![]() The green circles are for jumps where I pulled my legs up. Did I fool the sensor? Here is a plot of Nike+ reported vertical height versus the jump height as measured from the change in center of mass in the video. However, will this change be enough to pick up on from the hang time? I doubt it. This should correspond to a small change in the location of the center of mass. I started the jump with my arms extended above my head and ended with them down by my side. I did try one trick that theoretically should work. Of course, I failed to do this - mostly because I didn't think of it. You could do this if you landed before extending your legs. I would have to end my jump with my center of mass in a different location. The problem is that if I pull my legs up and then put them back down while still in the air, I will not change the hang time. Although this will make my feet seem to not have a constant acceleration, my center of mass would still have a constant acceleration. My first attempt to trick the sensor was to pull my legs up in the air. Although this guy's feet ended up 64 inches higher than they started, his center of mass only increased about 33 inches. Here is a previous analysis of an impressive 64-inch box jump. Let me show an example where the sensors would give the wrong value. If Nike+ uses just the hang time to calculate the vertical height, there could be a way to increase the time without actually jumping higher. In the other jumps for my video, I tried to cheat. ![]() I could probably fix this with a more careful video but it seems convincing enough to me that Nike+ uses the hang time to get the vertical height. I am going to attribute this difference to a video problem. What time interval does that correspond to? If I use that height in the above kinematic equation, I get a hang time of 0.54 seconds. With a quick unit conversion, that gives a height 0.36 meters. Close enough for me.īut what does Nike+ report? For that same jump, Nike+ gives a height of 14.2 inches. Using the above formula, I get a jump height of 0.306 meters. ![]() At what time did I actually leave the ground? I am just going to guess on the start and end times to get a hang time of 0.5 seconds. ![]() The time is a little more difficult since the video frame rate isn't super high. How high? How long? First, the "how high?" Looking at the starting and highest point of my watch, I get a vertical height of 0.319 meters. In this case, I used my watch (on my wrist). Since in this jump, I was fairly stationary with my body position, I can use any point to mark my position. The plot was created using Tracker Video Analysis - a free program to make the location of an object in each frame of a video. Here is my first jump where I was fairly stationary during the jump so that I would be similar to a point object. Why? I wanted to see if I could get my actual jump height to be different than the NikePlus value. You might notice that I jumped a several different ways. For any object with a constant acceleration just due to gravity, we can write the following kinematic equation (which isn't hard to derive, but I will skip that part). This constant force would mean the person has a constant acceleration. While the person is off the ground, essentially the only force on that person would be the gravitational force. If you assume that the person is a point particle, you could calculate the jump height. ![]() How would a computer determine how high you jump just from these force sensors? My guess is that the sensors in the shoes just record the time that there is no force sensor readings. Measuring Jump Heightįor this post, I will just look at the vertical jump height measurement. By recording the values and times that these force sensors are activated, you could get all three of the above measurements. Just from this, I suspect that the Nike+ has three pressure sensors in each shoe. The sensors then give you the following data: pressure, vertical jump height, the number of steps per second, and a measure of your NikeFuel. ![]()
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