Purpose This study aimed to investigate whether a lung volume-dependent decrease in the projected frontal area (PFA) contributes to reducing active drag in the front crawl. Methods Twelve competitive male swimmers performed a 15-m front crawl at 1.20 m·s-1 while sustaining one of three lung volume levels: maximal inspiration (INSP), maximal expiration (EXP), or intermediate (MID). The three-dimensional positions of the reflective markers attached to the swimmers` bodies were measured using an underwater motion capture system. Based on the body shape obtained from the photogenic body scanner, an individual digital human model was created using model vertices color coded into eight body segments. The time series of the volumetric swimming motion was reconstructed using the individual digital human model and motion capture data. The PFA of each body segment was calculated using image processing. The pressure drag index (PDI), defined as the value excluding the drag coefficient while simultaneously considering the PFA and horizontal velocity, was calculated for each body segment. Results There were significant interactions between lung volume and body segment on PFA and PDI (both P < 0.001). Specifically, the PFA and the PDI of the head segment were smaller in INSP than in EXP (P = 0.046); in addition, the PFA and the PDI of the trunk and femur segments were smaller in INSP and MID than in EXP (P = 0.003). Conclusions These findings suggest that the decrease in PFA caused by the increase in lung volume directly contributes to reducing active drag.
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