FK vs IK: Which to Use and When

FK vs IK. This is a question every animator stops to ask themselves. Knowing when to pick Inverse Kinematics (IK) or Forward Kinematics (FK) is essential to creating good animation. One of the fastest ways to clean up arcs, lock contacts, and speed up blocking is knowing when to use which and when to switch between them. Here’s a focused guide you can use on your next shot.

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What are FK and IK?

In animation, this is typically found on various body parts like limbs and throughout this blog post we will use an arm as an example. To an animator's perspective FK and IK appear as different types of controls on a rig that act differently.

Forward Kinematics (FK)

Forward Kinematic is the motion used in a chain of objects to obtain a specific position and orientation of the last item in the chain.

Take an arm as an example. If you want to wave, you might move your shoulder to raise your entire arm. Then your elbow to create a bend, to not raise your arm too high. Lastly, you rotate your wrist. When doing this on a character rig, you are animating using forward kinematics (FK). As you move the top element the subsequent elements of the chain are repositioned.

Inverse Kinematics (IK)

Inverse Kinematics is the opposite. It is when the motion of the last object in the chain, called the end-effector, is used to rotate the values of its parent objects to conform to the location you want the arm to be in.

In animation, there would a be a control on the wrist that you translate. While doing this you will see the elbow and shoulder rotate automatically to what the computer calculates is the best position based on the wrist location. Most of the time there is a second control pulled away from the elbow called a pole vector control. This is to help the calculation determine where the elbow should face during the automatic rotation.

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When To Use FK Versus IK

Know that we know what FK and IK are, the big question comes. When do we use them.

When To Use FK vs IK

FK is best used when you want to create natural and fluid arc motions. It is best when you want to exaggerate overlapping action. (If you do not know about the 12 principles of animation, we have another blog for that.) A few example situations:

• Arms swinging in a walk/run cycle

• Arcs. Swinging a sword or bat

• Waving

• Expressive gestures in acting

• Dance moves

• Spine or tails that need to wag

When To Use IK vs FK

IK is best used when you want to create a locked in pinning effect.

• Hand resting on a table, shoulder, railing

• Feet locked to the ground during walks and runs

• Character climbing down a ladder

• Character hanging from a ledge

• Holding a phone up to a character's ear.

• Hands pinned to hips

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Switching Between IK and FK

Yes — Animators often switch mid-shot. Imagine a character climbing down a ladder, removing one hands to wave at a friend below, and then place their hand back on the ladder to climb down.

Common workflow:

• Use IK for planting (hand on railing, foot on step)

• Switch to FK for swings (gestures, weapon arcs, expressive flourishes)

• Switch back to IK for re-contact (placing object, bracing against wall)

Tips for smooth switching:

• Use your rig’s Match IK/FK tool to avoid pops. (Not all rigs or productions have this. A quick google search for match fk/ik tool should get you to a tool you can use.)

• Always key both systems before the switch frame.

• Add safety keys one frame before/after.

• Bake a few frames if needed to lock the transition.

• Try to switch when the motion is still planted or still. Or switch when there is a large move from one frame to the next.

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Conclusion

For an FK vs IK animator, the golden rule is:

• IK = planting and precision

• FK = arcs and flow

• Switch when the limb’s relationship to the world changes

Mastering both (and knowing when to swap) is what makes animation look seamless, believable, and professional. Both are extremely powerful and necessary when used appropriately.