How do i mark time at faster tempos?

my shows first movement is 180 bpm and our third movement is 210 bpm. At slower tempos like 150 i mark time fine but when its at faster tempos like 180 or 210 i accidently start slowing down and dragging whatever i play. When i mark time i feel like im tensing my legs/knees when i do but i dont know how to fix it or mark time more efficiently any tips?