His name is closely associated with the exploration and rediscovery of medieval music in the 20th century, particularly the compositional techniques of the Ars Nova and the isorhythmic motet.
He joined the faculty of Strasbourg University upon Jacobsthal's retirement in 1905, first as a lecturer, and in 1910 as an associate professor of music history.
Friedrich Ludwig belonged to the school of thought among cultural historians that did not ascribe to the Romantic view that Baroque Polyphony was the only type of polyphony of highest worth; rather, he sought to explore its historical development and evolution, leading to a critical reassessment of earlier music.
For this purpose, he used the philology of High Middle German, the Romance languages, and medieval Latin, the chorale, and historic chronicles.
Ludwig's contributions to musical scholarship include his investigations into Organum, deciphering early neumatic notation (square note notation), the discovery of Rhythmic modes in the unison songs of the 13th century, and the systematic representation of compositions of the Notre Dame School and the motets of the Ars Nova.