The role of the muscles of the erector spinae - multifidus, longissimus and iliocostalis – is spinal stability, not to extend or move the individual joints that they cross over. The Individual muscles are in the wrong position with the wrong angles of force to extend the joint efficiently. They are too weak, with small tendons that attach to the thin bone of the spinous process and transverse process. Muscles that pull to flex and extend the joints they surround are much bigger, with strong tendons and strong attachments, think biceps and triceps for example.
