They hypothesized that the FDS retracts to the level of the A-1 pulley, where Camper's chiasm acts as a noose to ensnare the FDP tendon, diminishing FDP excursion. Flexor digitorum superficialis ...

[1] The mechanism of injury is thought to be forceful extension of a flexed digit. Avulsion of an isolated flexor digitorum superficialis (FDS) tendon is rare, and the exact pathology is uncertain.