With free weights and most exercise equipment resistance is constantthroughout the range of motion. You are limited to lifting whatever amount you can lift through the sticking point, that point where you are at a mechanical disadvantage. That will be the hardest part of the movement; the reminder of the repetition will be relatively easier.
With variable resistance the weight changes throughout the range. It should be lighter at the sticking point and heavier in the range where one has a mechanical advantage. Watch the numbers on the scale change in the video. This feature makes for safer more productive exercise. MedXstrength training equipment has variable resistance. We have extensive lines of MedX equipment both our Austin Fitness Trainers and New Orleans Personal Trainers locations.