If things are left to themselves, they tend to lump into a spherical form:
When nature can’t lump up, it groups isolated spherical lumps into circular patterns:
If I leave my gear lying around, neither of this is going to happen, unless I have a few hundred million years to spare. And the clients are not going to wait that long. Therefore, I have the unenviable task of finding the right way to lump my gear.
The spherical shape has two interesting properties:
- The force at one point is exactly equal to the force at the opposite end.
- Every point on the sphere has the same force and direction, so you’ll never feel any different from anyone else.
For practical reasons, we can’t get our gear into a spherical shape, nor is it ideal in more circumstances. But we can try to sneak in its advantages into our rig. Why bother?
It’s not the meaning we are accustomed to. In scientific terms, especially the field dealing with mechanics, a moment is:
Moment (M) = Force (F) x distance (L)
With so many forces acting on his arms, it’s no wonder our Vitruvian Man is upset.
Look at the left side of the image. Imagine a waiter holding a plate with his arm outstretched. The weight (for simplicity’s sake I’ll disregard the difference between force and weight) bearing down on his arm multiplied by the length of his arm is the Moment.
Since his arm is fixed at the shoulder, the force has the effect of trying to push his arm downwards in a spiral arc. This is similar to how a ceiling fan works. This fixed point is called the Fulcrum.
If a force is applied in the opposite direction, the arm will try to rotate in the upward direction.
The other kind of force is when it comes in at an angle, like the red force line in the middle. In simple physics, to make calculations easier, we break this force into two directions – one along the path of the arm, and the other wherever it may fall, always 90o from the first.
Imagine someone standing in a subway train, hanging off the top rails. Her body forces a downward force at an angle, and this force has the effect of pushing the body downwards (which is why she stays down!) and sideways. Try it yourself.
If the force is outwards, as in the right-most example, with the green force line, the same principle applies, except the direction changes. The force is split into two parts, one along the length of the arm, and the other 90o away. The force along the arm tries to pull the arm outwards, while the other component has an upward effect.
We’re not interested in the science and formulas here, just the overall ‘feeling’ of how this works.
Our body isn’t this simple, though. Other than the shoulder, we also have an elbow, a wrist and fingers. Each of this is a possible fulcrum. Take this for example:
Not the beer!
If he is holding the tray at a slight angle, one component of the weight is distributed along the line of the elbow. The second is pushed inwards. Where do you think this force acts? On the elbow, where else? The elbow can take the force along its length, but the other component puts a strain on it that it isn’t designed for.
Imagine, then, what might be happening to his wrist and forearm.
How many of you hold camcorders this way? How does it affect your wrist, forearm, elbow and shoulder? You don’t need a camcorder to learn all this. Hold your arm out without any weight, in every position you can think of. The natural weight of the arm will act as a force – How long can you take each position?
Assuming your feet are firmly on the ground, try these rules of thumb:
When your arms are above the height of your shoulders, it’s better to hang than to push upwards.
When your arms are below the height of your shoulders, it’s better to push than to pull (think about carrying shopping bags)
When your arms are straight out sideways or front-ways (like supporting yourself on a vertical post in a subway car), it’s better to push than to pull.
The idea behind balance
If a load must be carried at all, it’s better for the body to center that load directly over the spine – I’m not a doctor so don’t hold me to that. Each individual is different – some of us slouch a bit, we have different muscle and bone structures and different shoulder-head-eye positions.
It’s better for a force to act directly along any particular length (wrist, forearm, elbow, arm, shoulder, etc) than to split into two forces. When we slouch or change the angle of the force along the spine, the spine will take one force down its length (the natural way), and another in an outwards direction (the dangerous way).
Keeping your spine (back) straight is a defense against this happening. No matter how the forces act on the arms – weightlifting, pull-ups, carrying, pushing, pulling, arm-wrestling, racquet sports, writing, holding a camera, whatever – protecting the spine and the neck is crucial. A force can always come in at an angle against the spine, but a correct posture at least tries to avoid the regular day-to-day strains that it is forced to take.
A supreme example of this in practice is formal dancing or gymnastics. The right posture also happens to look graceful in motion. There are many techniques that attempt to teach correct posture, but I’m not going into details. None of this is meant to be medical advice, so please consult a doctor before trying any of this.
Ok, enough of the basics. How does all this meet the two properties of spheres we are trying to emulate? And how does all this tie in with the things we’ve covered in the chapter on Ergonomics?
Let’s look at rigs next.