It is now held that, on the contrary, it was a novel system in England when it was introduced after the Conquest by the Normans, who relied essentially on their mounted knights, while the English fought on foot.
An allusion is made to this in the coronation charter of Henry I (1100), which speaks of those holding by knight-service as "militibus qui per loricam terras suas deserviunt" (literally "soldiers who serve [or are subject to] their lands by means of armour").
[1] The chief sources of information for the extent and development of knight-service are the returns (cartae) of the barons (i.e. the tenants-in-chief) in 1166, informing the king, at his request, of the names of their tenants by knight-service with the number of fees they held, supplemented by the payments for scutage recorded on the pipe rolls, by the later returns printed in the Book of Fees, and by the still later ones collected in Feudal Aids.
[1] The disintegration of the system was carried further in the latter half of the 13th century as a consequence of changes in warfare, which were increasing the importance of foot soldiers and making the service of a knight for forty days of less value to the king.
The barons, instead of paying scutage, compounded for their service by the payment of lump sums, and, by a process which is still obscure, the nominal quotas of knight-service due from each had, by the time of Edward I, been largely reduced.
[1] For the financial side of knight-service the early pipe rolls have been printed by the Record Commission and the Pipe Roll Society, and abstracts of later ones will be found in The Red Book of the Exchequer, which may be studied on the whole question, but the editors' view must be received with caution and checked by JH Round's Studies on the Red Book of the Exchequer (for private circulation).
[1] The existing theory on knight-service was enunciated by Mr Round in English Historical Review, vi., vii, and reissued by him in his Feudal England (1895).