In response to the above.
During a long car journey, the body settles into a fixed involuntary posture for a long time. The muscles have to adjust themselves to accommodate the posture. The knees are bent while seated. The long duration keeps the leg muscles “fixed” in that position.
When one gets out of the car, the legs feel very stiff, the knees feel stiff as if they cannot straighten out.
People who drive a Porsche know this well. They get out of their Porsche as if dismounting a horse after a rough ride. I used to drive a Porsche 944 and I felt stiff after a long drive in it.