Abductor hallucis false motor points: electrophysiologic mapping and cadaveric dissection

False motor points (FMPs) can occur in intrinsic foot or hand muscles, causing spuriously prolonged distal motor latencies by misrepresenting the compound muscle action potential (CMAP) onset. We investigated the motor point (MP) and possible FMPs in abductor hallucis (AH) by three methods: (1) electrophysiologic mapping of the CMAP with a grid of approximately 29 G1 sites over AH (n = 20), including commonly used MPs just anterior to (Ant-MP) and posterior to (Post-MP) the navicular tuberosity; (2) electrophysiologic mapping with direct percutaneous threshold stimulation of AH (same grid as above); and (3) cadaveric dissection (n = 4). We found AH FMPs in 19 of 20 feet (2.7 FMPs/foot) which resulted in prolongation of the CMAP onset latency by 0.5-2.3 ms. Post-MP had a significantly lower mean threshold stimulus intensity than all other grid sites, including the FMPs. The anatomic MP of AH was consistently found just inferior and posterior to the navicular tuberosity. This study demonstrates that AH FMPs: (1) can be identified in virtually all feet; (2) do not correspond to the true MP (i.e., Post-MP); and (3) are likely due to superimposed compound action potentials from nearby muscles or nerves.