Many human movements involve stretch-shortening-cycle (SSC) actions of lower limb muscle-tendon-units (MTU), whereby an eccentric muscle action is immediately followed by a concentric action. Tendons comprise the majority of eccentric MTU actions, particularly as movement frequencies increase (1), which optimises concentric muscle force output. However, this hypothesis has not been tested during conditions which replicate functional performance. Hopping involves SSC muscle actions and provides a reliable model for examining mechanical properties of lower limb joints, such as joint stiffness, which correlates positively both with movement frequency and functional performance (2). Six well-trained males performed three, single-leg hopping trials (each lasting for 15 seconds) on an inclined sledge apparatus, at two frequencies (1.5 and 2.5 Hz). For each trial, five consecutive hops within ±5% of the prescribed frequency were analysed further. Three-dimensional motion analysis, ultrasonography of medial gastrocnemius (MG) and ground reaction forces were simultaneously and synchronously collected. Sagittal plane joint angles and joint moments were determined via a combination of motion data, force data and inverse dynamics. Ankle joint stiffness was calculated as the ratio of peak joint moment (relative to body mass) to peak joint angular displacement during the eccentric phase of the hops (3). MG MTU length was determined as a function of shank segment length and joint angle data (4). MG muscle length was calculated as MG fascicle length multiplied by the cosine of the pennation angle (Figure 1) and MG tendon length was determined by subtracting MG muscle length from MG MTU length (5). Dependent t-tests were used to compare mean differences between variables measured at both frequencies (p=0.05). Data represents the mean ± S.E.M. of three trials performed at both frequencies. Ankle joint stiffness doubled (0.08±0.01 vs. 0.16±0.02 Nm/kg/deg, p<0.05) and ground contact times shortened (329±0.21 vs. 254±0.01 ms, p<0.05) as hopping frequency increased from 1.5 to 2.5 Hz. MG muscle lengthening was either slightly less or non-existent (-3.67±2.86 vs. 7.15±1.66 mm, p<0.05) at 2.5 Hz compared to 1.5 Hz, whereas MG tendon lengthening was much greater (18.31±3.34 vs. 6.98±2.26 mm, p<0.05). MG muscle shortening velocity was similar at both frequencies (48.3±9.5 vs. 48.3±17.0 mm/s, p=1.00), however, MG tendon shortening velocity was far greater at 2.5 Hz (143.3±16.7 vs. 85.0±14.6 mm/s, p<0.05). Results suggest that single-leg hopping performed with increased ankle joint stiffness resulted in more efficient use of the SSC, as greater tendon lengthening and subsequent rapid recoiling allowed the MG muscle to operate within a more optimal range for maximising concentric force output, due to muscle force-velocity and length-tension relationships.