What is being tested from a short sitting position when resisting efforts to flex, abduct, and externally rotate?

The combination of resisting efforts to flex, abduct, and externally rotate from a short sitting position specifically targets the sartorius muscle. This muscle is the longest in the body and is responsible for hip flexion, abduction, and external rotation as well as knee flexion.

When the leg is positioned in short sitting, typically on a treatment table or chair, the sartorius can be specifically engaged as the individual attempts to move their leg in those three directions against resistance. This effectively tests the muscle's strength and functionality in a position that mimics some common daily activities.

In contrast, the iliopsoas primarily functions to flex the hip but does not significantly contribute to hip abduction or external rotation in the same movement context. The quadriceps are chiefly responsible for knee extension rather than movements involving hip rotation or abduction. The adductors play a role in hip adduction and stabilization but do not actively resist flexion, abduction, or external rotation in this position. Therefore, the resistance testing described in the question aligns best with the actions and functions of the sartorius muscle.