1680s, "pertaining to muscles," from Latin musculus (see muscle (n.)) + -ar. Earlier in same sense was musculous (early 15c., from Latin musculosus). Meaning "brawny, strong, having well-developed muscles" is from 1736. Muscular Christianity (1857) is originally in reference to philosophy of Anglican clergyman and novelist Charles Kingsley (1819-1875), who rejected the term. Muscular dystrophy is attested from 1877.
You have used that, to me, painful, if not offensive, term, 'Muscular Christianity.' My dear Sir, I know of no Christianity save one, which is the likeness of Christ, and the same for all men, viz., to be transformed into Christ's likeness, and to consecrate to His service, as far as may be, all the powers of body, soul, and spirit, regenerate and purified in His Spirit. All I wish to do is, to say to the strong and healthy man, even though he be not very learned, or wise, or even delicate-minded--in the aesthetic sense: 'You, too, can serve God with the powers which He has given you. He will call you to account for them, just as much as he will call the parson, or the devout lady.' [letter, Oct. 19, 1858, to a clergyman who in a review had called him a "muscular Christian"]