You need to use this muscle during squats, lunges, overhead presses, planks, and more — but chances are, you probably aren't. It's the transverse abdominis — TVA, for short — your deep abs that extend downward from your lower ribs toward your pelvis and around your sides toward your spine. Essentially, the TVA is muscular corset. This large sheet of muscle is super important for stability, posture, pain prevention, and functional movement. And sitting for extended periods of time is not helping — so let's get into how to fix this.
Here's a quick tip: get into tabletop position on all fours. Keep your back still but pull your navel to your spine; think about someone trying to punch you in the low abs (near your hip bone) and try to tense that area. You're feeling your transverse abs engage, and it's even easier to feel it work when you're exhaling; try that same move but on a forced exhale. Another bonus: when the TVA is working, it decreases the diameter of your waist.
Maintain that tension in your abs (when the TVA engages the other abs tend to turn on too), and try Bird-Dog — reaching your right arm and left leg out — to see if you can keep your abs engaged throughout the exercise. These two moves are a great way to train your TVA and also to reestablish that brain-body connection. Keep thinking about engaging your deep abs while you go through your regular exercise routine and even while you're at your desk; it's a great way to keep your muscles and your posture healthy.