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Power Kicks: The Physics of Martial Arts

Difficulty
Time Required Very Short (≤ 1 day)
Prerequisites None
Material Availability Speciality items (kick bag)
Cost Very Low (under $20)
Safety Minor injury possible

Abstract

"Ay Yaah!" echoes across the room while a loud "thud" signals a powerful kick striking the kick bag. Sound familiar? If the discipline, precision, and power of martial arts is your bag, try this project out for size. You won't be sparring with any opponent other than a swinging kick bag, but you'll learn a few powerful lessons about the physics of efficient kicking. No black belts required; just bring your best form and work up a little sweat while you use your feet to do fun science.

Objective

The goal of this project is to compare the power impact of different martial arts kicks.

Credits

Darlene E. Jenkins, Ph.D.

Sources

The idea for this project came from this DragonflyTV podcast:

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Last edit date: 2013-11-25

Introduction

Watch DragonflyTV taekwondo video
Click here
to watch a video of this investigation, produced by DragonflyTV and presented by pbskidsgo.org

In the project video, two young black belts in Tae Kwon Do blended their athletic talent with some high tech science to find out how to improve their kicks. They were curious to know what types of techniques produced the strongest kick. Their study focused on the effect of changing the foot position of the standing leg during a side kick. They used a special high tech sensor to monitor the pressure exerted down into the mat from the foot when it was turned at an angle or kept straight during a series of side kicks.

Check out the video to see what they learned and how they applied their scientific findings to their sport. Then read on to see how you can set up a simple, but equally instructive, series of experiments to examine the power of kicks. No black belts or fancy equipment needed here. Just you, your feet, a bag, and a buddy.

The basic plan of this project begins with selecting three different kicks–a front, side, and back kick–in your form of martial arts to use in your experiments. You'll then measure and compare how much power each type of kick generates by observing how far a kick bag moves after each kick. You'll experiment with doing each kick slowly or fast to see how velocity influences the ultimate power of a kick. You'll also try an experiment with a kick that involves a turn or spin to see how rotational momentum adds to or reduces a kick's power. Once you have gathered your data, you can study the muscles and movements of the legs and feet to see if, like the two athletes in the video, you can explain your results in terms of muscle movement and anatomy.

Before you start your experiments, do a little background research on concepts like center of gravity, inertia, mass, and momentum. Also, you should become familiar with the basic movements and muscle groups of the legs and feet. See a list of suggested search terms, research questions, and a bibliography in the next sections to get you started. You'll find that knowing a little science about your athletic art will help you interpret your results and understand why some moves and techniques work better than others in generating a powerful kick.

For example, according to Newton's First Law of Motion, the property of inertia tends to keep a body still unless acted upon by some outside force. This means in a martial arts kick, one must first expend energy to overcome the inertia of a stationary standing leg to position it for a powerful kick. The energy used to position the leg is stored momentarily as potential energy within the thigh, calf, and foot muscles. Potential energy quickly converts to powerful kinetic energy when the leg fires a percussive kick to an opponent, or in the case of these experiments, the bag. The speed (velocity) of a kick and the size (mass) of the kicker are additional factors that greatly influence the ultimate power in a kick.

As you do your experiments and collect your data, think about the science that explains the different power you get with your various kicks. You may discover how the science of physics and body mechanics can help explain the training techniques for effective kicks and strikes that have developed over thousands of years of martial arts history. And who knows, maybe a little "lab" work from this project will translate into better foot and leg work for you against the bag or your next opponent!

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

  • Physics of martial arts kicks
  • Center of gravity (center of mass)
  • Kinetic energy
  • Potential energy
  • Velocity
  • Momentum
  • Rotational momentum
  • Inertia (Newton's First Law of Motion)
  • Acceleration (Newton's Second Law of Motion)
  • Muscle groups of the hip, leg, and feet

Questions

  • How can you change a kick in martial arts to improve its impact?
  • What is the difference between potential and kinetic energy?
  • What factors increase the potential energy of a kick?
  • What do velocity, mass, and momentum have to do with executing a strong and efficient kick?
  • What principal muscle groups are involved in front, side, and back kicks? Which additional muscles are necessary for a turning kick?
  • Define center of gravity. Describe how it shifts when delivering each type of kick.

Bibliography

Here are some websites you might want to check out as you start your research:

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • Karate kick boxing bag (hanging or upright)
  • Workout room or open space to kick the bag
  • Assistant to evaluate the kicks
  • Notebook or paper
  • Pen or pencil

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Experimental Procedure

  1. Select your favorite front, side, back, and rotational kick for your experiments.
  2. Set up your kick bag, and review with your assistant the procedures of the experiments.
  3. The assistant will stand to the side of the bag and note how far the bag moves or swings after each kick. They will also reposition the bag to a still, vertical position after each kick.
  4. The assistant should give each kick a "power rating" based on how far the bag moves from the vertical position. For example, use a 0–5 scale such as 0 = no movement of the bag; 1 = slight wobble; 2 = slight swing (10–20 degrees) 3 = medium swing (up to 45 degrees); 4 = hard swing (45–90 degrees); 5 = bag moved > 90 degrees (or was knocked over).
  5. Warm up a few minutes by practicing each of the four types of kicks you selected for your experiments.
  6. Do the four experiments listed below. The first three experiments have two parts, a slow and fast version of each kick.
  7. In all kicks, use your best form, and try to keep the height of your kicks and the point where you hit the bag consistent. For slow kicks, concentrate mainly on good, consistent form, and don't worry about speed. For fast kicks, kick your leg as quickly as possible, but try to maintain good form at all times.
  8. For each experiment and speed, do ten kicks in a row. Alternate feet after each kick so you don't always use your "favorite" leg throughout an experiment.
  9. Record in your notebook the "power ratings" of each 10 kicks as they are done in every experiment.
    1. Experiment 1: Front kick–Slow and Fast versions
    2. Experiment 2: Side kick–Slow and Fast versions
    3. Experiment 3: Back kick–Slow and Fast versions
    4. Experiment 4: Rotational kick (Any kick that involves a turn before hitting the bag)

Analyzing Your Data

  1. Total the ten "power ratings" from each type of kick. Calculate an average rating for each kick type.
  2. Prepare a bar graph showing the average power rating for all kicks.
  3. Rank the kicks from highest to lowest according to their average power ratings.
  4. Did one type of kick stand out as most powerful, or were all the kicks fairly close in power ratings? Is this what you expected?
  5. How did speed influence the power of a kick, if at all?
  6. Reviewing the muscle groups in the legs and feet, describe how muscle group or size influences the outcome of a kick.
  7. For help with data analysis and setting up tables, see Data Analysis & Graphs.
  8. For a guide on how to summarize your results and write conclusions based on your data, see Conclusions.

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Variations

  • Does Mass Matter? Recruit a martial arts friend who is much smaller or larger than you. Ask them to do the same experiments. Compare your results to those of your friend. Are the results what you expected? Why or why not?
  • Do the Math. Research the mathematical equations that describe the force generated during the motion of a kick on a kick bag. Describe mathematically the different levels of energy or power that can be generated from the various kicks you did in your four experiments. Show mathematical examples of how speed, mass, distance from the bag, and rotational momentum may influence the power of a kick.
  • The Strike Zone. Use a speed bag and repeat these experiments with three different arm strikes instead of kicks. For example, compare front strikes to back strikes, or elbow strikes to hand and forearm hits. Compare these results to results from your first set of experiments with kicks. Explain your findings in terms of mass, velocity, momentum, and inertia.

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