Most fitness apps for iOS will teach you to stretch and exercise through specific demonstrations, but iMuscle takes it a step further by actually showing you the muscles in action. It works by providing a ton of animated diagrams that are accessible with just a touch.
When you launch the app, you just grope the part of the muscular body you want to work on and it'll stick a few pins in it. Each pin corresponds to a muscle (or muscle group) and its associated exercises and stretches. Touching any of the exercises or stretches will provide you with an animated demonstration, written instructions, and pictures of the affected muscles.
In addition to just getting a bunch of handy information, iMuscle lets you create users and assign each user workouts, so you can make a plan for yourself or share the app with others. As your perform your workouts, you can mark your progress in the app and see how you're doing.
Overall, iMuscle is a pretty handy guide with lots of useful information. The only downside to the iPhone app is that it's pretty much a scaled-down version of the iPad version so some of the buttons are a bit small and tough to touch accurately. Other than that, it's a great option for learning to exercise and stretch specific muscles in your body — or many of them, as the case may be.