my knee feels stiff after workout. could be an osteoarthritis flareup. what can I do

The short version: stiffness after a workout is usually normal joint response, not proof of arthritis, and exercise is the best-supported thing you can do for a knee that does have osteoarthritis.

A stiff knee after exercise is common and often just the joint's normal response to load. In people with mild knee osteoarthritis, a squatting exercise caused cartilage to compress slightly and then fully recover within about 15 minutes, with no significant difference from healthy knees [4]. That is the mechanism behind post-workout stiffness: fluid shifts in and out of cartilage. It is not automatically a flare.

If it is osteoarthritis, exercise is the treatment with the best evidence, not rest. A review of four trials covering 1,317 people found exercise reduced pain and improved function and quality of life [1]. A separate meta-analysis of 17 trials in 1,705 patients found resistance exercise reduced pain, stiffness, and improved function [3]. The stiffness outcome specifically improved [3]. Supervised exercise beat home exercise for pain and strength in one randomized trial [2].

What this retrieval cannot see: it did not surface head-to-head data on ice, bracing, or when stiffness signals something other than OA, like a meniscus tear or inflammatory arthritis. Those are real possibilities and this search does not rule them out.

My call: keep moving, do supervised strengthening two to three times a week, and treat brief post-workout stiffness as expected unless it lasts hours, swells, or locks. Confidence: moderate for exercise benefit in confirmed OA; low that your stiffness is OA at all, since no exam or imaging was done.