High-density surface electromyography(HDsEMG) decomposition has allowed us to study individual motor unit (MU) responses in great detail during voluntary or reflex muscle contraction. Being noninvasive in nature, the technique has been widely used for many different applications in both healthy and stroke impaired populations. Here for the first time, we demonstrate the use of a HDsEMG based MU decomposition technique to study the morphological changes in the recruited MU population after botulinum toxin (BT) injection in the biceps brachii muscle for spasticity management in chronic hemiparetic stroke survivors. Three stroke survivors were examined before and after intramuscular (biceps brachii) BT injections. The HDsEMG grid enabled simultaneous recordings over the entire muscle. We have recorded both force and the surface electromyogram (sEMG) during voluntary isometric contraction tasks. The HDsEMG was decomposed using the convolution kernel compensation (CKC) method. We report a 60% increase of the peak to peak amplitude of the motor unit action potential (MUAP) signals after the BT injection compared to pre-injection values. The overall generated muscle force and sEMG values decreased during this period compared to their pre-injection level. We discuss potential mechanisms that would result in the emergence of larger MUAPs in the weeks immediately following intramuscular BT.