Just ask Kardashian's trainer, Gunnar Peterson, who helped the truth star form up her well-known derriere. Of the various fitness strikes Kardashian does throughout their training periods, Peterson recommends lunges. "On a twisting lunge, you're working transverse plane, rotational plane," he explained in a Khlo-Fit video on the star's website. "You're blasting calves, you're hitting glutes. Up here [in the monkey row] you're getting shoulders, and obviously a little bit of biceps, plus by being upright, you’re automatically activating your core."