using neuronavigation.