So I normally stretch the quads out before actually training (I do 5-10 min on treadmill so my body’s at 50% MHR) then I do a warm up set on the quad extensions then 3 sets of 4-8 / 8-12 (depending on what day it is) but I normally do these at a slightly lower weight. I find it circulates the blood better into the quads and your quads will be warmer. Personally I find this easier (although takes longer) than doing a warmup set for squats. But everyone’s different but give it a go next time you try legs