## 4 Discussion
It has been reported that weight differences between shoes of more than 100 g per shoe lead to longer running times [1] and increased oxygen uptake of approximately 1% [50]. To avoid effects of shoe mass on physiological parameters, shoes with similar mass were used in the present study--the difference between the lightest shoe (S1: 172.4 g) and heaviest shoe (S2: 203.0 g) was approximately 30 g. Thus, the focus in this study was on the influence of footwear characteristics on the physiological parameters of MBS and cushioning. The influence of MBS on physiological parameters has been examined in several studies. However, the findings are not consistent. On the one hand, an increase in MBS leads to a decrease in energetic cost of running or improved RE [6; 13; 18]. In contrast, [PERSON] et al. [51] reported no positive effects on RE when increasing MBS. Furthermore, [PERSON] and [PERSON] [24] proposed decreasing MBS by cutting the carbon fiber plate of the \"Nike Vaporly 4%\" (six medic-lateral cuts) did not significantly affect RE. In a study by [PERSON] and [PERSON] [18], three identical shoes were used to investigate the influence of RE when adjusting MBS by inserting carbon plates with differing stiffness. The authorsfound a \"U-shaped\" curve relationship between MBS and RE and concluded that a running shoe with medium MBS improves RE compared to more flexible or stiffer MBS. In the present study, MBS ranged from 104 to 130 N/mm. Comparing the reported MBS values with the aforementioned studies is difficult because of the different test procedures used to determine MBS. We found no significant differences in physiological variables between shoe conditions. However, our data shows the same tendency as found by [6, 13, 18]. Shoes S2 and S3 (lower MBS) showed lower HR, rel. VO\({}_{2}\), and RE than S1 (highest MBS). One reason for similar performance results may be the additional influence of midsole stiffness. Adjusting the midsole stiffness to enhance running performance has been investigated in several studies [5, 21, 52]. However, adjusting midsole stiffness is also controversial. For example, [PERSON] et al. [53] reported lower oxygen consumption while running in shoes with softer midsoles. In contrast, [PERSON] and [PERSON] [54] provided data showing greater values for oxygen consumption when running in shoes with softer midsoles compared to harder shoes. Additionally, [PERSON] et al. [21] found no significant differences in rel. VO\({}_{2}\) or HR when running below and slightly above the individual ventilatory threshold in distinctly different shoe conditions (cushioning and energy loss). Our results show that neither MBS nor midsole properties influenced physiological variables in our study. It seems that runners adjusted their running styles, especially the foot strike pattern, during the 10 km run at 70% of rel. VO\({}_{2}\)max to reduce the load on lower extremities. However, RE, HR, or rel. VO\({}_{2}\) were not affected by the adapted running styles. Running at higher velocities (e.g., at 75 or 80% VO\({}_{2}\)max) could lead to greater differences in biomechanical and physiological parameters. This study has a few limitations that should be mentioned. First, our test group consisted mainly of recreational runners with a wide range of running speeds for the 10 km tests (8.0 to 14.2 km/h), since RE improvement be speed dependent [14]. The runners in this study preferred stiffer shoes when running velocities over 17 km/h. Future studies should focus on high-level runners who can achieve the required running speed. Second, in our study we did not collect data about the perception of footwear. Since there is evidence relating to improved running economy when wearing more comfortable compared to less comfortable footwear [55], follow-up studies should include subjective measurements. Third, compared to outdoor running, running on the treadmill shows some differences in terms of biomechanical parameters, such as sagittal foot strike angle or knee flexion at foot strike [56].