New quick and interesting fact that came to me originally courtesy of Alwyn Cosgrove but also shows up in this Nate Green article via Christian Thibaudeau:  pausing at the bottom of a rep makes the muscles work harder.  Thibaudeau makes the point in regards to squatting, but it actually works across the exercise spectrum.

Why is this? 

Picture yourself doing squats, or, even better, curls:  when it gets heavy, your tendency is to keep the muscle taut at the bottom of the movement and go right into the lift portion of the move after you've lowered it.  What you're trying to do there, Cheater-Head, is lean on the stretch-shortening cycle--the tendency of tendons, fascia, and other connective tissue, to snap back to a shortened state after being stretched. 

Thing is, when you relax, or even just pause, in the lengthened position--at the bottom of a curl or a squat, for instance--the stretch-shortening reflex can't do as much to help you.  In fact, after you've spent about two seconds in the stretched state, the elastic component is taken out of the movement.  Now the muscles have to do 100% of the work to get the weight up. 

"Bouncing" out of the bottom of a movement is always easier than pausing before you lift, but it's not without its uses:  athletes in many sports have to learn to decelerate a load and then rapidly accelerate it up again in an instant, so the quick-turnaround style of training can be useful to people in athletics.  It's also a good calorie-burner because you're going and going with almost no rest.

Taking the pause in the "down" position of a lift, however--along with lowering the weight under control--stresses the muscles almost exclusively.  Even without using more weight, you're going to stress the muscles harder if you take the pause. 

(As far as anyone knows, this guy's still stuck under his massive weight, and his partner's waiting for the signal.)

Once you get around to actually lifting the weight, though, it's best to lift as fast as you can.  More on that on a later post--but unless you're injured, lifting fast (or at least trying to lift fast--if you're using close to a maximal weight, you might wind up grinding out a rep anyway) is a good idea.

So if muscle-building--and, indeed, strength-building--is your focus, it's probably worth it to take that pause for effect.