Investigation of the factors influencing spinal manipulative therapy force transmission through the thorax: a cadaveric study

BACKGROUND: Spinal manipulative therapy (SMT) clinical effects are believed to be linked to its force–time profile characteristics. Previous studies have revealed that the force measured at the patient-table interface is most commonly greater than the one applied at the clinician-patient interface. The factors explaining this force amplification remains unclear. OBJECTIVE: To determine the difference between the force applied to a cadaveric specimen’s thoracic spine and the resulting force measured by a force-sensing table, as well as to evaluate the relationship between this difference and both the SMT force–time characteristics and the specimens’ characteristics. METHODS: Twenty-five SMTs with different force–time profiles were delivered by an apparatus at the T7 vertebra of nine human cadaveric specimens lying prone on a treatment table equipped with a force plate. The difference between the force applied by the apparatus and the resulting force measured by the force plate was calculated in absolute force (F(diff)) and as the percentage of the applied force (F(diff%)). Kinematics markers were inserted into T6 to T8 spinous and transverse processes to evaluate vertebral displacements during the SMT thrusts. Mixed-effects linear models were run to evaluate the variance in F(diff) and F(diff%) explained by SMT characteristics (peak force, thrust duration and force application rate), T6 to T8 relative and total displacements, and specimens’ characteristics (BMI, height, weight, kyphosis angle, thoracic thickness). RESULTS: Sixty percent of the trials showed lower force measured at the force plate than the one applied at T7. F(diff)¸ was significantly predicted (R(2)(marginal) = 0.54) by peak force, thrust duration, thoracic thickness and T6–T7 relative displacement in the z-axis (postero-anterior). F(diff%) was significantly predicted (R(2)(marginal) = 0.56) by force application rate, thoracic thickness and total T6 displacements. For both dependant variables, thoracic thickness showed the highest R(2)(marginal) out of all predictors. CONCLUSION: Difference in force between the clinician-patient and the patient-table interfaces is influenced by SMT force–time characteristics and by thoracic thickness. How these differences in force are associated with vertebral displacements remains unclear. Although further studies are needed, clinicians should consider thorax thickness as a possible modulator of forces being transmitted through it during prone SMT procedures. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12998-023-00493-1.