Cues and Kinematics: Brief analogy of animation and motor skills

In Computer Animation and Robotics (but we won’t discuss the latter since I honestly don’t know much about it), there are two ways of controlling kinetic and motor actions in actors, usually referred to as Forward and Inverse Kinematics. The purpose of both of them is to get joints/bones in characters to achieve a certain position, but the way they achieve this is different. Explained briefly, imagine if you have a character’s arm (or your own) with a fixed joint for the shoulder, elbow and wrist. In Forward Kinematics (FK), a motion you design would start from the “parent” joint, in this case the shoulder. As you rotate it your elbow and hand would follow along. When you have it in the desired position, you would then move the next joint in the kinetic chain (this time the elbow), and so on, until you reach your final desired position for the full chain.

In Inverse Kinematics (IK), the motion is instead dictated by the last part of the kinetic chain (in this case, the wrist/hand. If you were holding an object, like a sword, this would be the last part of the chain). You put it in the position you desire, and the other joints will automatically move to accommodate this.

Illustrating FK/IK. Note how the movement starts in different ends of the same chain. Image example produced by Rive. Used with permission.

FK and IK are both important and help animators achieve realistic and aesthetically pleasing results. IK is very helpful in cases where the animation has to fit the environment, like characters walking over uneven ground, just moving the characters torso and letting the feet find their own place. FK is instead useful in places where the environments and its contexts can’t help place the animation, or where the context would get in the way of it, like scenes where characters are gesturing while talking, or if they need to maybe brush something of their arm, or reload a gun.

In The Language of Coaching by Nick Winkelman, the concept of Cues for motor actions are explored. A cue is in this context described as a brief phase to focus attention on the movement the athlete is trying to perform. Cues are developed in the moment by the coach for the athlete and their movement, and you’ve probably found yourself both using them and having them used on you. Generally, it’s the last instruction given before a repetition or an attempt, but it’s likely they’re used in between reps as well. Examples of cues in fencing could be:

“Launch towards them like an arrow being shot” For a passing step or flèche-like action;
“When the opponent pulls back their sword, act as if a string is between your points and that your sword is immediately pulled with them” For immediately following with an attack when they recover or pull back;
As you strike, step around the opponent”; For finding a lateral angle on your opponent with your attack;
“When you cut, keep your elbows tucked close to your torso”. For delivering a cut without flaring your arms.

I can recommend the book for anyone interested in an extremely deep dive into developing cues, as it’s extensively covered.

Internal and External Focus

One thing you may have noticed in the examples above, is the difference in both verbosity and focus of the cues. Cues can be long or short, but shouldn’t aim to describe the entire motion the fencer is trying to do. Again, it should be about focusing the athlete on the movement and goal. Additionally, the examples vary their focus internally on the body (Tuck your elbows, step, etc) or externally on the environment (Follow their sword as if a string pulled yours, shoot out like an arrow to them). The book lists several studies where it was found that external cueing, that is, cues that focus on achieving something with regards to the environment, are superior in improving performance of the motion the athlete was attempting (relative to internal cues). There are a few reasonings as to why this is, but some general ideas are that with an internal focus, the athlete will consciously think about how the position their body to match with the cue (I.e., “get the elbow above your head”), which ironically takes their focus away from what they’re supposed to achieve. Internal focus can also lead to trying to achieve an ideal motion, instead of just exploring your own movement solutions. When the focus is external, either on the environment or the task that needs to be achieved, athlete performance consistently improves. (As mentioned, the book goes through a few studies, but if you’re curious for more, you can check out the works of Gabriele Wulf).

Looking back at our animation principles, we can see a similar idea to how it connects to the environment. A considered benefit in FK is the fact that it can exist without context and take no influence from the environment around it. It’s important when you need to characters to repeat certain motions perfectly, or perform without taking other actors (or physical constraints, like the gravity of the game world) into account. IK needs an environmental cue (even if it is just an arbitrary point in space) to relate to in order to work best. For this reason, games mix the methods, as some times you have a clear environmental goal (e.g. hands need to touch and follow along the movement of a crate realistically), together with something that happens without context, like their eyes blinking.

If you’re looking for an example of a game with only procedural/IK-driven animations, Rainworld is a great example. Recorded by the author.

Nevertheless, as “ecologists” we know our fencers act in direct relation to their environment, and that their actions don’t exist out of context (Do note that the cues I’m talking about here are not the same as the idea of “perception cues” as discussed in the link above). As we get all the information we need from the environment, we have less reason to tell (or get told) fencers where to place their bodyparts.

In a way, FK can be seen as internal cueing of where an actor places themselves, and IK can be seen as external cueing of where an actor places themselves. With appropriate goals and constraints, you can model a fencer’s behaviour. Let’s briefly explore this further with some more cues that are either focused internally or externally.

Internal FocusExternal Focus
As you strike, your arms should reach as long as possible.Get your sword to the intended target as soon as possible.
Extend your back leg as far as possible when you lunge.Push away from the ground as much as you can as you try to reach the opponent.
When you cut, keep your elbows tucked close to your torso.Imagine that you’re holding onto a ball between your arms as you cut.
Cross your arms as you cut around their parryPush the pommel out to the side as you cut to their head.
(Fun fact, the very last external cue is part of Lew’s instruction on how to do the duplieren)

Do keep in mind that cueing is just of course a part of the process. It should reinforce finding solutions to goals or motions that are known to the fencer. So in the example of the first cue, this would be given in the context where the fencer is already trying to reach someone with a cut, and knows this is what they’re trying to do, but is maybe struggling. Again, for an extensive dive into cueing and how you can work with them, I recommend reading Winkelman’s book. Learning how to do this efficiently is a process in and of itself, and it’s ok to make mistakes or finding a decent cue rather than the best one at the moment. There are anyways some lessons to take from the ecological approach and IK to consider when figuring out cues.

  • Is there something in the environment around me that I can tell the fencer to focus on? (Opponent, target area, a specific spot on the floor, their weapons, etc).
  • Can I make a comparison to an object outside their body? (Shoot like an arrow, swing like a pendulum, etc.)
  • Can I use something the fencer is holding? (Make your glove face that way, push your pommel down, etc.)

The last one goes to a particular point that external can still be attached to the body of the fencer, and this can be an efficient trick sometimes, like referring to sole of the shoes rather than feet, shoelaces rather than toes, or elbow pads rather than elbows. Even in this case it gives fencers an external goal to build their solution around.

In summary, I hope that this article and brief analogy serves as a nice introduction to how different kinds of cues may make your fencer organise themselves. Internal focus, and forward kinematics, share an attribute in that they are based around more absolute positioning of specific joints of a body. External focus, and inverse kinematics, instead share an attribute in that they’re trying to organise a larger structure with regards to an achievable goal.

With time, cueing fencers on an individual basis that pertains to their needs and specific situation will come easier as you practice. Until then, happy cueing!