2025-09-10

Teaching my regular classes at the Ross Farnsworth - East Valley YMCA.

Most of my students probably didn’t know they were signing up for math, physics (and even psychology) lectures when they joine my classes. 🤷 But, when “don’t do that” and “do it like this” both fail to work, you have to bring out the big guns and explain why it needs to be done a particular way.

Today’s little lecture was about “moment of inertia” - how much energy it takes to get something to spin. This comes up in all kinds of places in karate training, but I’ll give you the three that we work on most frequently:

• Every 270° “backwards” spin in kata:
  This correlates with my update from last time, where a student was watching a video of someone explaining how to do this turn. The person in the video was reaching out behind themselves with their leg, planting it, and then pivoting into place. That’s a fine way to teach teach someone who’s having trouble understanding where they’re supposed to end up after the turn, but when it’s time to do it “for real”, you need to compress, turn, and then expand. It’s much faster and prevents you from stepping blindly into an attack.

• Spinning back kick: The problem I often see with spinning back kick is that people don’t keep their thighs together as they spin. Instead, they allow the soon-to-be-kicking leg to “drift” away from the supporting leg. This causes the mass of that leg to move further away from the axis of rotation, which both slows the rotation and causes the kick to curve towards the target instead of moving straight in like a thrusting technique should. Remember, this is spinning back kick, not spinning hook kick. The latter is a perfectly good kick, but not what we want to see when we ask for spinning back kick.

• Moves 12, 14, and 16 in Heian Sandan:
  This is the part where you step forward from kiba-dachi to kiba-dachi, stomping your foot and blocking with your elbow. The most frequent mistake here is exactly the same as the mistake on spinning back kick - the tendency to allow the legs to come apart, almost like you’re wanting to do a mawashi-geri, when the feeling should really be like you’re wanting to do a mae-geri. (Except, in addition to slowing your rotation, it also leaves your groin exposed, but that’s anatomy, not physics.) To correct this, I have students stop halfway through each step with their knee raised as if they’re going to do a front kick and then stomp down into kiba-dachi instead.

TLDR: Keeping your mass close to your axis of rotation reduces the energy required to spin or, to put that another way, it allows you to spin faster with the same amount of energy. This is why figure skaters spread out their arms to slow a spin and bring them back in again to speed up again.