In examples, methods and systems for sensing tension in a timing belt are provided. In one example, a transmission device is provided that comprises a timing belt transmission configured to cause rotation of an output hub, and the timing belt transmission includes a timing belt with markings. The transmission device also includes a detector for detecting the markings on the timing belt as the timing belt is placed under a tension due to a load at the output hub, and a distance between the markings on the timing belt changes as the timing belt is placed under the tension. The transmission device also includes one or more processors for determining output torque of the timing belt transmission based on the distance between the markings on the timing belt or based on a time between detected markings.