We present the design of a motion analysis sensor using a flexible fiber and an electronic circuit. The fiber is made of carbon nanotubes and polydimethylsiloxane elastomer. The performance of the fiber is thoroughly characterized for motion detection and analysis. The measurements were performed with a developed electronic circuit designed to interface with the fiber. It can read the changes in the fiber impedance with an error of less than 0.2% and an accuracy of 10Ω. The system is validated using a robotic arm, and then on the knee joints. Results show that the overall system can detect movements between 0.2Hz and 1Hz. Low to excellent correlations between the signal amplitude and angles were found.