Can you train the lower abs in isolation?
Can you train the lower abs, build the inner chest muscles and shape the outer back muscle? It all sounds fascinating, but with different exercise variations, it's more of a pipe dream than a realistic goal.
The rectus abdominis (the straight abdominal muscle) is often divided 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. In the end, though, it's the whole muscle that contracts.
The shape of a muscle is genetically determined and can't 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, trying to bring the insertion and origin closer together. When a muscle fibre contracts, it contracts as a whole - it can't be active only in the middle or only at the ends. That's why it's not possible to build up a muscle in just one particular section of the fibre (beginning, middle, end).
So the lower abs are never trained in isolation.
Not only has it never been proven, it's actually unlikely that a narrow grip on the chest press really does train the medial (inner) part of the pectoralis major more effectively. To understand why, we need to look at functional anatomy.
The inner part cannot be functionally distinguished from the outer part.
Similarly, distinguishing the lateral (outer) and medial (inner) latissimus dorsi (the broad back muscle) by its fibre direction is probably not possible either. The fact that a narrow grip makes the "outer back" feel subjectively more engaged could, for example, be down to greater involvement of the teres major, which has almost the same function as the latissimus but sits a little further out on the back.
