If one has tension and stress and backache can one get a massage of the back, shoulders and neck by the opposite gender?
It was narrated that Ma’qil ibn Yassaar said: the Messenger of Allah said: “For one of you to be stabbed in the head with an iron needle is better for him than that he should touch a woman who is not permissible for him.”
Narrated by al-Tabaraani in al-Kabeer, 486.
Medical treatment by the opposite gender is 'permitted' during a medical emergency incase a doctor of the same gender isn't available. Islam differentiates between different options in a given situation (eg: choosing the woman over the yet to be born child incase of a medical emergency when all options have been exhausted).
Afaik back massages typically are done for non-fatal reasons and it is usually easy to find a masseur of the same gender.