You wouldn't drive without adjusting the seat and mirrors, so don't do sets on exercise machines in somebody else's seat settings.

The Fix:
In general, the rule is this: "You should be able to go through the full range of motion and maintain good posture with back and feet well supported," says Cedric X. Bryant, Phd, chief exercise physiologist and vice president of educational services for the American Council on Exercise. For example, with the leg extension, make sure your thighs are in the center and the backs of your knees just about touch the front of the seat. With any upright machine, put your head, back and hips in contact with the back pads.