hollow of knee
膕 refers specifically to the hollow or pit at the back of the knee joint.
膕 combines the flesh radical ⺼ with 國, which likely provides the sound. The character is a phono-semantic compound for a body part.
The flesh radical ⺼ indicates a body part, and the right side 國 suggests a bounded area. Picture the hollow behind the knee as a small, enclosed fleshy pit.
For カク, imagine a cackling sound coming from the hollow of the knee when you bend it: cack -> カク, and the knee pit makes a funny noise.