Reference: A Ahmadi, D Rowlands, D James, Deriving Upper Arm Rotation from Vicon to Enhance the First Serve in Tennis,
Journal of Science and Medicine in Sport 12, S29-S30
Abstract: Introduction: Upper arm rotation is a major contributor (>53%) to the speed of a first serve in tennis. Monitoring this rotation using standard models in marker-based systems is problematic. In this paper we present a vector-based method using Vicon's standard plug-in-gait together with geometric calculations and a mapping of the space to calculate upper arm rotations. Methodology: The method was developed using the trajectory of six markers on the upper arm, shoulder and pelvis in each Vicon frame. A protocol was developed to collect data from the Vicon for angle validation. The protocol consisted of a predefined range of motions involving shoulder abduction, shoulder flexion and limb rotation for various trunk positions. A case study of 70 trials and 10 repetitions for each trail has been investigated in this paper. Discussion and results: The method calculates the rotation angle based upon a local coordinate system. The upper arm plane in each frame is translated and rotated to the initial upper arm plane (frame one) which aligns the upper arm to an axis. It was found that in some conditions variations in the marker positions led to some inaccuracies. This variation was accounted for by applying a surface correction technique. The case study shows that upper arm rotation can be calculated to -8DG when a correction surface is applied to the calculations.