Muscle outlines from Body Muscles by Ivan Vulović, Apache 2.0 — regrouped and recoloured for Muscle Train.
Grip the bar firmly with one hand and hook only the fingers of the other hand over the bar next to it. Hang with your working arm straight, then pull your shoulder down. Pull with the working arm until your chin is above the bar, and press with the helping fingers only as much as you need. Lower under control to a straight arm and repeat, then do the same on the other side.
Let the working arm do the pulling; the other hand only helps where you get stuck. Use fewer fingers on the helping hand as you get stronger. Keep your body close to the bar and don't let it twist. Start every rep from a straight arm and don't drop at the bottom.
Muscle Train is actively being built. If something looks broken, or you have a suggestion, let us know — it really helps shape the app.