: Spinal flexibility radiographs are important in adolescent idiopathic scoliosis (AIS) for clinical decision-making. In this study, we introduce a new method, the 'quantitatively controlled standing fulcrum side-bending' test (CSFS test). This is a feasibility study; we aimed to quantify the applied force and track the temporospatial changes in the spine specifically by measuring the continuous change in the Cobb angle (in degrees) during lateral bending. : In this cross-sectional study, we included patients with AIS. Using a low-dose cinematic fluoroscopic technique, we captured the lateral bending of the thoracolumbar vertebral spine while inducing quantified lateral force on the ribs by a force gauge in a three-point fixation setup of controlled lateral bending. Trial registration number: H-1703423. : Twenty-one patients with small-curve AIS were included as subjects. All subjects performed the CSFS test adequately. They had small curves with a mean Cobb angle of 12.0 (range: 0.0-26.0, SD: 7.1). The measured median stiffness was 3.66 N/degrees (°) of the Cobb angle (range: 0.02-11.81) with a median coefficient of determination R of 0.58 (range: 0.002-0.92) by regression analyses. When analysed concerning the median-term clinical outcome of either progression/regression or stationary curves, various Cobb angle measurements and the other experimental parameters, there were no significant relationships. : The CSFS test is feasible to quantify the force applied and the temporospatial changes in the spine during lateral bending. The CSFS test has been utilised in basic research for mechanical characterisation of the scoliotic spine and has the potential of being implemented directly in patient-specific bracing to estimate the forces needed for brace correction and adjustment so as not to supersede the allowed skin pressure.