What is the PRIMARY cause of excessive knee flexion during the early stance phase for a patient with a transtibial prosthesis?

The primary cause of excessive knee flexion during the early stance phase for a patient with a transtibial prosthesis is related to the positioning of the socket. If the socket is positioned too far anteriorly, it leads to an imbalance in the forces acting on the prosthesis during weight acceptance. This anterior placement causes the body's center of mass to shift forward, which can result in the knee flexing excessively because the foot does not have adequate ground support to keep it stabilized in a neutral position. Essentially, the forward positioning of the socket can prompt premature knee flexion, leading to instability and difficulty during the stance phase.

In contrast, if the socket were positioned too far posterior or if the foot were in an inappropriate inset or outset position, these would likely affect other kinematic factors related to the alignment and function of the prosthesis, not primarily knee flexion in early stance.