The BOSU cross-body crunch is a dynamic abdominal exercise performed on a BOSU ball or other stability training device. It targets several of the muscles in the core, including the obliques and the rectus abdominis or "six-pack" muscles. It is usually performed in alternating reps, either for time or a number of reps, as part of an ab-focused workout or a circuit involving body weight or the BOSU ball.