Journal of Science and Medicine in Sport (2008) 11, 90—95 ORIGINAL PAPER Reconstruction accuracy in underwater three-dimensional kinematic analysis Vassilios Gourgoulis * , Nikolaos Aggeloussis, Panagiotis Kasimatis, Nikolaos Vezos, Alexia Boli, Giorgos Mavromatis Democritus University of Thrace, Department of Physical Education and Sport Science, Greece Received 31 May 2006; received in revised form 22 February 2007; accepted 22 February 2007 KEYWORDS Accuracy; Three-dimensional; Kinematics; Swimming Summary Accuracy of kinematic data is of decisive importance in motion analysis particularly when they are to be used for the calculation of kinetic parameters, like the propulsive forces in swimming. The present study investigated the accuracy of underwater three-dimensional (3-D) kinematic analysis using periscope systems and calibration volumes with different size. Two analogue cameras (60Hz) were used to record the images, along with two stationary periscope systems for the under- water recordings. The calibration of the recording space was performed using two different-sized calibration frames. The reconstruction accuracy was determined in static and dynamic conditions, both under water and out of the water. In static con- ditions, the reconstruction accuracy was determined by calculating the root mean square (RMS) error for the reconstruction of eight validation points on the calibra- tion frames, and in dynamic conditions it was determined by the percentage of RMS error of the reconstructed length of a moving rod. The results revealed that in static conditions the RMS error was greater during underwater analysis, due to refraction, and it was increased particularly in the longitudinal axis as the dimensions of the calibration volume were increased. The reconstruction errors observed during under- water recordings with both calibration volumes (transverse axis, 1.61—2.35 mm; longitudinal, 2.99—4.64 mm; vertical, 2.83—2.59 mm) may be considered suitable for three-dimensional kinematic analysis of swimming. © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. Introduction Three-dimensional (3-D) kinematic analysis is the most common method used for the assessment of the underwater stroke in swimming since the ∗ Corresponding author. E-mail address: vgoyrgoy@phyed.duth.gr (V. Gourgoulis). hand moves under water in all the three axes simultaneously. 1 In all motion analyses, accuracy of the kinematic data is important particularly if they are to be used to calculate kinetics of the motion. 2 In 3-D kinematic analysis the most popular tech- nique used for the transformation of the two dimensional (2-D) image coordinates into 3-D space coordinates is the direct linear transformation method (DLT). 3—6 With the DLT technique an appro- 1440-2440/$ — see front matter © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jsams.2007.02.010