METHOD FOR 2-DIMENSIONAL ANALYSIS OF ABNORMAL MEDIAL KNEE MOTION

Abstract

Purpose: A new method to quantify knee movement was developed to identify athletes with increased medial knee motion during the initial landing phase of a box drop vertical jump. Athletes with increased medial knee motion likely exhibit decreased joint control in the coronal plane and may be at an increased risk of knee injury. METHODS: The method to measure this motion was designed to be portable and convenient in order to analyze numerous athletes at a practice or a game location. A standard digital video camcorder (30Hz) was used to record an athlete performing a drop off of a 30.5cm box and immediately performing a vertical jump to a height of 80% of the athlete's maximum vertical jump height. A jump-training device (MX-1, MXP Sports) that employs a basketball suspended on a retractable cable was used to measure maximum vertical jump height and to position the ball at the testing height. Reflective markers were positioned at the lateral joint line of the right and left knee. The camcorder was positioned 6.5m away from the box and aligned parallel to the floor. Three trials were recorded for each athlete. Each athlete dropped off the box and performed a vertical jump and grabbed the ball at the top of the jump. The digital video containing all three trials was converted to an AVI file on a PC. Two frames from each trial, initial touchdown (IT, frame when contact is made with the ground after dropping from the box) and maximum motion (MM, frame when the largest medial/lateral motion is present during the landing phase) were transferred to a 2D digitizing program (tpsDig, F.J. Rohlf). A known distance of 30.5cm was digitized on the first frame to calibrate to actual cm measurements. The right (R) and left (L) knee coordinates were digitized for each frame (total of 6 frames) and the XY coordinates stored in an ASCII text file. RESULTS: A custom program was used to calculate knee motion (MKM=[(√((XLIT-XRIT)2+(YLIT-YRIT)2))-(√((XLMM-XRMM)2+(YLMM-YRMM)2))]) as the change in total distance of the right and left markers between the first and second frames of each trial and was then averaged across the three. CONCLUSION: This method can easily be applied to large groups of athletic teams to identify athletes with abnormal medial knee motion; and those athletes that might be at an increased risk of knee injury.