The 1896 diagram of Apatosaurus excelsus drawn out by Othniel Marsh depicts an animal with a very smooth body curvature from the head to the tail. The skeleton, according to Marsh, was not very rigid, making the tail and neck droop and leading to the assumption that the neck and tail were not heavily muscled; meaning that they were lacking the proper amount of muscle to hold them parallel to the ground. The neck, though parallel in this image, was assumed to possess an ability to snorkel, being held at a nearly perpendicular angle to the ground. Since that description of the neck was published it has been discovered that the bones of the neck, in order to sit at the perpendicular angle, were positioned incorrectly or broken. The much more parallel orientation is much closer to an actual articulation and "comfortable" posture for these dinosaurs. Additionally, the tail has been changed in posture as well, but that cannot be seen in this image.
The tail, as stated before, has been changed and here captured by Scott Hartman exactly as described. This specimen of Apatosaurus ajax has an upward kink at the base of the tail. The tail then extends horizontally posterior rather than deflecting downward until it drags on the ground. The tail musculature, and rigidity through tendons and ligaments, allowed for the tail to be used as a counterbalance to the neck and also as a weapon system in a pinch. The kink at the base creates a more stabilized area for muscle attachment above the pelvic girdle without fusion of vertebral neural spines in this area; this is not expressly noted in any paper I have read, but the arrangement suggests a non-fused but larger attachment surface when looking at it in profile.