## 4 Discussion  
This paper is the first to evaluate the validity and reliability of 2D video analysis of the h\({}_{\text{peak}}\) and L of vaults in artistic gymnastics. Compared to 3D analysis (validity), 2D analysis showed smaller \(\pm\)95% limits of agreement for h\({}_{\text{peak}}\) (\(\pm\)3.6%) than for L (\(\pm\)7.6%). Further, we found smaller random errors for h\({}_{\text{peak}}\) (CV% = 2.24%) than for L (CV% = 4.64%). In contrast, there was a tendency toward a systematic error of h\({}_{\text{peak}}\) (p = 0.06), but not for L (p = 0.43). Regarding reliability, the h\({}_{\text{peak}}\) and L of vaults can be repeatedly (intrarater reliability: h\({}_{\text{peak}}\): CV% = 0.44%, r = 0.99; L: CV% = 0.87%, r > 0.99) and independently (interrater reliability: h\({}_{\text{peak}}\): CV% = 0.51%, r = 0.99; L: CV% = 0.72%, r > 0.99) determined by 2D analysis. The results support 2D video analysis as a valid measurement tool--particularly for determining the h\({}_{\text{peak}}\), but slightly less so for the L, of vaults in gymnastics. Our findings are in line with those of [PERSON], [PERSON], [PERSON], and [PERSON] [9], who have shown that 2D analysis is a valid technique for accurately measuring the flight height of vertical jumps. Thus, under the described measurement conditions, an increase of at least 6.16 cm in h\({}_{\text{peak}}\) or of at least 10.59 cm in L, measured by 2D video analysis, can be considered a true performance increase. Differences in the validity between the h\({}_{\text{peak}}\) and L of vaults, as shown in the Bland-Altman diagram, may have several reasons. The most obvious is that 2D analysis measures the height and length of flight in the sagittal plane only. Accordingly, lateral displacement at the landing cannot be detected by 2D analysis and presents a possible confounding factor. This was shown by the significant relationship between lateral displacement upon landing and the difference between 2D and 3D determinations of L. This result supports research by [PERSON], [PERSON], [PERSON], and [PERSON] [1], who compared 2D to 3D analysis of joint angles during complex movements. They found that 2D analysis is a valid measure only when cameras are placed perpendicular to the segment of interest. This limitation of 2D analysis was less important for the measurement of h\({}_{\text{peak}}\), because the lateral displacement is at its maximum at landing, long after the attainment of h\({}_{\text{peak}}\). Further, since L was determined as the distance between the vaulting table and the ankle, even straddled legs at landing may result in a lateral displacement and may lead to a measurement error of 2D analysis. In contrast, h\({}_{\text{peak}}\) was measured vertically from the landing mat to the center of gravity. Therefore, the determination of h\({}_{\text{peak}}\) did not depend on the position of one single extremity but on the highest point on the center of gravity's smooth trajectory. Another reason for the less accurate measurement of L could be the quality of the 2D videos (resolution and frame rate). The still frame at landing was often slightly blurred due to the velocity of the recorded movement. Therefore, it was difficult in some cases to determine the position of the ankle precisely, and, for this reason, small measurement errors may have occurred. In contrast, the slightly blurred image did not affect the determination of the center of gravity for the measurement of h\({}_{\text{peak}}\), since the determination of the center of gravity depends on a global view of the body and not on a small single part of the body. One more reason for the larger inaccuracy in the determination of L may be the slight bend in the calibration rod when it was held horizontally to scale the image width (Figure 1). This would have