Peripheral Modulation of Muscle Stiffness and Spasticity

1. To test the effect of hyaluronidase injections on upper limb outcomes. The investigators will test the hypothesis that compared to placebo, human recombinant hyaluronidase injections will increase passive range of motion (ROM) in the most affected joint of the upper limb (primary outcome), isometric muscle strength or force generation capacity using Maximum Voluntary Contraction (MVC) on electromyography (EMG), reduce upper limb motor impairment as measured by the Fugl-Meyer (FM) scale, and improve function as measured by the Wolf Motor Function Test (WMFT). 2. To evaluate the effect of hyaluronidase on neural and non-neural components of muscle stiffness. The investigators will test the hypothesis that compared to placebo, intramuscular (IM) hyaluronidase will modulate the passive and potentially also the active component of the stretch reflex, reduce stiffness and improve arm motor control. 3. To determine the effect of hyaluronidase on intramuscular glycosaminoglycans (GAG) content. The investigators will test the hypothesis that compared to placebo, IM hyaluronidase will reduce the T1rho relaxation times in the biceps and triceps muscles of the affected arm.