This study examines how well the **Iovera®° system** helps people with **upper extremity spasticity** (muscle stiffness in the arm). The study will include 132 people: 88 will get the actual treatment, and 44 will receive a pretend treatment (placebo). The Iovera®° system targets nerves with a special device, using a local numbing medicine called *lidocaine*. The main goal is to see if this system can reduce muscle stiffness safely. Participants will be in the study for 13 weeks, with check-ins at 3, 7, and 13 weeks.

Eligibility: Participants must be 18 or older, have a stable muscle stiffness condition, and not have had certain treatments recently. They should be able to follow the study schedule and provide consent.

• **Duration**: 13 weeks with multiple follow-up visits.
• **Treatment**: Some will receive Iovera®° treatment; others will get a placebo.
• **Eligibility**: Must be 18+, have stable spasticity, and follow the schedule.