Synthetic fibre ropes have high tensile strength, a lower friction coefficient and are more flexible than steel ropes, and are therefore increasingly used in robotics. However, their characteristics are not well studied. In particular, previous work investigated the long-term behaviour only under static loading. In this paper, we investigate the elongation behaviour of synthetic fibre ropes under cyclic loading. In particular, we use ropes made from Dyneema DM20 (UHMWPE) and Zylon AS (PBO), which according to prior work have low creep. While Dyneema is more widely used, Zylon has higher tensile strength. We could show that under cyclic loading the Dyneema DM20 rope elongated more than 9% and kept on extending even after 500 cycles. Zylon exhibited a more stable and lower elongation of less than 3%.