Muscle outlines from Body Muscles by Ivan Vulović, Apache 2.0 — regrouped and recoloured for Muscle Train.
Hang from a bar with an overhand grip, hands shoulder-width apart; ideally, you should be able to hang with your body fully straight without touching the ground. Raise your straight legs until your toes touch the bar. Smoothly lower them back down, keeping your legs straight and trying not to swing.
Try not to use momentum: it's better to raise your straight legs short of the top but learn to do the movement correctly. If you can't do the exercise correctly yet, start with leg raises to an L-position or with knees-to-elbows.
Muscle Train is actively being built. If something looks broken, or you have a suggestion, let us know — it really helps shape the app.