## 2 Materials and Methods
### Experimental Protocol  
Participants were asked to avoid high intense exercise preceding and during the study. Prior to testing, they were given a demonstration of the proper technique of trunk rotations. The time of day and testing procedures were identical for all participants. The same researchers conducted the testing in all groups. After a warm-up, participants underwent familiarization trials, during which they performed standing rotations of the trunk in a slow and controlled manner while keeping the back straight. Then they performed two repetitions of trunk rotations to each side per load. Bars of different weights were placed on their shoulders behind the neck. The load increased from 1 kg by -5 kg up to a max. of 50 kg. They were asked to perform rotations of the trunk with maximal effort in the acceleration phase. Emphasis was placed on the correct body position during trunk rotations while standing. Participants stood with their feet wider than shoulder width apart and toes slightly pointed outwards while holding the bar on the shoulders with their hands. They rotated their torso from the right (or the left) side towards the opposite side, and then they slowly returned to the starting position. The test was then repeated for the opposite side of the body. They had to engage their core muscles to stiffen the torso and stabilize the spine. A laboratory assistant ensured that participants remained upright throughout the movement and that their head, chest and torso were aligned over the hips. During rotational movement of the trunk with a bar over the shoulders, torque has to be produced to accelerate or decelerate mass on both sides of the bar axis. At the same time, centrifugal forces arise. As long as the weights on both sides of the bar, as well as their rotation radii, remain the same, arising centrifugal forces will be of the same magnitude, although acting in opposite directions. This means they will be fully compensated without putting any stress on the trunk of the subject. In order to quantify the torque produced to accelerate (or decelerate) a rotating bar, one needs to \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Groups of Athletes** & **n (1)** & **Age (Years)** & **Height (cm)** & **Body Mass (kg)** \\ \hline Boxers and Thai boxers & 12 & 22.3 \(\pm\) 4.7 & 183.6 \(\pm\) 8.5 & 83.8 \(\pm\) 10.0 \\ Canoeists and kayakers & 19 & 24.7 \(\pm\) 3.6 & 178.6 \(\pm\) 6.5 & 72.7 \(\pm\) 6.4 \\ Golfers & 15 & 26.1 \(\pm\) 4.2 & 180.6 \(\pm\) 9.3 & 81.4 \(\pm\) 13.6 \\ Hockey players & 20 & 23.5 \(\pm\) 3.0 & 184.6 \(\pm\) 7.7 & 83.1 \(\pm\) 9.9 \\ Judo competitors & 14 & 21.9 \(\pm\) 2.7 & 177.3 \(\pm\) 6.8 & 75.6 \(\pm\) 7.5 \\ Karate and tae kwon do competitors & 27 & 22.9 \(\pm\) 2.8 & 179.1 \(\pm\) 6.0 & 76.3 \(\pm\) 8.2 \\ Tennis players & 17 & 23.2 \(\pm\) 3.3 & 181.4 \(\pm\) 6.5 & 73.9 \(\pm\) 10.7 \\ Wrestlers & 9 & 25.4 \(\pm\) 3.6 & 177.9 \(\pm\) 8.1 & 80.0 \(\pm\) 8.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Participant’s characteristics (mean \(\pm\) SD). know the mass of the bar and its rotation radius (distance between the center of gravity of rotating masses and the axis of rotation) and instant acceleration. Calculation of the center of gravity of the total rotating masses on both sides (consisting of the bar axis and weight plates) is based on their geometrical dimensions, assuming the same specific mass of the material (iron) used for the bar axis and weight plates. As the rotation radius and masses are set prior to the test, acceleration depends on the subject's effort. For registration of acceleration, the FiTRO Torso Premium (FiTRONiC, Bratislava, Slovakia) was applied. The equipment consists of an inertia measurement unit (a combination of a three-axis accelerometer and three-axis gyroscope) in a small box with an integrated USB interface connected to the computer. While inserted in the bar axis, the inertia measurement unit registers instant angular velocity of rotation movement (rad per second), which is sampled by the computer at a rate of 100 Hz.