If you have access to a TRX or other suspension kit, don't underestimate body saws; they're much more challenging than they look. Set the handles around one foot off the ground, and place the feet inside while face down. Assume a plank position, and start with the eyes in line with the fists. Slowly allow the arms to "guide" your body backwards until the eyes are in line with the elbows. Be sure not to break form by letting the stomach fall and the back arch. Keep the hips slightly elevated like Dean Somerset shows in the video, and remember to engage the glutes. This movement will challenge the anti-extension capacity of the abs and strengthen the lower back in the process, which can fix walking issues and pelvic tilts, not to mention basic exercise techniques.