You all may or may not have read a previous article of mine concerning the mind-muscle connection. In it, I heavily stressed the need to not cheat in a movement, that one should concentrate solely on the muscle of the day and try to eliminate any extraneous movement. I'd know like to play a bit of devil's advocate to myself here and throw in a way in which cheating to an extent can be used to your benefit. It all boils down to one word: torque.
Torque has a bit of a bad rap in bodybuilding. It's generally used in terms of the rotator cuff, the user citing reasons why a behind the neck press is back, that the torque on the cuff is awful. But that tends to limit the perception of the word as if it were a bad thing to have any sort of torque on a limb (much like how badly "momentum" has been demonized). To properly assess "torque", we need to define it.
So What Is Torque?
The simplest definition, provided by Merriam-Webster, is:
"A force that [produces] rotation or torsion."
Simply put, any time we have something rotating due to force, we have torque. In bodybuilding terms, any time a limb is rotating with weight on it, we're dealing with torque. This is what exists any time a weight is lifted.
So how is torque measured? Simple physics teach us that torque is a product of a force times the length of the torque arm. If one makes one side of a see-saw longer than the other, it takes less weight to lift up the fat kid on the other side. In other words, the further from the joint the weight is, the harder it is to push. This is why flyes are harder to do than dumbbell presses with equal weight; more torque.
The barbell curl is the lift most frequently cheated on, so it's the easiest to look at. Although the lever length (meaning your arm length) doesn't change, the apparent torque arm length does. If we were to graph out the movement of an arm on a Cartesian graph, it would roughly look like a half circle with the center at the origin.
Now, the picture looks a tad confusing, but it really isn't. The thin black line is the possible path of the arm, the thick black line is the arm itself as it moves along the path, and the blue, red, pink, green, and purple lines are the (x,y) components of the arm's position.
Remember that gravity only works in one direction, and for our purposes that would be in the negative x direction, so the apparent length of the torque arm is the y component. This is why every freeweight exercise has a "sticking point" where the weight feels the heaviest.
Occasionally a trainer will reason that this happens because that's where one muscle stops working and another starts, and to some extent it may be true for some movements, but the above explains it far better. I also left out illustrating the same things above the x axis because they simply mirror those below.
Now, returning to our example, realize that the above illustrates perfectly the path of our barbell/EZ bar/dumbbell, with the origin being the elbow. Why does the bicep relax when the arm breaks vertical? Obviously because our y component has broken into the negative territory, where there is no longer any stress on our intended muscle.
You'll also notice the sticking point is when the forearm is parallel to the ground. I'm sure we've all felt this phenomenon, mostly when swinging a weight up for a nice cheat curl, where the bar suddenly stops halfway up, and gets significantly easier once that point is broken. The same happens at parallel in squats, or when the upper arm hits parallel in the bench press.
What Can It Do For Me?
So what does this mean for us? How can all this mumbo-jumbo help us maximize our gains? Well if our goal is maximal overload, then this tells us that we want the most force we can get on our muscle. Where is this point? Well we just established that it's at the point where the y component is at its greatest (parallel to the ground), so we want as much weight as we can get there, as often as we can get it there.
It's great to move in a controlled manner, realize that the force on the muscle lessens quickly the further from the "tweak" point (as I call it). As such, we could, in effect, gauge the quality of our workouts by how frequently we push a weight through this tweak point. To an extent at least.
So what does this mean? Well, it means that a little bit of body English at the bottom of a movement isn't necessarily a bad thing. It also means that the negative can't be ignored at all, since the negative portion includes another pass-through of the tweak point, so we want to make sure to be applying as much force as possible there.
To an extent, this will fly in the face of any type of repetition timing program, since for use we want to pause for as little as possible at the top and bottom of the movement so we can get as many pass-throughs of the tweak point as possible. There's no need for a peak contraction, don't pause to feel the stretch, keep that weight moving at all times.
Time spent at the top or bottom of the movement could mean the difference between a failed rep and two more pass-throughs. Think about it, you're guaranteed to get the negative, it's the positive that's the question, and so quitting halfway through a rep robs you of quite a bit of stress on the muscle.
This also means that for some movements, doing the top half is beneficial over the bottom. It takes much more force to change a weight's direction from down to up than it does to let the weight fall again. This is most prevalent again with a barbell curl for those who have tried 21s.
Doing the 7 reps of the top half of the movement is considerably harder than doing them at the bottom. This isn't applicable for, say, the bench press, since the top half of the movement is mostly triceps, and for squats doing partials is death on the knees.
The simple lesson for this all (I realize not everyone will read it all), is that from a strictly physical sense, the point of greatest stress on a muscle is always at the point where the limb in use is perpendicular to the force, so spending time at the stretch and contraction are not necessarily helpful, though a full range of motion is always necessary.
For your next workout, feel free to do a bit of cheating on your skullcrushers or dumbbell curls, get through that tweak point, and feel it again on the way down. Don't waste energy feeling that contraction at the top or the stretch at the bottom. See how well your gains go after this. But above all, be safe about it.