Intramuscular botulinum toxin (BT) injections are used to alleviate the spasticity effects in chronic stroke survivors. The toxin injection works by means of chemodenervation of muscle fibers from their respective motor neuron (MN). Thus it most likely affects the territories of active motor units (MU) although the uniformity and territorial range is unreported and thereby unknown. Here, we have demonstrated the BT effect on motor unit territory area (MUTA) using a novel noninvasive high-density surface electromyography (HDsEMG) decomposition based detection and quantification method. The HDsEMG recordings were performed during isometric voluntary contractions about the elbow joint. We were able to analyze the effect of BT on MUTA of individual MUs with different MU recruitment thresholds in the biceps brahii (BB) muscle. We report on data from two stroke survivors before and after two weeks of their BT injection. Our results show that after the BT injection reduction of the MUTA was more prominent in larger MUs compared to the smaller MUs.