Bodybuilders have given us more than their fair share of memes. Who can forget “sun’s out guns out” or “do you even lift, bro?” and one of the all-time weightlifting classics, “Don’t skip leg day.” But unless you’re a dedicated gym rat, you might be wondering what exactly “leg day” is and why you shouldn’t skip it.

Let’s take a closer look at leg day — and how Apple Watch and various apps can help you build great “wheels” (bodybuilder talk for “legs”).