If you sit all day, you'll develop a big waistline (sources: Live Science, NPR, and US National Library of Medicine). The main way to prevent that is to move around often, even if it's just for a few mins .
Many of us are in similar situation as you (sitting for an extended period); so, we have an idea what you are going through.
To keep a flattened belly all day, try the following tricks:
- Sit straight in your seat all day. Try not to rest your back on the chair's seat or pull it forward. By doing this, you naturally engage your abs muscles. If you're sitting straight, you should feel the muscles contracting. Rest whenever you can, but try to get back in that position as much as you can. Whatever you do, resist the urge to slouch your back. It's bad for your back and waist line.
- Every hour or two, take a break and stretch your legs and back. If possible, perform at least 10 knee jumps to keep your juice flowing and work your abs muscles (at least once a day).
- Also, you can do sitting crunches on your chair. Rest your back on the chair (make sure your back doesn't actually touch the seat's back), stretch your legs (ensuring they don't touch the ground), put your hands behind your head, and do about 10 or more crunches. Do this as often as you can.
- When you get up (during break, rest, or lunch), perform (at least 20) standing crunches.
- If you're strong enough and not tired, you can perform some crunches, planks, or reverse crunches.
Do all those exercises intermittently during the day and your stomach will feel tight all day.
Tidbit: if you can add bodyweight exercises (planks, pull-ups, chin-ups, knee-ups, push-ups, reverse crunches,) to your weightlifting sessions, you should feel your abs muscles tightening more and more.