Motor somatotopy of extensor indicis proprius and extensor pollicis longus

After tendon transfer of extensor indicis proprius (EIP) to extensor pollicis longus (EPL), rehabilitation is initiated to enhance motor cortex reorganization. However, patients have been described showing thumb extension immediately after the tendon transfer. At cortical level, no evidence supports either of these assumptions. We noninvasively investigated motor cortical source locations of EIP and EPL muscles. Magnetoencephalography was used to identify motor somatotopic map in healthy right-handed participants, who performed voluntary extension at index metacarpophalangeal joint and thumb interphalangeal joint. Motor cortical representation of EIP was more medial than cortical representation of EPL, with mean Euclidean distance of 15.4±2.7 mm. Motor somatotopic map of EIP/EPL that was obtained by magnetoencephalography supports 'functional somatotopy' representation of the finger in primary motor cortex.