Inverse Kinematic (IK) chains can be created for bone structures. For example, the paperdoll's two arms and two legs all have IK chains, which can be turned on and off. The IK chain for each arm includes the shoulder, forearm, and hand bones. The IK chain for each leg includes the thigh, shin, and foot bones. By default, the IK for the arms are off, and the IK for the legs are on.
When IK is off, the bones move according to the bone hierarchy. For example, with the arms, you rotate or bend the shoulder, and the forearm and hand follow the movement. This follows the hierarchy of the bone structure, which runs from the shoulder to the forearm to the hand. When IK is on, the bones can be said to move in the reverse order of the hierarchy. For example, with the legs, you move the foot and the shin and thigh follow. This is the opposite of the hierarchy of the bone structure, which actually runs from the thigh to the shin to the foot.
For the arms, if you turn on IK, then the arms will begin functioning like the legs. The major movements will be controlled by altering the position of the hands. The forearm and the the shoulder would follow—just as the shin and the thigh follow the foot. However, the bending of the shoulder and forearm often look unnatural, and these body parts will need generally some alteration of the rotate, bend, and twist values.
There are generally two instances when I turn on IK for the arms. The first is when I want to precisely control the movement of the hand. I have done this with spear/sword thrust animations, but I primarily do this with archer animations. When the unit is ready to draw the bow, I want the hand to pull the string straight back. Therefore, I turn on IK for the arm pulling the string, and I move the hand backward in a straight line usind the hand's zTrans parameter dial.
The second instance when I'll turn on IK for an arm is when I want a hand to remain on an object. This is useful for two-handed weapons. You can parent the hand to the weapon, and wherever the weapon goes, the hand will remain in place. The exception is when the weapon moves so far away that the hand cannot reach; then the arm will stretch as far as possible but will eventually not reach anymore. This is also the situation you have with your cannon and the technique you would want to use. For the run animation, you would want to turn on IK for the figures' arms and place the hands where you want them on the cannon. Then parent the hands to the cannon. Then you could move the figures' hips, chests, etc., and the hands would remain unmoved in relation to the cannon. You could also bounce the cannon a little as it rolls and the hand would remain in place.
I used this technique with my Chinese Rocket Team unit. There is one figure pulling a wagon. Each of his hands are parented to one of the wagon's poles. When the wagon is rotated on its axle, the figures hands moved accordingly. I did this when the unit placed the poles on the ground, for example. I rotated the wagon until the poles touched the ground; the figures' hands followed. Then I was able to see how far I had to lower and bend the figure's hips, abdomen, and chest so that the figure could still reach the poles on the ground.
