The invention is an exercise device which creates a controlled and infinitely variable resistance profile using a pair of rods that are wrapped about a common axis with a varying pitch. A slide unit is attached to the rods with rollers. When the slide unit is moved in a linear motion, the movement of the rollers on the rods causes the rod, and a shaft on which they are mounted to rotate. In one embodiment, the rotation of the shaft is used to move one or more units of a set of stacked weights. In a second embodiment, a rotation motion of the shaft on which the rods are mounted, causes a linear motion to the slide unit which is attached to a set of stacked weighs.