This might not seem like a very big deal - I mean, hey, just grab the barbell and hammer it out, right? Not so. In fact, the rep is just as important of a variable as any other factor when you're working out. Just like you can use different exercises, different versions of a machine, or use different handles for a pulley - or rowing machine.
In addition, there are a few neat tricks you can do to crank up the intensity a little, achieving that last ounce of fatigue and burn.
Priority number one should always be to master the weight (unless you're consciously applying the principle of cheating, but even then you must remain in control of the weight). Never let the weight jerk YOU around.
It can be tempting to put some extra plates on the barbell when you have two buddies cheering, but if the consequence is drastically reducing the efficiency of your own workout - plus the added risk of injury - I say it's not even an option. Stay cool, and compete only against yourself.
With that said, let's use a simple bicep-curl as an example. When curling UP (positive phase), it should take about 2 seconds from having a straight arm to maximum contraction. Going back DOWN (negative phase) should take longer, 3-4 second.
The reason for this lies in the muscle's self-preservation instinct. If it can actively lift 100 lbs, it can statically hold 120 lbs, and can resist the weight in a negative phase (for a controlled descent) with 140 lbs (all figures approximate).
It's pretty simple: If it was the other way around, you'd be able to hurl 140 lbs over your head, only to have it come crashing down on you. To avoid this, the muscle has this extra reserve to tap when it's "protecting you from yourself", so to speak. That is what you're taking advantage of when putting more time to the negative phase!
Negatives And Forced Reps
To move further down the road on this discussion, there's a rather common method for boosting intensity called Negatives. I discussed this in-depth in a previous article, but I thought I'd quickly mention it here, to get the whole picture straight.
The principle is virtually the same as forced reps - they both make use of the same, basic ability of putting extra strain on the muscle in the negative phase when the muscle is technically too worn out to do much more in the positive phase.
In the gym, this either means that your training partner adds an extra push to make the weights heavier as you curl down, or helps you through the positive phase so that you can struggle against weight on the way down on your own. Only a nutcase would try to apply both principles at the same time though.
I read an interesting theory, Compensatory Acceleration Training (CAT) that suggested that you could get even more out of your reps than you can doing the classic, full-control rep. The essence of it goes something like that it takes more energy to accelerate the weight throughout the motion, which would result in better gains in strength and mass.
This could make sense, as the hardest part of, say a bench press, usually is to get the frigging bar moving upwards! Once you've got it off your chest and has decent speed, it's basically a "lazy groove" all the way up to the top, right?
This works just like it's easier to keep a car rolling that is already in motion, rather than start pushing a car that is standing still. But if you try to accelerate the rolling car, you quickly realize it takes just as much effort as to START the car standing still!
In the case of the bench press, that means that instead of settling for keeping the bar moving once you got it off the chest, you should strive to increase the speed almost all the way to the top. A word of advice though: Don't try to accelerate through the NEGATIVE phase, as you might find it troublesome to stop a 300 lbs barbell from chopping off your head.
Concentration Vs. Compound Exercises
Make a difference between concentrations vs. compound exercises. The purpose is different - the concentration is out to pinpoint one specific muscle, while the compound hits you like a sledgehammer to increase overall body power. Of course the focus of the reps are different as well. A dumbbell bicep-curl is a good example of a concentration exercise.
Here, your objective is to beat the crap out of the bicep, and the bicep only. Keeping the shoulder still, not yanking the forearm in any way, or moving the elbow around does this. Your mind should be in the bicep. A dead lift on the other hand, a prime example of a compound movement. Here, the key to success is to get a feel for overall balance and coordination.
For compound exercises you're using a vast variety of different muscles, so your effort should be to "set them off" in a synchronized manner, so no one muscle group receives more or less beating than the next one. Your mind should not be focused on individual muscles, but rather on your body as a whole. The key question is: Does this feel well? Am I in control? Am I balanced?