Muscular Christianity is a movement, originating in nineteenth-century Britain, associated with promoting physical health, athleticism, and active evangelism. Many critics say that C. S. Lewis, particularly in his fiction, drew from this tradition:
... Lewis's creed of clean-living, muscular Christianity, pipe-smoking, misogyny, racism, and the most vulgar snobbery. (Philip Hensher, The Independent column of 4 December 1998)
Not surprisingly, some readers embrace Narnia's pagan flavor in preference to its muscular Christianity. (Naomi Wood, God in the Details, in Revisiting Narnia, ed. Shanna Caughey, 2005)
... his polemical, somewhat muscular, orthodox Anglican Christianity ... (Terence Brown, C. S. Lewis: Irishman?, chapter in Ireland's literature, 1988)
Did Lewis have anything to say about "muscular Christianity" as an idea, and did he associate himself with the concept? I would certainly count him as a forceful defender of Christian ideas, but not as someone particularly interested in physical fitness.