Is trunk motion influenced by breast volume? The effect on joint moments at the hip, knee and ankle

Objectives: The relationship between breast size and factors impacting running biomechanics is poorly understood. This study examined the effect of breast volume on trunk motion and subsequent, lower limb joint moments. Design: Cross-sectional. Methods: Thirty female recreational runners (volume range: 91–1030 mL per breast) completed a 4-minute treadmill running protocol on an instrumented treadmill at a velocity of 10 km·h⁻¹ whilst kinematic and kinetic data were collected. Breast volume (mL) of each participant was quantified from a three-dimensional surface scan of their breasts. Results: The mean breast volume of study participants was 353 ± 196 mL (range: 91–1030 mL, per breast). Breast volume did not influence peak trunk to pelvis relative angle in any plane, however higher breast volume was linked to greater hip flexion moment. Greater peak trunk counterclockwise rotation was associated with less peak ankle plantarflexion moment (β [standardised effect] = − 0.64, p = 0.001). Greater peak trunk extension was associated with a greater peak hip extension moment (β = 0.46, p = 0.006). Conclusions: Whilst relationships between trunk motion and lower limb moments were observed, and a direct association between breast volume and hip flexion moment was observed, breast volume was not related to trunk mass. Either the absolute mass of the breasts may not be large enough to elicit changes in trunk motion or women with larger breast volumes have developed adaption strategies to counteract an increased trunk mass, eliminating any impacts of breast volume on trunk motion.