After nearly 20 years teaching fitness and a life practising sports, I have to say one of my favourite exercises to strengthen the abdominal muscles while teaching your body stability and strengthening your lower back is the Pilates 100 exercise.
This exercise is part of the Pilates method which targets your transverse abdominals. These muscles wrap around like a belt in front of your hip bones and around the small of your back. Think of these muscles like your own internal corset that holds in your internal organs and helps you sit, stand and walk every day.
The 100 targets those transverse abdominals, helping to achieve a stronger abdominal wall and a flatter stomach. What a winner!!!