Heat therapy can relax stiff muscles and help ease joint pain. Heat therapy is recommended for: soreness from over-exertion; and pain and stiffness that are caused by cramping / spasms.
Heat wraps, patches and packs are commercially available. A home-made version can be created by soaking a towel in hot water, and then wringing it out. Or, a towel can be soaked, wrung out, and then placed in a clothes dryer or microwave until very warm.
As dogs (and humans) get older, the walls of the blood vessels lose their elasticity, and circulation decreases. Additionally, the fat layer under the skin tends to shrink, and insulation from the cold becomes less effective. Interestingly, a large human study also found that internal body temperature slightly decreases with age, which presumably is true for other mammals like cats and dogs.
All of this means that older pets can be more sensitive to cold than younger animals. Heat therapy - heating pads, heating wraps / packs, and even heated clothing - can bring lots of comfort to our senior companions.