Weighted Dead Bug

Extend your arms straight overhead holding the weight, lying on your back with your knees bent to 90 degrees and shins parallel to the floor. Press your lower back flat into the ground and keep it there as you lower one arm and the opposite leg toward the floor, stopping just before your back wants to arch. The weight overhead makes it harder to keep your ribs down as your arm travels back, which is exactly what it's meant to challenge. Hold the extended position with tension through your midsection rather than letting your limbs go slack, then return to center before switching sides.