A sensor unit (18) adapted for mounting on a rubber band (17) for physical training. The sensor unit comprises attachment means such as a clamp (4, 5), means (10) for measuring the tension of the rubber band, a power source (1), a processor (11) and optionally a signal transmitter (12) for cable or wireless data transfer. In one embodiment, the means for measuring the tension is an integral part of the attachment means. In another embodiment, the means for measuring the tension is adapted to measure the mechanical resonance frequency of the rubber band.