I'm completely frustrated by this problem. For years, literally, my lower quads (vastus lateralis) just lock up almost every time i attempt cardio. I've tried drinking a gallon of water a day, i've tried slow warmups followed by stretching, and now i'm taking an electrolyte supplement prior to cardio and nothing is helping. Treadmill, bike, stepper, it doesn't matter. Within ten minutes or so, i cramp up. Has anyone else had a problem similar, or can anyone think of anything i've missed here?