Patients with lower limb dysfunctions have high risks of developing deep vein thrombosis (DVT) symptoms if they sit in a wheel chair for long periods of time. Developing DVT is easily caused from the standstill of venous blood flow. It is important for the patients to increase the velocity of venous blood flow in order to decrease the risk of developing DVT. Exercise of bending and stretching legs is one of the effective methods for prevention of DVT by increasing velocity of venous blood flow. Therefore, such patients can't move them lower limbs at will, in this study we focused on providing passive support through repetitive sit-to-stand and stand-to-sit (RSTS) motions. The purpose of this study is to develop RSTS motion support system for bending and stretching exercises. The system is structured from a linear mechanism and a control unit. We implemented passive exercise using PID control algorithms. A benchmark experiment was carried out with weights in order to evaluate basic performance of our system. We also experimented measurement of blood flow velocity with a passive RSTS motion for three able-bodied participants. The result of the first experiment showed that the system has the ability to support weight of 80 kg. The results of the second experiment have shown that both average and peak blood flow velocity in high frequency exercise were three times faster than that in resting. The increasing of venous blood velocity was caused from bending and stretching exercise using the developed system. Therefore the integrated system might have potential for decreasing the risk of DVT of wheelchair user by supporting a RSTS motion.