Triceps surae function can be modified by changes in knee joint angle through altering the effective contribution of the bi-articular gastrocnemeii. However, the impact on plantar flexor power from altering knee angle has not been studied systematically across a range of loads. Here, in 11 young men (25.7 ± 2.2 years), we determine the effect of knee angle on torque, velocity and power at loads ranging from 15 to 75 % maximal voluntary isometric contraction (MVC). Contractile properties were recorded with either the knee extended (170º) or flexed (90º). Despite similar voluntary activation (~97 %), peak twitch and MVC torques were 25 and 16 % lower in the flexed than extended knee (P < 0.05), respectively. Across all loads, subjects were 15-24 % less powerful with the knee flexed than extended (P < 0.05). In the flexed knee at relative loads ≤30 % MVC, impaired power was accompanied by 6-9 % slower shortening velocities than the extended knee. However, for the higher loads, limited torque production in the flexed knee was the key factor contributing to the generation of maximal power than for the extended position. This was supported by no change in velocity at higher loads (>30 % MVC) and a 15-22 % lower maximal rate of torque development across all loads. Hence, in a flexed knee position, which disadvantages the contribution of the gastrocnemeii, results in a left-downward shift in the torque-power relationship impairing maximal power production. Thus, the gastrocnemeii are not only a major contributor to plantar flexion torque, but also critical for modifying loaded shortening velocity and ultimately power production.