Stiffness Affects Thrust Divers generated significantly greater thrust (distance per kick) and velocity with the Attack. The distance per kick for the Attack fin was significantly higher (19%), while the Apollo was significantly lower (16%). The maximum distance per kick increased as a function of stiffness. The maximum velocity was highest for the Attack (15%). Although the maximal kick frequency was higher for the Apollo (13%), the distance per kick was significantly lower (14%). Flexible fins had to be used at higher frequencies as the distance per kick was significantly less. The stiffest fin (Attack) had the highest maximum thrust, and the diver covered the greatest distance per kick (1.19 m/k).