How do I train my lower abs?

Working the “lower abs”, the “inner chest muscle” and the “outer back muscle” in isolation sounds fascinating, but achieving it through different exercise variations is more fantasy than fact!

The training of the rectus abdominis (the straight abdominal muscle) is often split into a “lower” and an “upper” part. EMG measurements do show certain differences in activation depending on whether you curl up from the torso or from the pelvis, but it's the whole muscle that contracts as one.

A muscle's shape is genetically determined and cannot be changed through training. A muscle's fibres run from its origin to its insertion. Like an elastic band, the muscle always contracts along the direction of its fibres, pulling the insertion and origin closer together. When a muscle fibre contracts, it contracts as a whole. A muscle fibre cannot be active only in the middle or only at the ends. That's why it isn't possible to build up a muscle in just one particular section of the fibre (beginning, middle or end).

It has not only never been proven but is actually unlikely that a narrow grip on the chest press really trains the medial (inner) part of the pectoralis major more effectively. You only need to look at the functional anatomy to see why.

So the inner part cannot be functionally distinguished from the outer part!

Likewise, it's probably not possible to distinguish between the lateral (outer) and medial (inner) fibre direction of the broad back muscle (latissimus dorsi). The fact that a narrow grip makes the “outer back” feel subjectively more heavily worked could, for example, be down to greater involvement of the teres major, which has almost the same function as the latissimus, but is located
a little further out on the back.

These claims, and many others like them, are probably best explained by taking a good look at the functional anatomy of the human body – which often speaks for itself.