Such blocks are banned due to the risk of injury, particularly those to the knee and ankle.
[1][2] The penalty for a block below the waist is 15 yards in the NFL, NCAA, and in high school.
[3] In the NFL, blocking below the waist is illegal during kicking plays and after a change of possession.
It was during the 1970s that the rules prohibiting these blocks were instituted in various leagues.
Blocking below the waist was initially banned in 1970 in the NCAA after a unanimous vote.