What are the different variations of crunches and how do they target different muscle groups?

There are several variations of crunches, including standard crunches, reverse crunches, bicycle crunches, and oblique crunches. Each variation targets different muscle groups in the core. Standard crunches primarily work the upper abdominal muscles, reverse crunches target the lower abs, bicycle crunches engage the obliques and rectus abdominis, and oblique crunches focus on the side muscles of the core. Varying these exercises can help to strengthen and tone different areas of the abdominal muscles.