Kinematic Movement Analysis: Part One

Table 1: Comparison between 2D and 3D motion analysis⁽³⁾

Movement Analysis: A Structured Approach

As qualitative and semi-qualitative biomechanical analysis is fully or partially subjective, biomechanists propose a four-task structured approach to create order from possible chaos (see figure 2)^(2,6). 

2D Video Analysis: Practical Tips and Common Errors

Camera position
Camera position depends on the specific joint angles of interest and the plane of motion in which they occur (see table 2). For movements in the sagittal, frontal, or transverse plane, cameras are placed on the side, in front/back, or above the athlete, respectively. 

Table 2: Camera position for sprinting analysis

Camera frame rate
Frame rate is the number of images recorded per second (Hz or fps). The higher the number, the more individual frames are being captured in one second of video and the smoother the footage. Sporting movements like walking, running, slower jumps and throws, and many gymnastics movements are captured at sampling rates of 60 FPS or 50Hz⁽³⁾. Faster movements, such as baseball pitching or cricket fast bowling, are captured at frame rates as high as 250 FPS. If the frame rate is not high enough, essential aspects of the movement can be missed. Aliasing is undesirable when the movement frequency, i.e., input frequency, is greater than half of the sampling frequency, resulting in high-frequency movements being observed at a lower, aliased frequency. Therefore, the chosen frame rate should be at least double that of the movement frequency. It is important to note that resolution will decrease as the frame rate increases. Depending on the version of modern phones, the camera apps can capture between 30 and 60 FPS on the HD setting and either 120 or 240 FPS on the slow motion setting.

Shutter speed
Shutter speed is the time for which a camera’s shutter is open at a given setting, i.e., the amount of time the camera’s sensor is exposed to light⁽³⁾. Images will blur if the shutter speed is too slow; if it is too fast, images may be too dark (see figure 4). Practitioners should ensure that the shutter speed is double the frame rate. For example, at a preferred 60 FPS, the shutter speed should be set at 1/120th. Higher frame rates and shutter speeds allow more detail to be captured, especially when reviewing footage in slow motion.

Conclusion

Coaches, conditioning and medical professionals, and biomechanists perform motion analysis to improve athletes’ overall performance and mitigate injury risk or re-injury by improving the quality of movement(1). Although not as accurate and comprehensive as 3D motion analysis, practitioners prefer 2D video capture as it is cost-effective, user-friendly, and can yield useful insights into athletes’ movement patterns. The accuracy of the results is highly dependent on the accuracy of the 2D video capture methods.  Although not as user-friendly, optical motion capture (using light reflective marker or inertial measurement unit systems) methods are more accurate. The utility of these systems will be explained in part two.

References

1.    Am J Sports Sci Med. 2020;8(2)
2.    Qualitative Analysis of Human Movement
3.    Introduction to Sports Biomechanics: Analysing Human Movement Patterns. Second edition. Routledge, Taylor & Francis Group; 2007
4.    Int J Sports Phys Ther. 2017;12(2):163-172
5.    Phys Ther. 2020;100(10):1759-1770 
6.    Sports Biomech. 2007;6(1):109-118