Plyometrics: box jump variations by Modern Day Fitness Stiff leg jumps: We use this exercise to get the athlete to jump higher than the box, the. land without the use of hip flexion. Meaning you have to work harder to jump higher. What we are trying to do is get the hips as high as possible, meaning a more powerful vertical leap through triple extension. Sometimes you may see extremely good hip flexion to land on very high boxes. Good hip flexion doesn't truly reflect an individuals jump height.