Proponents of free weights and weight machines are very much like sheepmen and cattlemen in the old west: both feel strongly about their point of view and have inflexible attitudes toward the other side. Machines are convenient, high tech, comfortable and attractive. However, they move joints through fixed ranges of motion that can create excessive torque and unnatural stresses. Also, the exercises are artificial and do not transfer well to functional movement.
Angelo Cacchio and coworkers from the University of Rome in Italy and Illinois State University found that exercising on a machine that provided free motion similar to dumbbell and barbell exercises created greater activation of shoulder girdle muscles than a machine that forced movement in a fixed path. They concluded that unconstrained weight training is superior to fixed path weight machines because it builds intramuscular coordination that might transfer better to functional activities.