Optimizing Breast Motion Control in Plus-Size Sports Bras: The Nonlinear Impact of Strap Elastic Modulus

Optimizing sports bra performance is important for improving exercise safety and wearer well-being. This study systematically quantifies the biomechanical interaction between strap elastic modulus (834.6–3087.2 N/m) and vertical breast motion control in plus-size cohorts (C–F cups). Fifteen healthy participants performed treadmill running at 7.5 km/h, while breast kinematics were captured via 3D motion capture integrated with subjective comfort assessments. Results demonstrate a nonlinear mechanical trend between strap modulus and vertical breast displacement. Specifically, both logarithmic (B = 91.923 − 8.257 ln(E)) and power function (B = 221.44 E−0.268) models illustrate the observed nonlinear trend, characterizing the diminishing marginal utility of increased material stiffness. While elevating modulus significantly attenuates oscillation amplitude, the inhibitory effect plateaus at higher levels due to breast tissue viscoelasticity. Qualitative feedback indicates that excessive rigidity elevates perceived shoulder pressure, thereby compromising ergonomic comfort. Consequently, this research advocates for a systemic equilibrium between motion attenuation and wearer tolerance, proposing an optimal strap modulus bandwidth of 2000–2500 N/m for the evaluated 85D prototype.