Which single exercise can you recommend the most to train the lower abs at home without any required tools?
Sure, you can do dead bugs or similar exercises. A tip while doing these is to ensure your lower back is pressed into the floor at all times, don't let it arch (otherwise the hip flexor takes over the movement instead of the lower abs).
I like the following (called a leg drop or something):
- Lie down and then raise your legs towards the ceiling. Tighten abs.
- Slowly lower your legs until they’re a few inches above the floor (or as low as you can go without lifting the small of your back).
Repeat. Try to get a breathing pattern down (exhale when lowering legs works well). Do these until you feel the burn, count the reps and monitor how many you do as you progress.
(note, if lowering both legs is too hard you can lower one at a time, or you can bend at the knee. There are lots of ways to make it easier)