Stroke accidents affect a significant number of people per year all over the world. Sequels of these accidents usually affect the ability of the patients to perform daily life tasks. Rehabilitation plays a fundamental role in assisting those people to recover (to the extent possible). Nowadays, robotic systems are being increasingly used in this area. FISIOROB is one such solution being developed by an industry-research consortium in Spain. Sensing plays a fundamental role in FISIOROB for monitoring patient's muscular activity during the exercises executed as part of the rehabilitation process. In this paper, the design and experimental validation of a muscle activity monitor for the FISIOROB system are presented.