I first came across this exercise in the book Underground Secrets to Faster Running by Barry Ross. No matter how strong you think your core is, Ab 45s will humble you! Here’s how you do it.

Lie on your back with your knees bent and hook your feet under something stable that won’t move, like a machine, two heavy dumbbells, or get Helga the “strong as bull” trainer and part-time dominatrix to hold your feet down! Try to get your butt as close to your heels as possible. Extend your arms out fully overhead and keep them in line with your torso throughout the set.

Raise your head and trunk up to a 45-degree angle. Make sure to bring your arms along with you, again keeping them fully extended and in line with your torso. Hold that position for a 5-second count, then lower back to the floor. Take a breath at the bottom and repeat the process.

Build up to 5 sets of 5 reps with 5-second holds, and gradually work your way up to holding each rep for 10 seconds.