Usually you are going to want to lower the bar far enough down so that your arms go below paralel with the ground (touch your chest if you have the flexability to do so). If you are not going below paralel or stopping at paralel there is a problem with this because you are not effecintly strengthening the mucsle....why? It basically has to do with levers and force production. If we are looking at your arm as the force arm, the bar as the resistance and your shoulder joint as the fulcrum, this resembles a 3rd class lever. When dealing with a third class lever we know that the resistance has the advantage and the force is at a disadvange because of how close it is to the fulcrum (axis of rotation). When we de-rack the bar on a bench press lowering it is simple, but how far we lower it is going to decide how difficult the exercise will be. When our arms are paralel with te ground we are at the greatest disadvantge because it is at the moment that our force arm is the longest (our elbows are at a right angle making our upper arm paralel to the ground and fully extended). If we go below that point and begin our rep the bottom half is easy because the focre arm is less and is now closer to the fulcrum, as we extend upwards once we get to parallel it is the most difficult (hence the "sticking point") because the force arm is at the greatets length. Once we pass that point locking out is easy becuase our force arm is now shortening and moving closer to the fulcrum again. If you only do partials then you are not getting to that sticking point and letting your body drive through the hardest part of the motion ( the same can be said for squats).
the four variables which strength is relative to:
1)the range of motion we train in
2)the intent of speed of the exercise
3)the plane of motion we train in
4)the environment we train in
peace,
Patrick