Abstract: A supplemental resistance device attachable as an accessory to the weight stacks of weight stack exercise machines to reduce or eliminate the unweighting effects of changes in acceleration at the top of the lift and friction losses during the downward phase of the lift. The supplemental resistance device comprises pins for insertion at two different levels of a weight stack, plus an elongatable resistance mechanism affixed between the pins which supplements the resistance provided by the weight stack as the two different levels of the weight stack are separated from one another. The supplemental resistance device maintains a more consistent load on the muscles during the full cycle of each lift on a weight stack exercise machine.