Welcome to Science Buddies and thank you for your questions. You have a very interesting experiment. Human performance experiments, such as those related to sports and body motion and power can be quite tricky. Therefore defining your variables is even that much more important to define correctly. This will help you effectively design your experiment and measure your results.
To review the difference between the different types of variables, read the Science Fair Project Guide section on variables located here: http://www.sciencebuddies.org/science-f ... ndex.shtml
Independent Variable (IV) is the one thing about your experiment that you change. You should have only one IV. When you change the IV, you will observe what happens to the Dependent Variable. In your case, your IV is the Kick Style. From your post, you need to change between 6 different kick types and measure the power output of each.
Dependent Variables (DV) are those that will change in response to the change in IV. You can have multiple DVs. These are the things you are going to observe during your experiment. In your case, you already stated it in your hypothesis…the power output of each kick style. You need to devise a way to accurately measure this. Force of impact on a target would be a good one. Then you can relate the force vs kick and see which one had the greatest impact.
Control Variables (CV) are all those things that you need to hold constant during all tests. For example, if you were testing the effects of sunlight on a material and you tested one material at noon, and the other one at sunrise, clearly each material wasn’t tested under the same conditions. So Time of day would be a CV in this case. In your case, the experience level of the kicker, fatigue, flexibility, distance to target, height of target, height of kicker, lighting, temperature, time of day, etc… are all CVs. A single kicker performing the same kick over and over will obtain different results each time because of minute changes in stance, fatigue, etc…To get around this in your experiment; you correctly have identified multiple kickers performing the same 6 kick styles. It is important that they each perform them in the same order and from the same relative position to the target (for example: arms length away and shoulder height, etc…). This should give you an average force of impact for each kick.
I hope this helps.
"As the circle of light increases, so does the circumference of darkness around it."
~ Albert Einstein