Several situations contribute to tensing up of the legs when one walks. (1) Too much exercise. (2) Sitting for hours on end in a big soft sunken leather sofa that is not worth the money one had to pay for it and watching the idiot box for hours on end. (3) Weak lower back and weak knees. (4) Extreme lack of exercise. (5) After driving a luxury automatic transmission vehicle for hours over long distances, especially in one of those newfangled sunken bucket seats.
(1) Muscles get tired and fatigued after too much physical exercise. Walking requires the leg muscles to flex. When the leg muscles are too tired, they have to tense up in order to regain strength to walk.
(2) After remaining in a relaxed state, the leg muscles have to tense up in order to stand up and walk.
(3) The leg muscles have to tense up to compensate for the weak lower back and the weak knees.
(4) Extreme lack of exercise reduces circulation of fresh blood and fresh oxygen to the leg muscles. The leg muscles tense up to adjust to the lack of oxygen in the muscles. A corpse is called a stiff because it becomes stiff.
(5) Driving long distances is like sitting motionless for hours. Circulation to the leg muscles is either severely reduced or is cut off. The muscles react in the same way by tightening up when one begins to walk in order to compensate for the lack of oxygen in the muscles.