Bout Of The Flu. Severe Pains In Back, Upper Legs, Muscle Pain. Cured With Coffee. Cure? Symptoms?

My dad will be turning 80 and has always lived a healthy, active lifestyle. Earlier this year he suffered with a severe bout of the flu. He has recovered, but since that time wakes up in the morning with severe pains in the back of his upper legs. Deep-rooted muscle pain. He has found that if he has two cups of coffee, within half an hour the pain is gone and he is fine for the rest of the day.

He does not have pain during the night, just once he gets out of bed in the morning. He has seen a couple of doctors who suggested increased potassium intake and to stay hydrated. Neither of these have had such an impact on him like the coffee!! Have you heard of anything like this before?

Thanks,

Julia