Final answer:
The maximum amount of knee flexion used by elite volleyball players during a vertical jump is an angular motion variable, which falls under the study of biomechanics in the field of physics.
Step-by-step explanation:
The maximum amount of knee flexion elite volleyball players use when performing a vertical jump would be categorized as an angular motion variable. This involves the analysis of biomechanics, which is a subfield within physics that applies principles of mechanical physics to understand movements and actions of human bodies and sports performance.
In biomechanics, angular motion variables describe the movement around a central pivot point, like the rotation of the knee during a jump. It's important in sports like volleyball where the degree of knee flexion can significantly impact the effectiveness and height of the jump.