The body saw with Val Slides is a great way to progress you plank. You can use Val Slides, a slideboard, or even just saw your socks on a wooden floor. The sawing motion forces you to stabilize your midsection through a range of motion which is the true function of your core. This exercise helps to reduce lower back, improves your posture, and tightens your core, glutes, and upper back.
1) Assume a plank position on your elbows with your feet on the Val Slides.
2) Firmly push your forearm into the floor as your propel your feet backwards.
3) Pull yourself forwards using your lats and shoulder shoulders while maintaining a plank position.
4) Slowly return to the starting position and repeat for the desired number of repetitions
This exercise is great to use as a part of your warm up or insert it into the strength component of any workout. I often pair it with a lower body exercise or pulling motion in a circuit.