Computer Simulation of how Lumbar Spine Biomechanics are Influenced by Female Breast Mass and Motion

  • Melissa Jones

Student thesis: Doctoral Thesis


Introduction: The mass and motion of the breasts have been previously associated with changes in upper body kinematics and muscular activity, however, the internal loading, such as that on the lumbar spine, has rarely been considered and may provide an insight into the incidence of lower back pain reported by many women with a large (>1.9kg) breast mass. To date, musculoskeletal models have neglected the mass and motion of the female breasts, which may be an important consideration when calculating loading within the spine. The aim of this body of work was to investigate how dynamic breast mass affects lumbar spine moments using a newly developed, novel, female specific simulation model.
Methods: Breast magnetic resonance imaging (MRI), whole-body kinematic and ground reaction force data were collected for a single participant (age 27 years, height 1.64 m, mass 65 kg, UK bra size 34D, total breast volume 1220 ml). Breast MRI plus kinematics during standing, walking (1.3 m·s-1) and running (3.7 m·s-1) were used to develop and evaluate a new method of calculating breast motion centre of mass (COM). Breast segments, located at the breast COM were added to three different whole-body, subject specific musculoskeletal models: a no breast model with breast mass included within the torso segment; a static breast model with breast mass separate to the torso with no motion; and a dynamic breast model with separate breast mass segments which moved relative to the torso. Lumbar spine moments were then calculated using each model. The magnitude of breast masses and motion were altered within the dynamic breast model and the subsequent effect on lumbar spine moments determined.
Results: Modelling the breast as a point mass at the COM position resulted in a more posterior, medial and superior position, and a reduction (47% to 73%) in displacement during dynamic activities compared to the common method of reporting nipple position. Breast deformation resulted in a notable change in COM breast position compared to the nipple, however, the inclusion of non-uniform density had little effect on breast COM position (<0.001 m). Assuming no breast motion in the static breast model resulted in altered lumbar spine moment (up to 0.11 Nm⸱kg-1) compared to the dynamic breast model. Including the breast mass within the torso segment in the no breast model resulted in further changes compared to the static (up to 0.03 Nm⸱kg-1) and dynamic breast models (up to 0.11 Nm⸱kg-1). Lumbar spine moment increased (up to 0.15 Nm⸱kg-1) with increased breast mass during all activities, however, reducing breast motion had little impact (<0.01 Nm⸱kg-1) on lumbar spine moment.
Conclusion: Modelling the breast as a point mass located at the COM of a deforming breast improved upon using the nipple position to represent breast position by removing assumptions of breast deformation and density uniformity. Incorporating breast motion into female musculoskeletal models is an important feature for future research to ensure lumbar spine loading is accurately estimated within the female population during physical activity. Large breast mass (>1.9kg) can result in increases in spinal moments which can be associated with musculoskeletal pain; reducing breast motion is ineffective in reducing the lumbar spine moments which result from the breast mass.
Date of Award7 Jun 2024
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorChris Mills (Supervisor), Timothy Exell (Supervisor) & Joanna Wakefield-Scurr (Supervisor)

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