Physicists and engineers, among others, might be able to appreciate the care that went into this study of female spine motion during coitus:
“Documenting female spine motion during coitus with a commentary on the implications for the low back pain patient,”Natalie Sidorkewicz and Stuart M. McGill, European Spine Journal, Volume 24, Issue 3, March 2015, pp. 513-520. The authors, at the University of Waterloo, Canada, report:
PURPOSE: To describe female lumbar spine motion and posture characteristics during coitus and compare these characteristics across five common coital positions….
METHODS: Ten healthy males and females performed coitus in the following pre-selected positions and variations: QUADRUPED (fQUAD1 and fQUAD2 where the female is supporting her upper body with her elbows and hands, respectively), MISSIONARY (fMISS1 and fMISS2 where the female is minimally and more flexed at the hips and knees, respectively), and SIDELYING. An electromagnetic motion capture system was used to measure three-dimensional lumbar spine angles that were normalized to maximum active range of motion—a transmitter and receiver were affixed to the skin overlying the lateral aspect of the pelvis and the spinous process of the twelfth thoracic vertebra, respectively….
RESULTS: Based on the spine kinematic profiles of each position, the least-to-most recommended positions for a female flexion-intolerant patient are: fMISS2, fMISS1, fQUAD1, fSIDE, and fQUAD2.
Here’s further, graphic detail from the study:
(Thanks to Delia Bailey for bringing this to our attention.)