A limitation of projected frontal area as an indicator of active drag in swimming: Focusing on tibial and femoral segments

The projected frontal area (PFA) is a useful indicator of swimming drag. However, it is inherently limited because it only considers observable frontal areas from a frontal view. To address this limitation, we determined a new indicator, the projected and occluded frontal area (POFA), which includes occluded frontal areas relative to the swimming direction. This study aimed to examine the difference between the PFA and POFA, focusing on the tibial and femoral segments during front crawl. Twelve competitive male swimmers performed a 15-meter front crawl at 1.20 m·s⁻¹. The three-dimensional positions of the reflective markers attached to the swimmers’ bodies were collected using an underwater motion-capture system. The body shape of each swimmer was obtained using a body scanner. Two types of digital human models were created: a whole-body model with vertex colors divided into eight body segments and a segment-specific model extracted from the whole-body model. To reconstruct identical motions in both models, the joint angle data obtained through inverse kinematics computations using motion-capture data and the whole-body model were applied to the segment-specific models. The PFA and POFA were determined through image processing of a series of parallel frontal images from whole-body and segment-specific models, respectively. The PFA of the tibial and femoral segments was substantially smaller than the corresponding POFA (p < 0.001), with underestimation ratios of 86.1 % and 42.3 %, respectively. These results suggest that PFA is not a fully reliable indicator for evaluating swimming drag, at least in the tibial and femoral segments.