An ongoing controversy in strength training is the relative benefit of free weights versus weight machines. Gyms typically emphasize weight machines because they are more profitable and considered safer than free weights. Free weights and machines are not the same, even when they work the same muscle groups. For example, squats and leg presses appear similar, but squats are a closed kinetic-chain exercise, while leg presses are open chain. Closed kinetic chain means that the lower or upper body stays in contact with the ground during the movement. Free weights require more spinal stabilization, which increases the load of the exercises. Free weights also trigger a greater neural-hormonal response— according to a study led by Aaron Shaner and Disa Hatfield. Elevated anabolic hormones appear to promote muscle hypertrophy and strength. The researchers found that testosterone and growth hormone levels were greater following the squat than the leg press, even though the perception of effort during the exercise was equal. At least for large muscle, lower body exercises, free weights increase anabolic hormones more than weight machines.
Source: Journal Strength Conditioning Research, 28: 1032-1040, 2014