Swimtraxx One is designed to be as small and as comfortable as possible. We worked together with a wide range of athletes from the Olympic to the recreational level to make sure the size and shape of Swimtraxx One are optimized for comfort.
It can easily be mounted onto the swimming goggles, and the general feedback is that after a while you can not feel you are wearing it. It's like getting a new pair of goggles.
For some swimming goggles, such as the Arena Cobra, you can not attach the Swimtraxx One directly to your goggles in a position that allows the heart rate sensor to make contact with the skin. That's why you can also wear Swimtraxx One under your swim cap. Be sure to give the sensor the same orientation as if it were connected to your goggles!