Since the early 1800s, people used weight training machines for exercise. Alan Bradley, author of "The Illustrated History of Physical Culture" states that weight training machines were very popular with the upper classes, who thought them very fashionable and stylish. Weight machines have evolved since then, and today's weight training machines fall into one of a number of basic types.
Plate Loaded Machines
Common in bodybuilding and sports performance gyms, plate loaded machines are heavy duty and designed to closely replicate free weight exercises. Standard barbell plates are loaded onto bars attached to lever arms, which pivot on a single axis. These machines tend to have independent actions allowing your limbs to work independently of one another. Popular with users who like to lift heavy loads, plate loaded machines offer a safe alternative to many traditional free weight exercises as even if the weight is dropped, you are not in any danger of being crushed by the weight.
Pin Selected Machines
This is the most common type of weight training machine. Weights are pulled up guide rods by cables attached to lever arms and rotating pulley cams. The weights are adjusted by inserting a metal pin below the weight you wish to lift. Many variations on this type of design exist, but the principle is the same in all of them. Some manufacturers use offset cam wheels to alter the weight as the level arm passes through its range of movement. Your muscles have a distinct "strength curve," parts of an exercise movement where you are stronger than in others. Offset cams aim to spread the load evenly throughout the movement to make exercises more effective. The first company to use this technology was Nautilus who, in the 1960s and '70s, led the way in weight training machine design.
Some manufacturers choose not to use weights in weight training machines. Electromagnetic machines are controlled by computer and the magnets provide a very precise breaking effect that simulates lifting weights. Most electromagnetic machines have no eccentric phase in their exercises. You do not lengthen your muscles against resistance but only shorten. Machines are designed so that two exercises are integrated into one single movement. Leg extensions/leg curls, chest press/seated row, shoulder press/lat pull down and biceps curl/triceps extensions are all examples of electromagnetic exercise pairings. The idea behind the elimination of the eccentric phase of the movement is to try to minimize post-exercise muscle soreness and make workouts more time efficient by halving the number of exercises that have to be performed.
Powered by compressors, hydraulic machines are light, portable and relatively cheap compared to traditional weight training machines. Often used in weight management gyms where the machines are set out in a circuit, these machines allow for very rapid weight adjustment and are virtually silent in use. Hydraulic machines tend to be easier on the joints as the resistance has a slight "spring" to it so joints are not shock loaded. Beginners and those people intimidated by traditional machines often favor this type of equipment as no one knows how much weight you are lifting as only you can see the display indicating the resistance being used.