OBJECTIVE We investigated the architectural organization of groups of neighboring motor units (MUs) in normal brachial biceps and tibialis anterior muscles by analyzing morphological landmarks of their MU action potentials (MUAPs). METHODS EMG signals containing multiple MUAPs were recorded using a monopolar needle electrode during moderate isometric voluntary contractions. The MUAPs were identified using computer-aided decomposition, and averaged. For each MUAP the onset, spike, terminal wave, and slow afterwave were identified; then the location of the MU's endplate and muscle/tendon junction were estimated from the latencies of the spike and terminal wave with respect to the MUAP onset. RESULTS The analysis revealed a variety of architectural organizations, including single and multiple endplate zones, MU fractions, pennation, intramuscular aponeuroses, and centrally and non-centrally located endplates. CONCLUSIONS This type of morphological analysis of the MUAP promises to be informative for studying normal MU properties as well as evaluating MU reorganization in disease.