It seems the term “muscular Christianity” was coined in the 1850s in a review of a novel by Anglican priest and author, Charles Kingsley. Across the Atlantic, Theodore Roosevelt was a keen advocate (read more here).
It was an age where industry was mechanising many processes, leaving working people more time for leisure than before. There were also threats of war with several nations, and key voices of the day proclaimed the need to raise up young future leaders. These, they said, needed to combine the moral character of Christianity with physical strength and fitness.
A friend of Kingsley, Thomas Hughes, author of the much-loved novel Tom Brown’s Schooldays, distinguished between “musclemen” (athletes without Christian faith) and “muscular Christians”. “The only point in common between the two is that both hold it to be a good thing to have strong and well-exercised bodies. Here all likeness ends. [The Christian belief is] “that a man’s body is given him to be trained and brought into subjection, then used for the protection of the weak and the advancement of all righteous causes.”
The writers of the research paper discuss the role of Muscular Christianity thinking in, for example, the foundation of the Young Men’s Christian Association (YMCA) and even the modern Olympic Games, begun by Baron de Coubertin in 1896. They also cover opposition to the concept by equally weighty figures like the poet Gerard Manley Hopkins, who foresaw the physical emphasis outstripping morality and the aspects of the heart.
I recommend the article for your own reading and further study. I would also appreciate hearing any comments you may have.