## 2 Materials and Methods
### Instrumentation and Testing Procedures  
The measurement tool was OptoGait (Microgate, Bolzano, Italy) [20]. The photoelectric cell system was used for biomechanical analysis of movement, including step length, contact time and frequency. The device consists of two bars (transmitting and receiving) emitting light using Light Emitting Diodes (LED)and a computer with software (OptoGait Software Version 1.13.4). The system records the time and area of interruption of contact between the strips, which allows the estimation of spatiotemporal parameters. The bars are placed on the sides of the Monark medical treadmill (Monark, Sweden) at the same level as the contact surfaces [21]. The validity of the OptoGait system for the evaluation of spatiotemporal gait parameters in healthy adults was demonstrated in a previous study [20]. All parameters were measured bilaterally.  
Before testing, participants completed the short version of the International Physical Activity Questionnaire (IPAQ) [22]. The IPAQ consists of questions about the amount of walking, participation in moderate and vigorous activities and sitting time. The questionnaire describes physical activity in METmin/week (Metabolic Equivalent of Task). This unit is used to estimate the metabolic cost of physical activity (PA) (energy expenditure as reflected by oxygen consumption) [23].  
Participants held a backpack on their shoulder or in their hand while walking on a treadmill. We selected these loading positions due to their varying distances from the global center of mass. We decided to focus on those two positions because adding more positions can elongate walking time and result in fatigue, which can influence results. The backpack's weight was calculated as 10% of the body weight, and for loading, dumbbells were used. The backpack's position was adjusted by tightening the straps so that the lower edge of the backpack was at the level of the upper part of the sacrum [24] (Figure 1).  
The examined person wore sports clothes and was without shoes [25; 26] during gait measurements on the treadmill. Barefoot walking eliminates discrepancies related to different types of footwear and does not require any standardization of shoe parameters. The participant first walked on the treadmill for approximately 5 min to warm up, selecting their preferred comfortable speed for normal walking [21]. Then, they walked for approximately 1 min during each trial to familiarize themselves with the conditions, of which 30 s was recorded [27]. Order of trails: (1) control trial--unloaded walking at preferred speed (no treadmill incline and with +5\({}^{\circ}\) incline); (2) backpack walking at preferred speed without treadmill incline (backpack on shoulder, backpack in hand); (3) backpack walking at preferred speed with +5\({}^{\circ}\) incline (backpack on shoulder, backpack in hand). There was one trial for each condition, with continuous recording of gait parameters for 30 s. Participants carried the backpack on their preferred side, which was established based on  
Figure 1: (**a**) Backpack with marked dimensions. Height, 38 cm (green line); width, 28 cm (red line); depth, 15 cm (yellow line). (**b**) Adjusting the backpack position to the participant so that the lower edge of the backpack was at the level of the upper part of the sacrum and the backpack fitted tightly to the back.  
questions about their preference for carrying bags/backpacks during their daily activities (Figure 2).