yesterday when i was doing some heavy deadlifts i noticed in the latter sets and reps of my 5x5 i had to fight the urge to push through my toes and concentrate really hard on trying to keep good form and use my hams, lower back and glutes. is this because my posterior chain was tired out and my body wanted to use somewhat fresh quads to lift the weight? also, when i felt this happening, my lockout was different because the bar drifted forward when i put more weight on my toes. i didnt straighten my legs and back simultaneously, but i'd get my legs straight and find i still had to adjust (only a couple inches, nothing dramatic) using my lower back to reach lockout. is this my lower back just overcompensating for my legs? luckily i found a good workout partner who pays close attention to form and pointed it out to me as soon as the reps happened, and i felt it as well. i was just curious to know the reasons for this. and finally, is it even possible to keep perfect form during the last reps in your final sets when you are really straining hard?