Stacked weight machines have an integrated weight stack that provides resistance for an exercise; you select the desired resistance by moving a selector pin in the stack. Hydraulic exercise equipment does not have any external resistance source -- hydraulic cylinders create resistance as you push against the machine levers. Although hydraulic machines have some advantages, they are missing some vital components necessary to improve your strength and fitness.
Weight stack machines are more common than hydraulic machines and are found in virtually all commercial gyms and fitness centers. These machines have a stack of weight plates that connect to the arm or footpads via cables. Slide the weight selector pin under the plate marked with your chosen resistance. Depending on the machine and manufacturer, the weight plates are in 5-, 10-, 15- or 20-pound increments.
A hydraulic weight machine doesn't have external resistance and you do not get to choose your resistance. As you push or pull against the arm or foot levers, hydraulic cylinders provide counter-resistance. The harder and faster you push, the more resistance you create. However, you can only push so hard or so fast, which limits the amount of resistance. On a standard hydraulic weight machine, you do not know the weight in pounds that you are lifting for any given exercise.
Hydraulic machines have the advantage of being able to perform two exercises on one machine. Stacked weight machines only allow for a concentric muscle contraction, which is when you shorten the muscle against force; such as curling a dumbbell and working your biceps muscles. On a hydraulic arm machine, you curl the handles to work your biceps. Instead of lowering the weight when you uncurl, you have to push the handles down; this motion works your triceps. On a weight-stack biceps machine, the uncurling is called an eccentric contraction and your biceps have to work to lower the weight slowly against gravity.
Hydraulic machines do not provide an eccentric contraction, which is the lowering phase of an exercise. Although many lifters neglect this phase, it is an important component of strength training. Elderly subjects who performed only negative, or eccentric, contractions improved their strength and balance and reduced their fall risk more than those who participated in traditional strength training, research published in 2003 in the "Journal of Gerontology" found. You can use more weight during the eccentric, or lowering, phase of an exercise than you can during the concentric, or lifting, phase of an exercise. Lifters who use only eccentric contractions to train also improve their one-repetition concentric lift for the same muscle, which demonstrates the importance of an eccentric contraction for improving your strength and muscle mass. Weight machines with a weight stack provide an eccentric muscle contraction; hydraulic machines do not.