Dead Bug Variation with Swiss Ball
Good core exercises put you in a position where you have to work to maintain your neutral spine, this exercise does just that. Neutral spine is the natural position of the spine when all 3 curves of the spine-cervical (neck), thoracic (middle) and lumbar (lower) are present and in good alignment. This is the strongest position for the spine when we are standing or sitting, and the one that we are made to move from.
Use a 55cm stability ball, press your right knee and left arm into the ball, engaging your hip flexors and your lats, extend the opposite arm and leg being mindful of maintaining "neutral." Hold for 2-3 seconds and then repeat opposite side. Start with 5-6 repetitions and build to 15 repetitions.