Tackling below the waist

[Middleman]

I'll save you the trouble.

From the NFHS Football Rules:
Rule 2, Section 44 TRIPPING, "Tripping is the use of the lower leg or foot to obstruct an opponent, who is not the runner, below the knee."

Rule 9, Section 3 ILLEGAL BLOCKING, Art. 7 ..."A player shall not trip an opponent who is not a runner."

"Tripping" a runner (which by definition is not tripping) is a legal means of using "legs ... by a defensive player ... to bring him [the runner] to the ground." That is in the definition of a Tackle (see 2-41).