Inclusion Criteria:  - For the knee arthritis groups: Chronic knee pain with evidence of knee osteoarthritis - radiographically grades 2-4  Gender: Males and females will be recruited. Language: only English-speaking subjects will be included.  Exclusion Criteria:  Inflammatory arthritis (e.g. rheumatoid arthritis) Inability to walk and climb stairs unassisted  History of knee replacement or knee surgery  Recent intra-articular injection of steroid (must be greater than 3 months) or other medication (e.g. hyaluronic acid, platelet rich plasma; must be greater than 6 months) History of arthroscopic knee surgery within the last 3 months (>3 months does not exclude participant) History of knee radiofrequency nerve ablation or ligation Current opioid or antidepressant use Cognitive impairment affecting the ability to provide informed consent, understand directions, and participate in study procedures  Uncontrolled or unstable medical disorder preventing participation in study procedures  History of brain surgery Tattoos on forearm or knee Pregnancy Patients with chronic pain conditions that are more severe than their knee arthritis pain, eg complex regional pain syndrome, fibromyalgia Patients whose most painful knee is excluded for another reason (e.g., recent surgery on most painful knee) For the pain-free control group: a history of chronic pain history of intra-articular steroid injections in the last 3 months history of platelet enriched or hyalauronic acid injections in last 6 months