Bicycle crunches are a way to exercise the abdominal muscles without having to use any equipment, according to the American Council on Exercise. Some people choose to use a mat while doing these exercises, but that isn't a requirement.
To do bicycle crunches, a person must lay flat on their back with both feet planted on the floor and hands behind the head. Lift both legs while keeping the knees bent. Pull one knee toward the opposite arm pit while lifting the arm and shoulder on that side. Use the abdominal muscles to lift the upper body and head while the arm and shoulder is up. Go back to the starting position and repeat the movement using the other knee and shoulder. Alternate back and forth for 10 to 20 repetitions, instructs 30 Day Fitness Challenges.