Abstract: In this study, a flexible sensor module was developed that can be applied in the activation training of spinal muscles. Silver films were sputtered onto flexible substrates to produce a flexible sensor. Assuming that spinal muscle elongation is positively correlated with the variations in skin surface length, real-time resistance changes exhibited by the sensor during simulated trainings were measured. The results were used to identify the relationship between resistance change and skin surface length (the dependent variable). In addition, ultrasound measurements and electromyography were used to verify the feasibility of the proposed sensor. Furthermore, the developed module can facilitate assessments of the movement accuracy of users during training. Inaccurate movements cause drastic resistance changes in the films of the flexible substrates, and the results are displayed on a screen. Thus, people using the developed sensor in spinal muscle activation training can adjust their posture to the appropriate position.