The projected frontal area (PFA), which predominantly determines active drag (Gatta et al., 2015; Morais et al., 2020; Washino, Murai, et al., 2022), is decreased by an interaction between a higher vertical body position and a smaller trunk inclination during front crawl (Washino, Murai, et al., 2022). The vertical centre of mass (CoM) position, as an index of vertical body position, increases linearly with swimming velocity (Washino, Yoshitake, et al., 2022). In contrast, trunk inclination is not associated with swimming velocity (Washino, Yoshitake, et al., 2022). Taken together, the PFA is expected to have a negative relationship with swimming velocity, but the relationship has not yet been clarified due to limitations of measurements and analytical methods. For calculating PFA during swimming, two conventional methods have been employed: prediction models based on anthropometric features (Morais et al., 2011; Zamparo et al., 2009) and manual tracing of a series of perspective frontal snapshots of a swimmer (Gatta et al., 2015; Morais et al., 2020). However, these methods have inherent problems. The problem with the former method is that the formulas do not account for differences in body shape between swimmers but use a reference value, which causes measurement errors. The problem with the latter method is that it requires manual operation, which causes inter- or intra-operator variability in the calculated data and is time-consuming. To solve these problems, we have developed a new method for reconstructing volumetric swimming motion to reliably calculate PFA by combining an underwater motion-capture system with digital human technology (Washino, Murai, et al., 2022). Using our developed method for calculating PFA, we aimed to examine the relationship between swimming velocity and PFA during front crawl, (Washino, Murai, et al., 2022). We hypothesized that there is a negative relationship between swimming velocity and PFA, in contrast to a positive relationship between swimming velocity and vertical CoM position observed in the previous study (Washino, Yoshitake, et al., 2022).
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