## 2 Materials and Methods
### Measurement and Data Processing  
The primary source of power for a variety of sports, which can give speed or acceleration to performed techniques and manage their magnitudes, are movements of the trunk and limbs [46]. Due to that, the kinematic characteristics of the RP were obtained using two custom-made wireless KS devices. The metrical characteristics and reliability of the device were confirmed [8]. KS were positioned on the hand and the back of the athlete as follows: (i) on the upper side of the fist performing the RP, between the second and fourth metacarpal bones, and (ii) in between the second and third lumbar vertebra, respectively. The positioning was based on the kinetic chain of technique and factors, excluding any source of disruption for KS or an athlete, as well as providing the most valuable data [26]. The device attached to the hand consisted of a 6 DoF sensor LSM6 DS33, a 3D accelerometer, a 3D gyroscope, and a microcontroller with a communication WiFi module. The accelerometer's dynamic range was \(+/-16\) g, and the dynamic range of the gyroscope was \(+/-2000\) dps. The acceleration signal was captured with a sampling frequency of 200 Hz and transmitted to a personal computer via a WiFi module using the UDP protocol. A particular phase of RP preceding the impact is characterized by a progressive increase in kinematic quantities, making the operating range of the devices and sampling frequency sufficient. The device attached to the location corresponding with the center of gravity on the athlete's back operated in the same way as the one on the hand but used a different type of sensor. It had a built-in absolute orientation sensor BNO055 and measured linear acceleration at a lower sampling frequency of 100 Hz.  
In order to provide additional check-ups during post-processing, two cameras were used to record the testing: the GoPro HERO 6; GoPro Inc., San Mateo, CA, USA (frequency 100 fps, resolution 1920 \(\times\) 1080 p) and the Logitech C920 HD PRO; Logitech Inc., Lausanne, Switzerland (frequency 15 fps, resolution 1280 \(\times\) 820 p). Cameras were placed on a 1.3 m tripod, 2 m laterally (on both sides) in relation to the athlete. Such positioning was considered adequate because it met two conditions: (i) enabling enough space for uninterrupted task performance in the field of view and (ii) recording punch performance from start to end point. Additional check-ups were possible on account of combining images from different viewpoints and sensor signals, allowing movement phase recognition and differentiation [47,48]. Heterogeneous but temporally synchronized data (Figure 2) acquired from four devices decrease uncertainty in technique evaluation. Data were stored in video (.avi and.MP4) and LabView (.tdms) files. Two wireless sensors sent signals to the LabView program. The signals from the sensors and the two cameras were synchronized and recorded. Post-processing and further analysis were carried out with MathCAD 7 numerical calculation software. A Butterworth 5 th-order low-pass filter with a cut-off frequency of 40 Hz was used to analyze the signal. Shifting the signals at the reference moment of analysis--an impact--was the initial step in calculating the variables of interest [9,26].