## 2 Materials and Methods
### Sensor Positioning  
To our knowledge, there have been no comprehensive studies with particular recommendations of using wearable kinematic sensors for testing in karate. Therefore, several requirements were taken into account concerning the position of the sensors before the final positions were decided upon: (i) the kinetic chain of motion; (ii) eliminating sources of sensor disruption and connectivity problems; (iii) avoiding the disruption of an athlete's performance; and (iv) the most affordable and inexpensive solution. Of course, the main question was asked: which position would give the most satisfactory data? The decision was based on biomechanical facts and empirical knowledge, including:  
Kinetic chain--The kinetic chain (Figure 2) represents the link between the superimposed body segments, including the ankle of the back leg, knee, hip, shoulder, elbow and  
Figure 1: The experimental set-up included two cameras and a laptop wirelessly connected to two sensors attached to the athlete.  
wrist of a punching fist [10; 36]. The joint sequence of the lower limb is connected to the joint sequence of an arm through pelvis rotation, i.e., the body. Therefore, this is the last reference point that includes all the relevant kinetic and corresponding temporal events of the lower part of the kinetic chain.  
The most optimal point for the most representative data--The main idea of the presented method is getting the most accurate data through the simplest approach: placing as few sensors as possible on the most informative locations (i.e., the last optimal point in the kinetic chain for both the upper and lower parts of the body). For the upper limb, for obvious reasons, it is the fist. As for the lower part, it is the centre of gravity. The reasons for this are explained above.  
Technique free from interference--The athlete has to feel free from disruption of any kind [37]. In other words, field research in realistic conditions is relevant only if each and every condition of execution is in accordance with the usual training and/or performance environment. In order to achieve this, it was necessary to determine appropriate sensor location empirically and get feedback from the athletes. The chosen location provided conditions in which wearable technical equipment was not obstructive to the athletes.  
Sensor output--Sensors placement must not affect sensor output [3]. During test trials, no such problems occurred at the chosen location. The measured kinematic quantities at the selected body attachment point did not exceed the sensors' dynamic ranges.  
Fixation--The location of the sensor has to be suitable for safe attachment and has to ensure fixation during the explosive movement. These conditions could be met by combining an appropriate fastening kit and the best sensor location. The security positioning and higher output production of sensors were ensured due to the usage of a waist belt with a tight-fitting pouch and a glove [38]. The particular fastening kit was chosen not only for  
Figure 2: The kinetic chain of a reverse punch and the orientation of the sensors.  
reasons of secure attachment, but also because it allowed for the sensors not to be in direct contact with the athlete's skin during testing (i.e., not exposed to the effects of elevated body temperature or sweat).  
After much deliberation, the exact positions of the sensors (Figure 2) for measuring the kinematic parameters of GTC were: (a) between the 2 nd and 3 rd lumbar vertebrae, on the back of the athlete and (b) between the os metacarpal II and os metacarpal IV bones, on the dorsal side of the punching fist. In order to maintain the accuracy of the devices, the commonly used method to reduce bias was applied in the field before testing. The average bias measurement interval was 10 s. The sensors were calibrated in a laboratory.