Studies suggest that the level of muscle activation may influence force reproduction. The purpose of this study was to develop unpowered assistive clothing without actuators or electrical devices and examine its efficacy for reducing voluntary muscle activation and improving force reproduction in the upper limbs of healthy young adults. The resulting SEnS was made of elastic fabric and designed to produce assistive shoulder flexion moments that can partially mitigate the required activation of the user’s shoulder flexor muscles. To examine its efficacy, voluntary muscle activation and force reproduction performance was examined with and without using the SEnS by comparing the amount of activation in the shoulder flexor muscles and the error in force reproduction in the upper limb. The results show that wearing SEnS reduces muscle activation and force reproduction error. These results suggest that the accuracy of force reproduction can be improved substantially by partially mitigating the shoulder moment and associated voluntary muscle activation using the SEnS.