Intentionally Light-Loss Carbon-Optic Fiber (COF) Twisted Sensor for Calf Strength Sensing via Monitoring Vastus Medialis

Flexible sensing exhibits application scenarios in the measurement of muscle deformation and wearable robots. Muscle deformation during exercise is multidirectional, and therefore one of the essential challenges is capturing muscle deformation in multiple directions. In this study, we proposed a wearable sensor of twisted carbon fiber and optical fiber [polymethyl methacrylate (PMMA)] embedded in the fabric, and by converting the light intensity loss to voltage loss, the sensor has the capability to detect muscle activation during movement. The sensor mainly consists of a light-emitting diode (LED), a photodiode to provide and convert light to a voltage signal, a twisted carbon-optic fiber (COF) structure to ensure bending without elongation during deformation under force, and an amplification filter circuit to modulate the signal. A commercial tension sensor was used to verify the relationship between the activation of the vastus medialis and force during calf flexion and extension. Three subjects were employed to complete the experiment, and a correlation coefficient of 0.99 was obtained as well as an experimental average correlation coefficient with a standard deviation of around 0.01. Although influences of the manual manufacturing process are apparent, as well as individual reasons of different subjects, the experimental results suggested that the presented COF sensor can be effectively applied in finding the relationship between vastus medialis activation with tension in calf flexion and extension. We also indicate that the sensor is not sensitive to differences between the right and left legs. This work paves a new way for the design of sensing systems, for example, prostheses and exoskeletons.