With all the different types of strength training equipment, you practically need a PhD to understand it all. Both machines and free weights have their advantages and disadvantages. So which type of equipment is better? Here's the boxing-match tale of the tape.
Round 1: Resistance
Weight Machines: Weight machines use variable resistance, which changes resistance throughout the range of motion. To accomplish this, weight machines have geometrically-shaped cams integrated with a pulley system that change the length of the lever arm of the external weight (the perpendicular distance from the weight to the machine's axis of rotation).
The cams are shaped so that at weaker joint positions, the lever arm of the external weight is shorter, making it easier for you to lift the weight. At stronger joint positions, the lever arm of the external weight is longer, making it more difficult to lift the weight. Manipulating the lever arms through which the external weight is applied allows weight machines to place more stress on the muscles at the angles at which they are capable of producing greater forces. However, given the differences between the length of people's limbs and their ability to produce force at different joint angles, not all machines may be able to match their resistance to your strength.
Free Weights: With free weights, the resistance on the muscle remains constant throughout the joint's range of motion. When you lift a five-pound dumbbell, it is five pounds at all parts of the lift—beginning, middle and end. Since there are points in your joints' ranges of motion at which your muscles are stronger and points at which they are weaker, and the amount of weight you can lift is limited by the weakest point, free weights only serve as a strong enough training stimulus for those weaker joint positions.
Advantage: Weight Machines
Round 2: Movement Specificity
Weight Machines: Most weight machines allow only single-joint exercises with movement occurring in a single plane. Since weight machines guide your movement, they don't recruit muscles other than those specifically targeted by the machine. This apparent lack of freedom does confer one advantage—weight machines allow you to isolate specific muscles or parts of muscles, which is valuable if you want to shape a specific body part.
Free Weights: Movements using free weights occur in three dimensions. The added task of balancing free weights in the three-dimensional plane recruits other ancillary muscles. However, the greater instability of free weights may require the need for a spotter, which would not be necessary when using weight machines.
In contrast to weight machines, free weights allow you to perform multi-joint exercises and therefore more closely mimic the activities of daily life and other specific activities for which you may want to train. Multi-joint movements also confer greater neural benefits in terms of acquiring specific skills. While multi-joint, free-weight exercises require a higher level of skill and may take some time to learn, this slight disadvantage is more than compensated for by the gains in movement specificity.
Advantage: Free Weights