They are two ways to pose a chain of bones. Forward kinematics (FK) rotates each bone from the root outward and the children follow. Inverse kinematics (IK) moves the end of the chain and solves the joint angles to reach it.

When should I use IK vs FK?

IK suits arms and legs, where you care where the hand or foot lands. FK suits arcs and follow-through, like tails and antennae. Most character work uses both, often on the same limb at different moments.

Does After Effects have IK?

Not built in; animators use scripts such as DUIK. Premation has bones with a two-bone IK solver and FABRIK for longer chains, and IK/FK switching per chain.

Glossary

IK and FK

Name: Premation
Author: Premation

Two ways to pose a chain of bones. Forward kinematics rotates each bone and the children follow. Inverse kinematics moves a target and the chain solves to reach it.

Also called inverse kinematics, forward kinematics, IK/FK switch.

In After Effects

After Effects has no built-in skeleton or IK. Its Puppet tools bend a mesh with pins, and IK chains are usually added with third-party scripts such as DUIK, which build them from nulls and expressions.

In Premation

Per chain, in the Rigging panel. Switching between them keeps the current pose, so you can pose in IK and polish in FK.

Step by step

Select a single shape or image layer and press Ctrl/Cmd+B for the Bone tool.
Click to place the root, then click once per joint to draw the chain. Or use Auto-Rig in the Rigging panel (Window ▸ Panels ▸ Rigging) for a biped or quadruped with IK chains and controllers.
For IK, add a target at the end of the chain and move it; the solver finds the joint angles that reach it.
For FK, rotate a bone and everything downstream follows.
Switch the chain between IK and FK in the Rigging panel. The pose is preserved, and the mode itself can be keyframed.
Set the playhead and pose the rig; keyframes are written.

Tips

Use IK when the end of the chain has a job, such as planting a foot or holding a prop. Use FK when the motion travels outward from the root, like a whipping tail.
For a walk, keep a leg in IK while the foot is planted and switch to FK while it swings, so you are not fighting the solver.
Pose with controllers rather than grabbing bones; a controller links to a bone or IK target and the solver reads it directly.
If a joint pinches, fix the weights: the weight-paint brush for broad regions, Pick Vertex on the bone overlay to type one vertex's weight.

Walkthrough: 2D character rigging and animation

Read the docs: Bones and IK

Questions

What is IK and FK?: They are two ways to pose a chain of bones. Forward kinematics (FK) rotates each bone from the root outward and the children follow. Inverse kinematics (IK) moves the end of the chain and solves the joint angles to reach it.
When should I use IK vs FK?: IK suits arms and legs, where you care where the hand or foot lands. FK suits arcs and follow-through, like tails and antennae. Most character work uses both, often on the same limb at different moments.
Does After Effects have IK?: Not built in; animators use scripts such as DUIK. Premation has bones with a two-bone IK solver and FABRIK for longer chains, and IK/FK switching per chain.

Related terms

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