Fill a tube sock with cheap, plain, completely raw rice, tie it off so the rice can't come out, microwave for 2-3 minutes and use as a heating pad. You can add few drops of essential oil if you'd like, which will help with the burnt popcorn/Brussels sprout smell that builds up over time. Follow the protocol for heat therapy- only use in 10-20 minute bursts, or the stiffness could be worse the next day. If your pain and stiffness continue for more than a week or gets worse, it's time to see a doctor.