Natalie Sidorkewicz carefully analyzed spine and hip motions of ten couples each having sex in five positions. Sidorkewicz wrote it up, as her master’s thesis :
“Lumbar Spine and Hip Kinematics and Muscle Activation Patterns during Coitus: A comparison of common coital positions,” Natalie Sidorkewicz, Master of Science thesis in Kinesiology, University of Waterloo, 2013.
The main objective of this study was to describe male and female lumbar spine and hip motion and muscle activation patterns during coitus and compare these motions and muscle activity across five common coital positions. Specifically, lumbar spine and hip motion in the sagittal plane and electromyography signal amplitudes of selected trunk, hip, and thigh muscles were described and compared…..
Ten healthy males (29.3 ± 6.9 years, 176.5 ± 8.6 centimeters, 84.9 ± 14.5 kilograms) and ten healthy females (29.8 ± 8.0 years, 164.9 ± 3.0 centimeters, 64.2 ± 7.2 kilograms) were included for analysis in this study. These couples had approximately 4.7 ± 3.9 years of sexual experience with each other. This study was a repeated-measures design, where the independent variables, coital position and condition, were varied five times, respectively. Recruited participants engaged in coitus in five pre-selected positions (presented in random order) for 20 seconds per position first in a simulated condition, and again in a real condition….
To determine if each coital position had distinct spine and hip kinematic and muscle activation profiles, separate univariate general linear models) followed by Tukey’s honestly significant difference (HSD) post hoc analysis were used.
Here’s detail, concerning the missionary position, from the study:
Sidorkewicz also wrote it up in a form that’s about to be published in a journal:
“Male Spine Motion during Coitus: Implications for the low back pain patient,” Natalie Sidorkewicz, Spine, vol. 39, no. 20, September 15, 2014.
BONUS: Hip, Safe Sex Motion Study