Abstract Aim: Elastic tendons have been suggested to attenuate fascicle lengthening during eccentric contractions; however, there is no in vivo evidence to sup-port this hypothesis. Therefore, the aim of this study was to determine whether patella tendon stiffness modulates vastus lateralis (VL) fascicle lengthening during eccentric contractions in males and females.Method: Vastus lateralis and patella tendon properties were measured in males and females owing to previously reported intrinsic gender differences in tendon properties. During maximal voluntary eccentric knee extensions, VL fascicle lengthening and torque were recorded at every 10° (range of motion 20–90°).Results: A significant correlation between maximal pa tella tendon stiffnessand change in fascicle length (r = 0.476, P = 0.023) was observed. Similarly,there was a significant correlation between maximal Young’s modulus andchange in fascicle length (r = 0.470, P = 0.049). As expected, patella tendon stiffness and Young’s modulus were significantly higher in males compared with females (P <0.05). Interestingly, change in VL fascicle length during the eccentric contractions was significantly greater in males compared with females (P <0.05). Based on patella tendon moment arm measurements, VL muscle-tendon unit elongation was estimated to be significantly greater in males compared with females (5.24 and 4.84 cm respectively).Conclusion: The significant difference in fascicle lengthening during eccentric contractions may be partly explained by the significantly higher patella tendon moment arm, patella tendon stiffness and Young’s modulus found in males compared with females. The current study provides in vivo evidence to support the hypothesis that the tendon acts as a ‘mechanical buffer’ during eccentric contractions.Keywords eccentric exercise, fascicle lengthening, gender, tendon stiffness.