To build an Functional Electrical Stimulation (FES) system that assists individuals with spinal cord injury (SCI) to improve sitting balance, one needs to determine multidirectional stiffness of the trunk during sitting and demonstrates that this stiffness can be influenced by FES. Therefore, the purposes of this study were:​ (1) to develop a novel protocol to quantify and identify the multidirectional stiffness of the trunk during sitting;​ and (2) to test the effect of FES on the multidirectional stiffness of the trunk. 15 able-bodied subjects were recruited. Each subject was perturbed in 8 different directions using 8 linear motors during quiet sitting with and without FES applied to his front/back trunk muscles. Stiffness in the lateral directions was the highest and FES increased stiffness of the trunk in the anterior/posterior direction. These results can be used to develop an FES protocol for individuals with SCI to improve their sitting balance.