## 4 Discussion

Table 3: Mean differences (MD) and standard deviations (SD) between t_evVel_Gonio and t_evVel_Gyro of the four velocities and the three footwear conditions: Adidas AdiStar (AD), Asics Gel Nimbus 12 (AS), PUMA Speed 600 (PU); [PERSON]’s correlation coefficient was interpreted as trivial (\(r\leq 0.1\)), small (\(r\leq 0.3\)), moderate (\(r\leq 0.5\)), large (\(r\leq 0.7\)), very large (\(r\leq 0.9\)), and extremely large (\(r>0.9\)). and the angular velocities were measured at this position in the frontal plane directly. However, the flex arm of the goniometer--which was attached at the posterior lower leg--can be slightly influenced by skin displacement during initial ground contact, especially at higher velocities and with harder shoes. Furthermore, [PERSON] et al. [10], [PERSON] et al. [8], and [PERSON] et al. [14] used lower sampling rates to compare optoelectronic systems and gyroscopes, resulting in less data, which could have influenced their results. In our study, data of both devices were recorded with a high sampling rate of 1000 Hz, and an effect due to different or too low sampling rates can therefore be excluded. Due to the very large and extremely large positive correlations, and the strong relationships between evVel_Gyro and evVel_Gonio, we can confirm hypothesis H1. T_evVel_Gyro was determined later than t_evVel_Gonio for all footwear conditions and all locomotion speeds. This offset differs between footwear conditions and locomotion speeds. Surprisingly, two trends were observed. First, the largest MD was found for the walking condition (for all three footwear conditions). Second, the hardest footwear condition (AD) revealed the largest MD and PU the lowest MD for all running velocities. Thus, this offset seems unrelated to rearfoot stiffness or Heelheight, and another unknown factor must be influencing t_evVel_Gyro. In addition, very large and extremely large positive correlations as well as strong relationships of t_evVel_Gyro and t_evVel_Gonio were found. Therefore, our hypothesis H2 can be confirmed. One limitation of the current study is that only rearfoot strike runners were investigated. Further studies should also investigate midfoot and forefoot strikers. Especially during initial ground contact with the forefoot first, the changed orientation of the gyroscope can lead to a crosstalk of the sensitive axes. This can influence the subsequently derived maximum angular velocities. In conclusion, the results of this study are necessary when interpreting peak angular velocities for different footwear conditions and locomotion speeds which were measured with a heel-mounted gyroscope in the frontal plane. The results of the present study showed strong relationships between evVel_Gyro and evVel_Gonio for locomotion at 1.5, 2.5, 3.5 m/s, and at individual speed, and for three different footwear conditions. Furthermore, strong relationships of t_evVel_Gyro and t_evVel_Gonio were found for all 12 conditions. Based on our results, it can be concluded that gyroscopes can accurately determine peak eversion velocity and time until peak eversion velocity during a variety of conditions (e.g., to compare the influence of footwear conditions on evVel in prolonged field runs). The authors would like to thank [PERSON] for proofreading the paper as a native speaker. This research was supported by Puma(r) Inc., Germany. The publication costs of this article were funded by the German Research Foundation/DFG and the Technische Universitat Chemnitz, Chemnitz, Germany, through the funding program Open Access Publishing. [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON], [PERSON] and [PERSON] analyzed the data; [PERSON] contributed reagents/materials/analysis tools; [PERSON], [PERSON] and [PERSON] wrote the paper. The authors declare no conflicts of interest. The funding sponsors had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.