Very few heavy rock bands were as good as Led Zeppelin in adding contemporary funk and R&B influences in their heavy-guitar maelstrom. While Sly & the Family Stone and Stevie Wonder had been fusing rock & roll and R&B under the "funk" catch-all for years, and rock & roll bands like the Rolling Stones and Faces had always heaped a healthy dose of boogie into their rock, few hard rock/heavy metal bands seemed to care as much about danceability and the sexy side of rock & roll as Zeppelin.
Taking the first half of Wonder's classic "Superstition" riff, Clavinet keyboard and all, John Paul Jones manually loops and lays into it, providing "Trampled Under Foot"'s incessant rhythm and theme. John Bonham provides his pummeling as a stalwart foundation, while Jimmy Page doubles the main riff on distorted guitar, layering multiple other, part-specific guitar textures throughout the arrangement. The lick is unrelenting. Indeed, there is only one main change, the wah-wah guitar seventh chords that ascend at the end of the refrains -- there is no real chorus, per se. Only a midsong breakdown, repeated once, offers respite from the assault.
Vocalist Robert Plant offers his predictable bluesman sexual innuendo, though this time he seems inspired by T. Rex's Marc Bolan, riffing on a woman-as-hot-rod metaphor: "Greasy slicked down body/Groovy leather trim/I like the way ya hold the road/Mama, it ain't no sin/Talkin' 'bout love/I'm talkin' about love/I'm talkin' 'bout/Ooh, trouble-free transmission/Helps your oil's flow/Mama, let me pump your gas/Mama, let me do it all." There must be something ironic about the lyrical content being assembled under the songs title, a reference from the New Testament: "You are the salt of the earth; but if the salt has become tasteless, how will it be made salty again? It is good for nothing anymore, except to be thrown out and trampled under foot by men." (Matthew 5:13) "When you see Jerusalem surrounded by armies, then recognize that her desolation is at hand...and Jerusalem will be trampled under foot by the Gentiles until the times of the Gentiles be fulfilled" (Luke 21:20, 24b). How any of this ties into Zeppelin's song is anybody's guess. The title probably just sounded cool at the time.
The Zeppelin recording is a heavy stomp, due in no small part to Bonham's trademark pulverizing style. But rarely has a hard rock band showed as much attention to the hip-shakeability aspect of rock & roll as to overloaded guitars, shrieking vocals, and loud drums. Only Aerosmith and AC/DC come to mind as other bands who rocked as hard, while retaining the same amount of boogie. Generally you get either one aspect or the other. Plant tempers his high-register vocals with an Elvis Presley-like quivering croon, a chesty bellow. There is a great tape-echo-like stereo delay effect on his vocal track. Bonham seems to have learned something from the polyrhythmic beats of James Brown and his drummer John "Jabo" Starks. And, of course, Jones almost completely apes Wonder's heavy Clavinet riffing, playing plenty of off-beat licks.