Fore-arm and hand, different positions I
The execution of the hand in art is frequently poor. Before starting to draw or paint, it is wise to have a close look at its form, how it closes and how it opens up again, how the fingers are built, if there are marks relating to the professional activity of the person etc. Once the general outlay is created, it is helpful to compare for basic accuracy with a presentation in a mirror setting.
The white lines in figures 3,4,5 point to muscles involved in the extension and abduction of the thumb over-passing the tendon of the brachioradialis.