Movement. It depends on which muscles are sore and when they are sore. If they only ache a day or two after strenous exercise, this is called delayed onset muscle soreness- doms- and is completely normal. Generalized achiness responds best to exercise- increased blood flow to the muscles brings them more oxygen. Stretching, ice, and heat can also help. See your doctor if not improving.
Diagnose - soreness. It depends why the muscles are sore. The best therapy for muscles is exercise! i can not emphasize this enough. But we need a diagnosis. Inflammation can couse muscle soreness. This could be due to polymyalgia rheumatica in older people - how old are you? This could be due to vasculitis. Of note polymyositis does not ache. It causes weakness! the most common cause is fibromyalgia.