## 1 Introduction  
Video analysis is common in elite sports and covers different areas of application. In individual sports, especially acrobatic sports, video analysis is used to compare and improve complex skills [1]. For quantifying biomechanical aspects of such skills, such as score-relevant kinematic variables of vaults in artistic gymnastics, 3D motion capture (3D video analysis) is used. [2, 3]. By means of video analyses, [PERSON] and [PERSON] [4] created reference values for kinematic parameters, indicating the safe execution of vaults. To make use of such reference values, measuring certain kinematic parameters in routine training becomes crucial, so that gymnasts' performances can be compared with the requirements of certain vaults. In regard to monitoring training progress, using video analysis during routine training is intriguing [2, 3].  
The gold standard for complex kinematic analyses is 3D motion capture, with which movements can be analyzed in sagittal, frontal, and transversal planes simultaneously [5]. However, precise 3D motion capture is associated with considerable financial, spatial, and temporal issues, and is therefore uncommon in routine training settings [6]. Two-dimensional video analysis (2D analysis),if sufficiently accurate, could offer a more practical alternative for measuring kinematics during training, especially since advancements in smartphone and tablet technology are continually making high-definition video capture and analysis more convenient. Aside from convenience while capturing motion, 2D video is faster, cheaper, and requires less prior knowledge compared to 3D analysis [1; 7]. Nonetheless, although multiple studies showed moderate validity compared to 3D motion capture and high intrarater reliability of 2D analysis when examining joint angles during simple movements, such as single-leg squats and running [1; 5; 6; 7], the validity and reliability of 2D video analysis for complex skills in acrobatic sports are unclear and must be scientifically assessed before this method can be recommended for gymnastics. The validity and reliability of 2D analysis were deemed sufficient for measuring the flight time and height of vertical jumps, when compared to optical measurement systems (Opto Jump, Microgate, I) [8] or to a force plate [9]. Further, 2D analysis was deemed valid (ICC = 0.8) and reliable (ICC \(>\) 0.85) for measuring joint angles during the more complex skill of baseball pitching, as long as the camera was placed properly [1], so the possibility of it being useful in gymnastics seems worthy of investigation.  
In summary, the studies mentioned above offer justification for using simple, time-saving, and cost-effective 2D analysis for simple lower-extremity movements based on good validity and reliability compared to gold-standard methods. However, in contrast to movements in previous studies, artistic gymnastics involve highly complex movements that are performed with high accelerations and velocities. Therefore, the aim of this study was to investigate criterion validity (comparison of 2D to 3D analysis) and both intrarater (test-retest) and interrater (equivalence of two assessors) reliability of 2D video analysis for the peak height (h\({}_{\text{peak}}\)) and length of flight (L) of vaults in artistic gymnastics.