Machines are usually less effective than free weights because free weight require you to balance more and this factor makes the preacher curls with dumbbells more complete. however, there’s one disadvantage all free weight and cable biceps curl variations have that a machine doesn’t – gravity!
During any style of biceps curl, the point in which the muscle is maximally stimulated, is when the lever arm is at its longest. This occurs when the forearm is at a 90-degree angle with the load vector.
On the other hand, the biceps curl machines is designed with a CAM system, the CAM is set up to offer you a much more consistent resistance throughout the entire range of motion.
The preacher curl machine is an exercise that hits the muscle from all points of the elbow flexion range.
Another benifit, is if you are a begginer and don't have a proper form yet, a Machine will be better and give you less chances to cheat.
How to do
Using a machine preacher curl machine that has a weight stack, adjust the weight to an appropriate level for your strength.
Grab the bar securely with both hands using an underhand grip (palms facing upwards).
Keep your back straight with your feet planted firmly on the floor and maintain proper posture throughout the movement.
Slowly curl the bar upward as if you are trying to touch the handles of the bar to your chin (but don?t actually touch it).
Return the bar back down very slowly to the starting position.