It depends on whether one has been seated in economy class or business class or first class on the airplane on a long cross country flight (6 hours) or on a long international flight from San Francisco/Los Angeles to Taipei or Shanghai (13 and a half hours) or on a long international flight from New York/Los Angeles to Sydney, Australia (13-16 hours).
Legs would feel tight after sitting in the economy class seat. This is because of a lack of leg room and leg stretching room.
Legs might feel tight after sitting in the business class seat because of the length of time that one remains seated. Thrombosis may set in when a clot forms in the legs and travels to the heart. A famous TV newsman returned from a vacation trip to Rome and had a heart attack after his telecast and died. The cause, although doctors at the time were reluctant to associate his long flight back from vacation from Rome with his heart attack only a few days later, the long flight was to me the definite cause of his fatal heart attack.
First class seats usually have plenty of leg room, especially if one is seated in the front row of the first class section upstrais on board a Boeing 747.
The combination of a lack of movement, the lack of stretching the legs, the heavy body weight that bears down on the butt while seated, will reduce circulation and blood flow and oxygen flow to the legs. The muscles of the legs react by tightening up to conserve oxygen.