Runtastic already offers dedicated apps for ab and bun workouts, and now the company takes aim at your lower half. Leg Trainer delivers over 50 exercise videos that'll allow you to select activity based on goals and fitness level. There are pre-selected routines -- like the 7-Minute Workout or Junk in the Trunk (yes, seriously) -- or you can choose individual exercises to build your own. Get ready to squat, lunge and kick your way to toned legs. Similar to the Six Pack and Butt Trainer apps, a 3D avatar guides you through the process so you know exactly what you should be doing. The Leg Trainer app also works with the Apple Watch, so you can follow the virtual trainer there or keep tabs on progress and workout stats. It also leverages the wearable to let you know when to start/stop a set and when the rest time between sets is over on tops of tracking heart rate. Perhaps the real question is what you gon' do with all that junk? All that junk inside your trunk?