Since very few structures within the human body are constrained to three degrees of freedom of motion, we will need to understand rotational mechanics as well as translational mechanics. The simplest type of rotation, and the type that is most common, is rotation that occurs about a fixed axis. Rotation of a propeller on an airplane is an example of rotation about a fixed axis. If the axis of rotation passes through the body, points that are not on the axis travel in circular paths centered on the axis, while points on the axis remain stationary. When we talk about rotational motion of a vertebra, we will describe it by referencing a point on the anterior surface of the vertebral body.
In order to protect the spinal cord from injury, cervical vertebra C1 is limited in its ability to:While translation is not sequence dependent, rotational motion is sequence dependent. This means that the start and end points of a series of rotations are dependent upon the sequence of the rotations. The final orientation of objects being rotated is also dependent upon the sequence of the rotations.
The practical implications of this can be life changing. Suppose that you are facing straight ahead and then bend over to pick up the cat. But the cat is a moving target and you have to rotate a few degrees to the left before you catch it. Unfortunately, the flea bag tries to escape, scratches you, and you straighten up before rotating back to midline. This rotational sequence places the vertebrae back into something other than their original starting position, which is probably accomplished by straining some soft tissue components. For the young person, no big problem. However, for the middle aged person it is likely to result in injury.
The strain patterns most commonly associated with persistant low back pain are a consequence of bending and twisting or sidebending, and then attempting to resume the upright position diagonally. The vertebrae do not end up in the same position in which they started.
A quick way to easily observe how rotation is sequence dependent is to do the following:
- Stand and place your arm by your side with your palm facing your thigh.
- Keeping your arm straight, rotate just your shoulder joint until your hand is pointing forward and your arm is parallel to the floor.
- Now rotate your shoulder to the right through an angle of 90 degrees.
- Finally, drop your arm back to your side.
Your have rotated your shoulder through three 90 degree arcs: first about the X axis, second about the Y axis, and finally about the Z axis. Notice that your palm is no longer facing your thigh, but is now facing forward with the side of your hand against your thigh.
Now, lets go through the same three 90 degree rotations, but let's do them in a different sequence to see what happens.
- Stand and place your arm by your side with your palm facing your thigh.
- Keeping your arm straight, rotate just your shoulder joint until your hand is pointing to the right and your arm is parallel to the floor.
- Now rotate your shoulder to the left through an angle of 90 degrees so that your arm is pointing forward.
- Finally, drop your arm back to your side.
Your have rotated your shoulder through three 90 degree arcs: first about the Z axis, second about the Y axis, and finally about the X axis. Notice that your palm is no longer facing your thigh, but is now facing backward with your thumb against your thigh.