Pull-ups and pulldowns are great exercises for really building those lats and your back. Just like any other movement, however, when done inproperly it can lead to injury and lack of results. Dr. Joel Seedman goes over one of the mot common mistakes when it comes to pull-ups and pulldowns- overpulling.
Like any movement, there is an ideal range of motion you are looking for. What people commonly do with this exercise is over step that range of motion and pull too close to the clavicle. Instead, you want to pull more towards the sternum and lower chest. By doing so, you are not pulling back, you are tucking your elbows, retracting your shoulder blades and getting good thoratic motion.
Ensure your elbows remain tucked in and really centrate that glenohumeral joint into the optimal position. You will see that your natural stopping point actually ends up being a few inches above your chest- that is the ideal range of motion.
In terms of your shoulders, avoid them being depressed- they must be retracted and medially rotated towards the spine when you reach the bottom. The stopping point is going to be much sooner than if your shoulders were out of position or if you have internal rotation of the shoulders.
The key to mastering pull-ups and pulldowns is understanding the ideal range of motion. You want a nstural range of motion, not an excessive range of motion.