Okay. What the fuck are you talking about? What upward resistance? Do you mean gravity? When you lift free weights, you have to control 3 axises at once, x,y,and z. You have to fight gravity along with controlling 3 dimensions of space all at once. There is no harder fucking thing to do than to control a free weight. Smith machine exercises are not equal to free weights, and 315 on a smith is not 315 on a squat rack.
See with a barbell plus weights there is no upward resistance except the weight. But w the smith rack you are fighting the weight plus the resistance of the cable mechanism