Last week, had the odd experience of not being able to get my posture right.
I realised how hard it was for me to sit on one machine simultaneously and concentrate on a) having my legs extended and feet apart b) reach forward and grab the arms but to *at the same time* keep my back straight and shoulders back.
I kept leaning forward (rather than simply reaching whilst maintaining this correct posture)
I kept being stopped to correct my body position, because that wasn't isolating the correct set of muscles (in this case lower back), rather I'd end up pulling with my upper back and shoulders making the task 1) disproportionately easy 2) a complete waste of time for training the lower back.
Several attempts later I finally 'got it' (I think) but my position felt really unnatural and awkward.
The moral of this tale is: it just reminded me what a trial it is trying to program my brain to accept an new motor skill.