Muscular weakness is one of the major impairments limiting motor function following a hemispheric stroke. The objective of this preliminary study was to examine possible motor unit (MU) structural changes in paretic muscle post-stroke as a measure by which to assess neural and/or biomechanical mechanisms of paresis. A surface electromyogram (sEMG) recording and decomposition system was used to record sEMG signals and extract single MU activities from the first dorsal interosseous muscle (FDI) of three hemiparetic stroke survivors. To characterize potential MU structural changes, an estimate of the motor unit action potential (MUAP) amplitude and duration was derived using the spike triggered averaging of the sEMG signal. Our preliminary results reveal MUAPs with systematically smaller amplitude and longer duration in the paretic muscle compared with the contralateral muscle of three tested stroke subjects with varying degrees of motor impairment. The changes in MU properties such as reduced MU size and a reduction in the muscle fiber conduction velocity could contribute at least in part, to muscle weakness post-stroke. The sEMG recording and decomposition system combined with our spike triggered averaging technique has the potential to provide an assessment tool for muscular weakness post-stroke.