Plenty of players have complained in recent years about low hits. Most would rather take a high hit (and a possible concussion) when defenseless than a low hit (and a possible knee injury).
Collectively, the players have not yet made a push to expand protections for defenseless receivers to include low hits. Per a league source, the NFLPA plans to study after the season ends all data regarding knee injuries before deciding whether to make a push for the establishment of a strike zone.
For more on this story visit: Mike Florio, NBC Sports Pro Football Talk NFL: Low Hits, Union Will Study In Offseason by Sports Talk Florida