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How long should a rep last? Yet again there are many theories ranging from four seconds down and then four seconds up to as fast as you can. I have tried them all and I have also noticed no real benefit from either.
The best I have found is medium pace using NO MOMENTUM at all. This means not using gravity or allowing the weight to go down too fast. You see in gyms all the time people who are trying to lift too much bouncing the weights of their chests usually when they are bench pressing. Or the classic bicep curl rocking style as if they are meeting a Chinese head of state!
You must have control both on the positive and negative range of your lifts. You should keep control of the rep speed throughout until the weight becomes too heavy and you force the last rep out.
A rep must be taken to the full range of motion where the weight is lowered to the most it will go to safely and then back up to the starting point. I see people regularly who do bicep curls and do not let the bar lower to a fully extended length, instead they lower it about three quarters of the way and then lift it up quickly because they are using too much weight.
You must exercise through the full range of motion and to full extension of your body part to receive full contraction of the muscle. Yes it is harder but the other way will yield poor results.