When lifting weights (squat, bench press, deadlift, whatever) is it better to do the motions as slowly as possible or fast and explosive? I've heard both opinions.
Please support your answer.
Note: Or, the answer may be: do X fast and Y slow
If you're lifting heavy enough to be doing sets of 5 (as per Starting Strength), the bar speed is not going to be very fast. You should try to move the bar as fast as possible while maintaining good form and control, but once you actually start approaching your 5-rep maximums (after a few SS workouts), the bar will not be moving very quickly. Your final reps will likely be quite slow, but not by choice. Moving weights more slowly than you are able to just for the sake of moving them slowly is training muscular endurance, rather than strength and power, and would be wasting energy better spent on the next rep.
The descent needs to be a managed a little more carefully. Rippetoe says, "Bar speed is too slow when the descent produces fatigue. Bar speed is too fast when it actually adds momentum to the load on the bar on the way down, so you must decelerate against both the weight on the bar and the effect of its excessive velocity." That should apply to the bench press, press, and squat. For deadlifts, he says "setting the bar down fast is actually okay... setting the bar down slowly uses up too much gas that could be better used in picking up your next rep."
If your goal is developing power (increasing the rate of force development, not just the magnitude of force development), then it's even more important to move the bar quickly. This is discussed in Mark Rippetoe's Practical Programming for Strength Training. At the intermediate stage, some lifters choose to focus on power, incorporating speed sets at lighter weights (to ensure they can move the weight quickly for the entire set).
It depend on what you are aiming aim for when weightlifting. You can be aiming for power training in which you have to push the weight quickly. This training is better to do with lighter weight. Your body won't grow big that much but you will develop stamina and power.
Lifting slowly will help you get buff and grow muscle mass. In this way of exercising, you need to carry heavier weight to stress your muscle so it will be able to grow which will increase your strength but decrease your speed.
So there are pros and cons for lifting weights fast or slow. It depends on what your goal is for the training. For me, I do a mixture of them as my muscle shapes are more uneven so I do not want to get that buff.