## 2 Materials and Methods
Additionally, the testing was carried out during clear sunny days when there was no snowfall and wind that could affect the results. The measurement was carried out on days when it was not too cold and the temperature did not exceed \(-5\) degrees Celsius, in the morning hours to ensure identical lighting conditions. A 30 m wide corridor was set up on the ski slope which was closed for other skiers to exclude the possibility of additional disturbing factors in the form of visual and auditory stimuli. The participant stood in the middle of the corridor with their back turned to the top of the ski slope and their head was in a fixed position resting on a stand of adjustable height to ensure identical conditions for all participants. To isolate the disturbing sound stimuli and to exclusively test the reaction time to the upcoming peripheral visual stimulus, the participant wore earplugs. Furthermore, there were Witty photocells at the bottom of the corridor on the left and right side. The participant held the Witty console in their hands, and by pressing the stop button, stopped the timer that was started in the moment when the skier passed through the space bounded by photocells on the left or right side. The stated time required to perceive the visual stimulus from the moment the skier passed by until the time the participant stopped it is the noted reaction time. Figure 1: (**a**) Testing protocol using only ski cap; (**b**) testing protocol using ski cap and sunglasses; (**c**) testing protocol using ski helmet and ski goggles. Before the actual testing, all participants underwent a trial (two descents of the skier from each side) to familiarize themselves with the testing protocol. When the subject was positioned in a defined position, at the signal of the researcher (who was outside the video circle of the subject), in the given corridor on both sides behind the participant's back, a skier who represented a potential danger for a collision on the ski slope descended. The descent of skiers was carried out alternately and in random order 10 times (5 on the left and 5 on the right) for each defined combination of wearing protective headgear. To limit the skier's movement speed, instructions were given to make a parallel turn from edge to edge of the corridor. At the signal of the researcher, immediately before entering the space bounded by photocells, the left or right side was chosen for passing. The 1 st study [21] we published on visual field impairments while wearing various combinations of protective headgear was carried out in a more controlled environment (in laboratory conditions). In this study, we focused on real situational conditions which could give us an insight into what really happens on the ski slope when wearing the same combinations of protective headgear that were tested in the laboratory setting. Statistical Analysis: With the use of the G\"power program 3.1.9.7 (University of Dusseldorf, Dusseldorf, Germany), the sample size was calculated (n = 32) that was needed for the testing procedure with a statistical significance of \(p<0.05\); statistical power 0.95; effect size 0.30; two groups (helmet users vs. non-users); and three measurements (tested conditions). Statistical analysis was performed with the use of Statistica 14.0.1.25 (TIBCO software, Inc., Palo Alto, CA, USA). Basic descriptive parameters (mean--x; standard deviation--SD) were used to describe variables for each group and tested condition. To determine differences between helmet users and non-users in each tested condition, 2 \(\times\) 3 (helmet*condition) mixed-model repeated-measures ANOVA was used. A post hoc test was performed for further analysis of variables with determined significant interactions. The level of statistical significance was set at \(p\leq 0.05\).