Martial art karate: Determination of maximum G-forces
Author: Adrian Kurmann, Lucerne University of Applied Sciences and Arts, Engineering & Architecture
As part of a project module at the Lucerne School of Engineering & Architecture, karate techniques are to be verified and evaluated using sensor technology. The aim is to measure the maximum acting forces.
Initial situation
In order to determine the maximum G-forces during karate movements, the author has a GoPro Hero Black 10 and an MSR165 data logger with acceleration sensor at his disposal. With the data logger, it is possible to determine accelerations of up to ±200 g. The accuracy is ±5 g in the measuring range of 16-100 g and measurements can be taken with a resolution of up to 1.6 kHz. The GoPro serves as a reference measurement of the acceleration. The Viana.net framework is used to analyze the videos and determine the acceleration. The accuracy of this reference measurement is checked by means of error calculation. Several measurements and video recordings are carried out on each test subject. One measurement is taken on the legs and several measurements are taken on the arms. Most of the measurements are filmed and analyzed with the GoPro.
Test procedure
The MSR165 data logger is available with several sensors. Only the acceleration sensor up to ±200 g is used for the following measurements. To record the acceleration values, an ADXL372 MEMS acceleration sensor is used, which is installed in the housing of the MSR165. The measurements of the sensors used are measured using capacitors, which change their capacitance in proportion to the acceleration. To ensure that the acceleration sensor can be used optimally, the parameters for the measurement setup used must first be set. In order to generate as many measuring points as possible in the short measuring time, measurements are taken with the highest resolution of 1.6 Khz. To carry out individual measurements, the measurements can be started or stopped with a button. The measurements are saved with a time stamp in the sensor.
During the measurements, each beat is carried out four times. It can be seen that the highest accelerations occur in the leg movements of all karatekas. If you look at the start and end sequences of the individual measurements, you can see that the maximum G-forces occur during the deceleration of the respective movement. This is due to the limitation of the human body, which is reached at the end of the stroke. It should be noted that the measurements on some movements must be considered inverted due to the coordinate systems. The maximum measured accelerations are: Leg: ≅ 42𝑔 (𝑀𝑎𝑒 𝐺𝑒𝑟𝑖, 𝑇ℎ𝑖𝑠) ± 5𝑔 Arm:≅ 35𝑔 (𝐻𝑎𝑟𝑎𝑖𝑜𝑡𝑐ℎ𝑖 𝑈𝑘𝑒, 𝑀𝑎𝑟𝑘𝑢𝑠) ± 5𝑔. The resolution or measurement speed of the measurement data produces a reproducible curve even for movements with high acceleration. The maximum values are confirmed with several measurements. With a movement duration of 0.5 s, the movement is saved with 800 measurement data.
In order to validate the values of the acceleration sensors, a reference measurement is carried out using video analysis. To ensure that the measurements from the acceleration data logger and the video analysis can be compared, the coordinate systems of the two systems must be equated or observed (see study).
Summary
The MSR165 data logger and the reference measurement using video analysis were used to determine the maximum G-forces. The maximum accelerations that occur during karateka are in the range of 40-50 g. The acceleration values of the video analysis must be considered with a loss of information due to the possible errors and the processing methods. For this reason, it is only possible to determine the maximum values using video analysis to a limited extent. However, it is possible to recognize the course of the acceleration as a function of time. The data from the acceleration sensor produces clear diagrams that allow analysis. With a resolution of 1.6 kHz, 800 data points are generated for movements of 0.5 s, whereby the measured maximum values are confirmed with several measurements. Only the accuracy of ±5 g can make it difficult to compare movements. A closer look at this accuracy and its influences is necessary for a precise comparison. In order to further evaluate the data of the MSR165 sensor and compare it with the real acceleration of the body part, the movement of the sensor on the skin and its influence on the measurement must be examined more closely.
