Upper and Lower Extremity Exercise and Exercise-Induced Hypoalgesia in Knee Osteoarthritis

Participants with knee OA (Target n=40) Inclusion Criteria: We will recruit participants with knee OA, using the National Institute for Health and Care Excellence's clinical diagnostic criteria, which does not require radiographs. The clinical diagnostic criteria include: * age ≥45 * activity-related knee pain * either no morning joint-related stiffness or stiffness that lasts ≤ 30 minutes. * knee pain at least 4 on a 0-10 pain scale * knee pain as a chief complaint if having multiple pain * understand English Exclusion Criteria: * peripheral neuropathy or other sensation loss on the body sites for pain measurements (i.e., the wrist, knee, the forearm) * use of medical devices electrically active (e.g., pacemaker) * chronic use of opioids * pregnant women * serious and uncontrolled concomitant disease, including cardiovascular, nervous system, pulmonary, renal, hepatic, metabolic, hormonal, endocrine, gastrointestinal or epileptic disease * rheumatoid arthritis, ankylosing spondylitis, and any neurological disorders that prevent the study procedure * cognitive impairment * history of a knee replacement surgery * inability to perform exercise due to severe pain or other symptoms * any intervention procedures for knee pain in the last 3 months Inclusion and exclusion criteria for pain-free controls (Target n = 20) Pain-free controls are not having a pain-related medical condition. They have to speak and understand English. Pain-free controls are excluded if they had experienced a pain episode, caused by musculoskeletal injury or otherwise in the previous 3 months.