# Survey on Video-Based Biomechanics and Biometry Tools for Fracture and Injury Assessment in Sports
## 1 Introduction
### Video-Based Biomechanics  
Biomechanical analysis can be performed by either video and/or kinematic sensors. In video analysis, 3D and 2D are typically found in the literature, as well as depth detection images [5; 69; 68]. In these types of procedures, in some cases, optical markers are used to detect different body parts and thus detect their position in real time in a given coordinate frame [70]. Some of the most used sensors in the literature to represent human kinematics are: electromagnetic, acoustic, and inertial sensors, Kinect platform (RGB, depth sensors, IR, accelerometers, and microphones), as well as mobile applications [70]. While different sensors provide useful information, video analysis is typically preferred by non-specialized customers because the outputs of the models, expressed as angles or coordinates which complement the videos, are easier to understand [6].  
Video analysis for biomechanical purposes has been increasing in recent years. Depending on the complexity of the desired model, this type of analysis can be performed manually, by manually selecting the positions of relevant body parts at different video frames, or automatically, by involving image recognition/processing algorithms to perform this procedure in an automated manner [71]. Although manual digitizing is not often considered as a difficult process, when analyzing large videos with several frames, it becomes a time-consuming task which could be solved in a more efficient way by an algorithm. Other more simple video analysis have been reported in the literature to assess concussions in Australian football [72] and bicycle accidents [73], however also require manual supervision and thus, are time-consuming.  
3D Video analysis involving multiple cameras allow to perform three-dimensional reconstructions of the environment, thus obtaining three-dimensional position, speed, and acceleration measurements in human models. 3D video analysis is usually referred to in biomechanics as the gold standard due to its precision and degree of information provided [6]. However, expensive, and specialized software and equipment are needed to perform this type of analysis. Even more so if the analysis is performed using optical markers.  
Biomechanical analysis through 2D video offers a relatively simple and low-cost solution to analyze human motion, as it can be easily performed using cellphones, and low-cost cameras. Hence, making biomechanical analysis feasible to general users outside laboratory-like environments, which makes it appealing for risk assessment and training in sports, medical and industry fields. The downside of video analysis is that proper setup about the angle and environmental lighting of the films need to be considered to obtain reliable measurements. However, many studies in the literature have reported successful biomechanical analyses using 2D video protocols, such as race walking, vertical jump squats, baseball pitching, martial arts [6; 68; 70; 74]. Some of the studies also compare the performance of 2D vs 3D video analysis and report no significant differences between both when using proper filming setups [5; 74; 75].  
Marker-less image processing algorithms offer a solution for automated biomechanical assessment for low-cost video analysis. Some of the reported algorithms include image processing techniques such as model reconstruction from multiple views using silhouette recognition algorithms [71] and 2D stick-like models of the human body based on polynomial regression [76].