The best muscle relaxer I know of is a very common hemeopathic remedy that can be found at most health food stores-- Magnesia Phos. It's cheap and it works!!!
It says on the front for Menstrual Cramps on the Hyland's container. But, it is used for any muscle cramps/ spasms/ pain that is relieved by heat--- back muscles, intestinal muscles, abdomen, chest, neck, shoulder, arms, legs and of course menstrual cramps.
My wife has used it for many years for hiatal hernia/ esophagus & back spasms. She has carried it in her purse for about 25 years and would not leave home w/o it --- cause you just never know when and where you may need it!!!