Force-Vector Diagrams: How They Can Help!

One of the most heavily debated topics in bodybuilding and weightlifting in general is the concept of form. The key? The Vector-force diagram. Learn why...
Hey guys. Before we get into the article, I need to address something. I know a lot of you guys have been e-mailing me and I haven't answered ANY. I apologize wholeheartedly, but the college network I'm on has made sending e-mails very difficult, and it rarely works. I have to go through a whole lot to send them out. If you're signed up on the message boards here, send me a private message there, I'm more likely to be able to respond. Sorry again, and here we go!

The Concept Of Form

One of the most heavily debated topics in bodybuilding and weightlifting in general is the concept of form. It is particularly important in bodybuilding as the various theories are all in order to maximize muscle activation. EMG tests can be helpful, but simple physics can help us decide both what exercises to use and how to perform them to maximize results. The key? The Vector-force diagram.

Physics is rarely considered in bodybuilding, chemistry and biology taking precedence. But in this case, it's physics to the rescue. So what exactly IS a VF diagram? Basically, a VF diagram is one in which every force during a lift, from forces due to gravity, the muscles activated, and even friction are put into play. For our purposes, normal forces and friction will be ignored, as they aren't that important for our goals.

Now how is a VF diagram constructed? We'll use basic line drawings because it's quick, I'm using MS Paint, and I'm a terrible artist. First start with a simple diagram of the exercise. This is of the bench press, looking down on the lifter's head parallel with the floor. The yellow areas are the lifter, the bottom gray is the bench, and obviously the bar is the gray bar with white weights on either end. Three diagrams will be used, narrow grip, wide, grip, neutral grip. Neutral is shown below.

From here, the important forces are added. Rather than using the direction of the muscle's pull, we'll use the resulting direction of the limb's movement. This is more useful information as it gives a better understanding of the relationship between the resistance's direction and that of our force. Note that since the bench press involves a bar, it's weight can be considered as concentrated at its center, the weights being on the ends makes no difference.

The result looks like this:

The arrows will always be perpendicular to the limb that is in motion, so they will not be pointing in quite the same directions at other points in the movement. But, it's easy to see that the arrows on the hands (the tricep movers) will never have a positive vertical component (an upward force), whereas the elbow arrows (representing chest movement) will ALWAYS have a positive vertical component.

What does this mean? This means that the triceps will be a stabilizer ONLY, that they are not a prime mover at this width. This can tell us that this "elbow width" grip is ideal for the barbell bench, as anything further in will give the triceps force a PVC (this is how I'll refer to the positive vertical component from now on). Notice also that at this point in our bench press diagram, the direction of the resistance (the bar coming down) is DIRECTLY in line with the chest movement, only opposite.

Remembering our trigonometry, this gives us an important relationship. The amount of force a muscle is giving against a given resistance is directly related to the magnitude of the resistance (the weight, w), and the angle between them (?). This gives us the equation:

F = -w*cos(?)

The angle is calculated by putting the vectors tail to tail. The answer is negative because the weight is in the opposite direction of the force against it. Now this method can also be used quite effectively in finding which exercises are worth using at all. Take the preacher curl for example, diagram below:

The white is the bench, the yellow is the arm, the circle the side-view of a dumbbell. Again we put the lines of force in, resulting in this:

Note that the angle between these two is roughly 45 degrees. Putting that into our equation with 35 lbs of force, we get a force of F = -w*cos(?) = -(35)*cos(135) = 24.74. The biceps themselves are delivering more than 10 lbs of force under the chosen weight at this point. And it increases the further the movement goes until it turns to zero when the lower arm is vertical. The PVC here is maximized when the lower arm is parallel to the floor and decreases from there. Thus, the preacher curl is far from an ideal movement.

Now that gives us something very important. The biceps move the lower arm in an arc, but our diagram shows that radial movement changes the amount of force the muscle can generate (when using free weights, machines use cams so the force doesn't change).

This means that an ideal biceps curl does NOT involve keeping the arms pinned to the sides, as that involves starting with nearly zero force against the muscle, a "tweak" point (as I've said before) in the center, declining to nearly zero again at the top. Here the elbow is the left-most end of the yellow line. The upper body has been excluded for clarity:

So what can be done here? Consider the barbell drag curl. In that, the bar moves vertically ONLY along the body. The PVC is always there, and is closer to maximized through the whole movement. Keep the bar a few inches away from the body to keep rear delt movement in check. The yellow is the body, vertical line the torso, with the elbow slightly behind the body.

VFDs can be used with literally any exercise, and can provide a good deal of help in choosing which to use for maximal development. If muscle overload is considered time under tension (F*t), then we can get F*t = -w*cos(?)*t. Now this is a simplified look at things, there is also the idea of contracting and extending a muscle (this method would give equal answers for both full range reps and static holds), but it still gives an indication of which movements offer more muscular overload than others.

Be Sure To Also Check Out:
The Omnimorphic Training Guide (OTG)!

Hope this helps!