Sawasdee krub, Khun Thanapon
I fly TG's 773 quite frequently on long haul flights, and I get painful knees and legs too.. I believe it may be because the seat is too hard, pressing your legs too much.
They may or may not be replacing the seats in the 773 soon as part of the fleet enhancement program.
Anyway, TG is a great airline, so hope your next experience is a better one