IK Targets and Reach

1The problem IK solves

Say you want a character to touch a doorknob. The forward way is to rotate the shoulder bone, then the elbow bone, then the wrist bone, adjusting each in turn until the hand lands. Change your mind about where the hand goes and you redo all three. That is forward kinematics, and it is exact but painfully slow for limbs. IK lets you skip straight to the goal: move the hand, and the joints above it sort themselves out.

2A quick refresher on bone chains

IK works on a chain of connected bones: shoulder to elbow to wrist, or hip to knee to ankle. You build these with Add Bone, dragging from the start of each bone to its tip, where the selected bone becomes the parent of the next. The top bone of the chain is a root or attaches to one, and each bone below inherits from the one above. IK needs this clean parent-to-child chain to know what bends when.

3Tip-drag with Manipulate Bones

The quickest taste of IK needs no setup. Pick Manipulate Bones and drag a bone: dragging its body rotates it and the children follow, but dragging its tip runs IK toward your pointer, bending the whole chain to chase where you drag. So grab the tip of the wrist bone, pull it toward the doorknob, and the elbow and shoulder flex automatically to make the hand meet it.

4Set an IK target on the end bone

For lasting reach, give the chain a goal it always chases. Select the end bone of the chain (the wrist or the ankle), open the Bones panel, and set its IK target field from None to the bone that should act as the goal. Now moving that target bone bends the whole limb to follow it. Instead of posing three joints, you pose one target and the arm obeys.

5Move the target, bend the limb

With the target set, animating the limb becomes simple: you move only the target and the elbow and shoulder solve themselves to keep the hand on it. Slide the target across the stage and watch the arm extend, fold, and swing to stay attached. This is the payoff of IK. One control drives a whole limb, and your keyframes live on the target instead of scattered across three bones.

6Reach farther with Max stretch

Sometimes the target sits just out of reach and the limb comes up short. Once an IK target is set, the Max stretch field appears in the Bones panel. Set it above one hundred percent and the chain is allowed to lengthen slightly to close the gap, so a hand can stretch to grab something a hair too far away. It is a small, elastic cheat that keeps a reach from looking stiff and stubby.

7Plant feet and hands with the target

IK is not only for reaching, it is also for staying put. Set an IK target for each foot and, while the character's body moves, keep that foot's target pinned to the ground. The leg then bends and straightens automatically to keep the foot planted, so it does not slide like it is on ice. The same trick pins a hand to a table or a grabbed rope while the rest of the body shifts around it.

8Keep elbows and knees bending the right way

IK sometimes solves a chain into a backward elbow or a knee that folds the wrong way. Guard against it with Limit rotation on the joint bone: tick it, then set a Min angle and Max angle so the elbow can only bend within a human range. Now no matter where the target goes, the joint stays anatomically sane. It is a one-time setup that saves you from chasing flips forever.

9Pose the target and key it

To animate with IK, you pose the target rather than the limb. Use the Pose tool to drag the target bone on the stage, and with Auto-key on it keys at the playhead automatically. Step forward, move the target to its next position, and it keys again. Because the limb solves to the target, you are effectively animating a hand path directly, which is a very natural way to think about a reach or a punch.

10When to reach for IK, and when not to

IK is the right tool when the end of a limb has a job: touching, holding, planting, or landing on a mark. It is the wrong tool for loose, floppy motion like a waving arm or dangling hair, where forward posing or bone dynamics feels more alive. A good rig often mixes both: IK on the legs so feet plant, forward posing on an expressive gesturing arm. Pick per limb, not per character.

11Troubleshoot flips and pops

If a limb suddenly snaps or flips mid-move, IK has found a second valid solution and jumped to it. The cures are the ones you already have: Limit rotation to forbid the bad bend, a slight pre-bend in the rest pose so the joint knows which way to fold, and keeping the target from crossing straight through the root. A chain that starts already slightly bent almost never flips.

12What good reach looks like

Good IK is invisible. The hand lands exactly where it should, the feet grip the floor without sliding, and the elbows bend like elbows, not like broken sticks. There is a touch of stretch when the character truly strains for something, and everything settles without a pop or a flip. When a reach reads as effort and a step reads as weight, the IK is doing its quiet job.

13Build the habits that make IK easy

Three habits make IK painless forever. Build clean chains with correct parenting so the solver has a proper ladder to work down. Pre-bend every joint slightly in the rest pose so it never flips. And set Limit rotation on elbows and knees the moment you rig them, not after they misbehave. Do these three things up front and IK feels easy instead of a fight.

IK questions