Knee injuries in World Cup freestyle skiing. Dynamic analysis of injury and non-injury situations for the assessment of release criterions in an "intelligent" boot-binding-release system (PhD)
Video analysis of the mechanisms for ACL injuries
Serious knee injuries, such as anterior cruciate ligament (ACL) injuries, are a growing cause of concern. Therefore, much attention has been given to non-contact ACL injuries in team sports. However, the mechanisms of injury are poorly understood and controversy exists on the loading patterns involved, which limits our ability to develop improved and targeted prevention programs. A complete description of the mechanisms for a particular injury type in a given sport needs to account for the events leading to the injury situation (e.g. playing situation, player and opponent behavior), as well as include a precise description of whole body and joint biomechanics at the time of injury.
The aims of this study were to develop and validate a new model-based image-matching technique for three-dimensional reconstruction of human motion from uncalibrated video sequences. This method was then used to estimate kinematic characteristics of three typical ACL injury situations from basketball, downhill skiing and European team handball. Furthermore, the accuracy and precision of kinematic estimates from video sequences of situations resembling those typically leading to ACL injuries, using simple visual inspection, was tested. We also tested if accuracy and precision could be improved by a training program. Finally, the mechanisms of ACL injury in 39 basketball cases were described.
Paper I: A new model-based image-matching technique for three-dimensional human motion reconstruction from video sequences was developed, and its accuracy was assessed using traditional motion analysis as a gold standard. This method involves manual matching of a skeleton model to the background image utilizing the commercially available 3D modeling software Poser®. A laboratory trial was conducted with one test subject performing jogging and side step cutting, while being filmed with three ordinary video cameras. This provided three single camera matchings, three double camera matchings and one triple camera matching for each of the motions. The test subject wore 33 reflective skin markers and was filmed with a seven-camera, 240Hz motion analysis system. Root Mean Square (RMS) hip and knee flexion/extension angle differences were less than 12° for all the matchings. Estimates for ad-/abduction (<15°) and internal/external rotation (<16°) were less precise. RMS velocity differences up to 0.62 m/s were found for the single camera matchings, but for the triple camera matching the RMS differences were less than 0.13 m/s for each direction. The kinematic estimates, in particular for COM velocity and acceleration, are clearly better when two or more camera views are available. This method can potentially be used to arrive at more precise descriptions of the mechanisms of sports injuries than what has been possible without elaborate methods for 3D reconstruction from uncalibrated video sequences, e.g. for knee injuries.
Paper II: A four-camera basketball video, a three-camera European team handball video and a single-camera downhill skiing video were analyzed, using the new model-based image-matching method described in Paper I. When the match was considered satisfactory, joint angles as well as velocity and acceleration of the center of mass were calculated using Matlab®. In the basketball and handball matchings, the skeleton and surrounding models were successfully matched to the background through all frames in all camera angles. Detailed time courses for joint kinematics and ground reaction force were obtained, while less information could be acquired from the single-view skiing accident. In conclusion, the model-based image matching technique can be used to extract kinematic characteristics from video tapes of actual ACL injuries, and may provide valuable information on the mechanisms for ACL injuries in sports.
Paper III: Using a traditional surface marker based infrared, 240 Hz, 3D motion analysis system, we recorded running and cutting trials from three test subjects. Six international researchers were asked to provide estimates of kinematic variables from 27 video composites from one, two or three ordinary cameras, systematically varying viewing angles and time point of analysis. The analysts thereafter went through a training program where 35 similar composites were analyzed, and feedback on the kinematics as measured by the 3D motion analysis system was provided on a group basis. Finally, the pre-test was repeated to test for accuracy and precision. The mean error for knee flexion was -19°, indicating a consistent underestimation. Hip flexion was underestimated by 7°, but the standard deviation between the analysts was 18° on average, indicating poor precision. Substantial errors were also found in the accuracy and precision of the other estimates. Only small group effects were seen from our training program. Based on these findings, we concluded that results from studies using a simple visual inspection approach to describe joint motion should be interpreted with caution.
Paper IV: Six international experts independently analyzed 39 videos (17 male, 22 female) of ACL injury situations from high school, college and professional basketball games. Two pre-defined time points were analyzed; initial ground contact and 50 ms later. The analysts were asked to assess the playing situation, player behavior and joint kinematics. There was contact at the assumed time of injury in 11 of the 39 cases (five males, six females). Four of these cases were direct blows to the knee, all in men. Eleven of the 22 female cases collided or were pushed by an opponent prior to the time of injury. The estimated time of injury ranged between 17 and 50 ms after initial ground contact. The mean knee flexion angle was higher in females compared to males, both at initial contact (15° vs. 9°, p=.034) as well as 50 ms later (27° vs. 19°, p=.042). Valgus knee collapse occurred more frequently in females compared to males (relative risk: 5.3, p=.002). Preventive programs to enhance knee control should therefore focus on avoiding valgus motion, and include distractions resembling those seen in match situations.”