When someone locks out on any lift, or uses a "full range" of motion on any lift, the parts at which you are locked out, or pull to far back, are a little bit more of "resting time" for your muscles. As said above those short range lifts keep pressure on muscles at all times making them harder. Try doing curls without going all the way up or down, you can feel where it tightens and stops, if you keep your motions within the areas that are "tight" you will see that you can only rep the same wieght as you usually could 12 times maybe 6 or 7... Thats just something I have always incorporated into my workouts.