Fallon and Johnson pretended to be The Fungo Brothers -- tube sock-clad siblings who apparently were using the Thighmaster, or the "Thighcruncher" as they call it, before everyone else.
"You'll know it's working when you feel the burn between your legs," Johnson explained, as the audience howled. Fallon didn't have quite as much success.
"I'm cramping up," the funnyman said.
Later on, Fallon pulled up another clip of The Fungo Brothers, this time as they were feeling the burn while dancing to Chubby Checker's 1960 classic, "The Twist."
The pair also made more than a few double entendres about the oft-parodied Shake-Weight device, as seen in the below video.
"Hercules" opens in theaters on Friday.