Abs are not built with crunches. What produces a firm stomach and a strong back is core stability – the ability of the abdominal girdle to resist movement rather than produce it.

The ab wheel is by far the most effective tool on the market for this. A single movement engages the rectus abdominis, obliques, transversus abdominis, lats, and shoulders. It is also one of the most demanding: start on your knees, with a short range of motion, and only extend the movement when your back remains perfectly stable.

The classic mistake to avoid: arching your lower back at the end of the movement. If your lower back sags, you've gone too far. Reduce the range of motion rather than forcing it – it's the best way to progress without injuring yourself.

A mat of at least 4 mm is highly recommended: floor work puts a lot of stress on the elbows, knees, and hips.