Quantitative underwater 3D motion analysis using submerged video cameras: accuracy analysis and trajectory reconstruction

TLDR

The study investigates the applicability of underwater 3D motion capture using submerged video cameras for accuracy analysis and trajectory reconstruction. The authors calibrated submerged cameras with static points (DLT), a moving wand (bundle adjustment), and a moving 2D plate (Zhang's method), then recorded fingertip trajectories of four trained male swimmers performing butterfly, breaststroke, and freestyle. The calibration achieved sub‑millimetre accuracy (0.96 mm for the wand, 0.73 mm for the 2D plate) far better than classical DLT (9.74 mm), and reconstructed fingertip trajectories were largely symmetric, matching Maglischo's model and revealing inter‑ and intra‑subject variability, thereby advancing quantitative 3D underwater motion analysis.

Abstract

In this study we aim at investigating the applicability of underwater 3D motion capture based on submerged video cameras in terms of 3D accuracy analysis and trajectory reconstruction. Static points with classical direct linear transform (DLT) solution, a moving wand with bundle adjustment and a moving 2D plate with Zhang's method were considered for camera calibration. As an example of the final application, we reconstructed the hand motion trajectories in different swimming styles and qualitatively compared this with Maglischo's model. Four highly trained male swimmers performed butterfly, breaststroke and freestyle tasks. The middle fingertip trajectories of both hands in the underwater phase were considered. The accuracy (mean absolute error) of the two calibration approaches (wand: 0.96 mm – 2D plate: 0.73 mm) was comparable to out of water results and highly superior to the classical DLT results (9.74 mm). Among all the swimmers, the hands' trajectories of the expert swimmer in the style were almost symmetric and in good agreement with Maglischo's model. The kinematic results highlight symmetry or asymmetry between the two hand sides, intra- and inter-subject variability in terms of the motion patterns and agreement or disagreement with the model. The two outcomes, calibration results and trajectory reconstruction, both move towards the quantitative 3D underwater motion analysis.