Demanding exercise is what stimulates positive change. Well designed exercise equipment will often use changes in leverage to make exercise more demanding and, interestly at the same time, safer.
Wedge a crowbar under a heavy object and apply pressure. Your leverage decreases and difficulty level rises the closer you get to the heavy object. MedX exercise equipment (the kind we use at Austin Fitness Training and New Orleans Fitness Training) utilizes the same principle; while the weight stays the same throughout the range of motion, the leverage and difficulty level change.