Probably the single most common pitfall we have seen in the early posing assignments is a character rig with a painful-looking, broken back. In an effort to strengthen the line of action of the spine and try to create a more dynamic pose, many students, ourselves included when we started, will start to play around with the spine controls blindly and push things here and there hoping for the best. But unless you keep the basics of anatomy in mind, the most likely result will be a spine that may have a strong line of action, but ends up feeling like the character’s back was trampled by a herd of wild horses. (Okay it won’t be that bad but the end result would be the same health wise)
Anatomy for the sake of Animation
To begin understanding how the problem occurs, we must first take a look at the anatomy of our character. To the right, is an image of Stewart with an approximation of his skeleton as well as where the FK spine control curves (the red curves) are placed on the rig. The first thing to pay attention to is that the two top FK controls lie directly on the character’s ribcage. This means that if we were to take those controls and over rotate them, we would in effect be breaking poor Stewart’s ribs, which in reality would most likely end up puncturing some internal organs. Not a pleasant way to go.
This doesn’t mean that you should stay away from these controls. The ribcage is designed to be flexible, otherwise it would simply be a solid bone. This makes these controls great for pushing the line of action in a character, but they should be used carefully. 
Continuing down the spine to the 3rd FK control curve, we come across the ideal control for getting the most rotation in the spine. This curve deals with the lower back which is where we humans bend from and without a ribcage or pelvis to get in the way, it gives us a very wide range of motion (we definitely recommend you stand up at this point and try bending and rotating to see just how your spine feels and where it bends from. Your body is your own best reference when it comes to doing character animation!)
With these top 3 controls you can now get most of the spine shapes that you’ll need. Just keep in mind that the higher you go in the spine, the less it will rotate. This means that the lower back control will typically have the most rotation of the spine, followed by the middle control, and the top control will have the least amount of rotation.
And lastly we have the 4th FK control and the COG (the wobbly shaped one around the hips) control curves which both deal with the hips. Now these cause a lot of confusion for many students starting out since they both control the hips. Dealing with these two controls (the hip and the cog) comes down to personal preference. In our experience, we like to use the COG for all major hip motions and animation and only as a last resort or as a way of pushing a pose will we touch the hip FK control. This method tends to give much more natural motion and poses since the COG rotates the hips as well as the entire spine which is how humans work, while the hip control only moves the hips which can cause the body to start feeling disconnected.
Speaking of the hips, this is where we will get the absolute most rotation in our body. For example if you were to bend over to touch your toes, your back bends quite a bit, however without the hips, you would be lucky to get your hands past your knees. The hips will rotate the entire body forward, back, or side to side in huge amounts and unfortunately are often forgotten by students. The hips are the root of nearly all our movement and are often called the driving force in the human body, so they should be one of the first controls to start with on most poses.
Recap
In short, what all of what we just said means is that:
- The hips are our main rotation point so the cog control is the go-to control for getting the body rotated and into position.
- The lower back control is the most flexible portion of the spine and will rotate the most of all three spine controls
- The middle control is positioned near the base of the ribs which gives it some flexibility however it won’t be able to rotate as much as the lower back.
- The top spine control will have the least amount of rotation because it’s positioned near the top of the ribcage where our spines are the least flexible.
This has been quite a list of rules and guidelines and by now rotating the spine might seem like a complicated ordeal. Luckily there is a simple trick to getting a solid base for a pose. Simply select all three spine controls and rotate them together and you’ll get a solid starting point for your pose without having to fight all the controls separately. Then once you have the pose you like, all you need to do is reduce the top spine control and push the lower back control and you should have a nice clean back pose with a strong line of action.
IK Spine
Another common pitfall of newcomers to the Stewart rig is posing out both the IK and FK spine controls of the rig. By default Stewart has two sets of controls for his spine. There are the FK controls which we talked about above (the red control curves) and the IK controls (the yellow control curves). In almost any shot you’ll always want to choose one set of control to animate and leave the others alone. This will help to avoid any counter animating and broken poses, plus it means having to use less controls!
When first starting out, we highly recommend sticking to the FK spine since they function much more closely to the human spine, allowing you to create more naturalistic poses much more easily.
The IK spine on the other hand tends to be much faster to pose out, however it often leads to stretched or squashed poses if you aren’t careful and can be very difficult to manage if you aren’t used to how IK works.
Conclusion
We hope this has been helpful to you and hopefully makes the spine a bit more manageable.
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