The ability of the quadriceps and calf to simultaneously work together to extend the knee and plantarflex the ankle is an important skill. These three exercises train that ability. The goal of these exercises is to rise as high as possible on the bottom leg toes. The top leg then lifts into hip flexion, whilst the bottom leg aims to stay high on the toes and not unlock the knee. Calf fatigue usually hits quickly as the box height increases.
As the height of the box increases (as below), it becomes more difficult to keep the lower leg knee extended.