Round 3: Strength Gains
Weight Machines: Since the assessment of strength requires the use of either weight machines or free weights, the outcome of any experiment comparing the two will likely favor the type of training that uses the same kind of equipment as the strength measurement. Studies testing strength (one-rep max) using equipment or a type of exercise different from that used in training have found similar gains in strength between weight machines and free weights.
Free Weights: Studies testing strength using equipment that is the same as that used in training have found that there is a greater carryover of strength obtained from free weights to weight machines than the converse and that training with free weights elicits superior strength gains compared to training with weight machines. The superior results of free weight training are likely due to their greater specificity of movement patterns, force application and velocity of movement.
Advantage: Free Weights
Round 4: Workout Flexibility
Weight Machines: Weight machines don't provide much flexibility in designing or performing workouts. While manufacturers of weight machines try to accommodate as many different body sizes as possible, there's still a limit to how many adjustments can be made in the seat or arm settings, rendering the exercise to be performed in a relatively fixed position. Since each weight machine is designed for a specific exercise, only the intended exercise can be performed with each machine. Training progression can also be problematic because many weight machines are restricted to 10- to 20-pound increments.
Free Weights: As their name implies, free weights don't limit the position of the exercise. Rather, they allow you to make adjustments in position based on your specific biomechanics. Unlike with weight machines, the number of exercises that can be performed with only a couple of dumbbells is nearly unlimited. Since dumbbells and barbell plates come in many different weights, free weights allow for small changes in training load. With free weights, you also have a greater opportunity to engage many large muscles, which means a greater metabolic cost, which can have a greater effect on your fitness and body composition.
Advantage: Free Weights
More: Periodization of Strength Training
Round 5: Injury Risk
Weight Machines: Although injuries can occur when using weight machines, they are less likely given weight machines' fixed positions and guided movements.
Free Weights: Training with free weights presents a greater injury risk compared to using weight machines because of their free range of movement. Research has shown that most weight training injuries occur with intense or aggressive free weight training. If you focus on performing the exercises correctly or work with a trainer who can monitor your workouts, the risk of injury is no greater than when training with weight machines.
Slight Advantage: Weight Machines
Overall Winner: Free Weights
Despite the inability to alter their resistance at stronger joint angles and the slightly greater risk of injury, free weights still have the overall advantage because of their training specificity, workout flexibility and superior stimulus for increasing strength. While weight machines allow you to train muscle, free weights allow you to train movement.
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