Day 13 works your abs from every angle it can find in two minutes. The circuit is crunches, cross crunches, cycling crunches and leg circles - 30 seconds each - and the four together move through straight-line flexion, rotation, alternating rotation and a full circle, in that order.
The progression across the card is one of increasing coordination. Crunches are the simplest thing your abs do: shoulders up, shoulders down. Cross crunches add a twist to that same curl. Cycling crunches add the legs to the twist, alternating elbow toward opposite knee while the other leg extends long - now three things are happening at once and your midsection has to run all of them. Leg circles then remove your upper body from the job entirely: with the shoulders lifted and both legs held straight off the floor, you draw slow circles with the feet, and the abs work continuously to keep the lower back down while the legs travel through positions that keep trying to pull it up.
Keep the crunches honest - a small range done properly beats a big one powered by your neck. In cross crunches, rotate the shoulder toward the opposite knee rather than the elbow. In cycling crunches, slow down enough that each extension is complete; this exercise is usually done twice as fast as it should be. In leg circles, keep both legs together and the circles smooth, and stop the circle small enough that your lower back never leaves the floor.
Straight lines, diagonals and circles - your abs get worked from every direction on the compass.
Make it Harder: Widen the leg circles and take the bottom of each circle lower toward the floor.
Make it Easier: Make the leg circles small and keep them high above your hips.
