Lepley, AS, Joseph, MF, Daigle, NR, Digiacomo, JE, Galer, J, Rock, E, Rosier, SB, and Sureja, PB. Sex differences in mechanical properties of the Achilles tendon: Longitudinal response to repetitive loading exercise. J Strength Cond Res 32(11): 3070–3079, 2018—Sex differences have been observed in the mechanical properties of the Achilles tendon, which may help to explain the increased risk of injury in men. However, the response and recovery of tendon mechanics to repetitive loading exercise, as well as sex-dependent responses, are not well understood. The purpose of our study was to compare Achilles tendon mechanical properties between men and women before, immediately after, and 60 minutes after a repetitive loading exercise. Seventeen female (age: 24.0 ± 3.9 years; height: 167.4 ± 6.9 cm; and mass: 64.9 ± 8.5 kg) and 18 male (age: 23.9 ± 2.4 years; height: 179.2 ± 5.09 cm; and mass: 78.4 ± 8.7 kg) recreationally active individuals volunteered. Using isokinetic dynamometry and diagnostic ultrasound, baseline levels of Achilles tendon force, elongation, stiffness, stress, strain, and Young's modulus were assessed before 100 successive calf-raise exercises using a Smith machine at 20% of participant body mass. Outcomes were reassessed immediately and 60 minutes after exercise. Women exhibited less Achilles tendon force, stiffness, stress, and modulus compared with men, regardless of time point. Both sexes responded to repetitive loading exercise similarly, with immediate decreases in mechanical properties of the Achilles tendon from baseline to immediately after exercise. Tendon properties were observed to be equal to baseline values at 60-minute postexercise. Baseline differences in tendon properties may help to explain the disparity in injury risk because both sexes responded to and recovered from exercise similarly. Future research should aim to include additional time points (both leading up to and after 60 minutes), and assess tendon responses to more sport-specific activities, while also including patients diagnosed with Achilles tendon injuries.