It is no big secret why we use the term “biomechanics” when we speak of the spine. The reality of the spine and its function is that it is a piece of complex machinery. We see the application of levers, pulleys, and fulcra in the stress distribution function of the spine. For example, take the small weight of a can of soup. Hold it tightly to your body wall and appreciate how light it is. Now fully extend your arm and try to hold the can parallel to the ground. Can you feel the difference in the stress at your shoulder? The same weight, just over a longer lever, increases the stress at the fulcrum. In this case the lever is your arm and the fulcrum is the shoulder girdle.
The spine is a machine which also depends on material science.
We describe functions such as tensile strength, shear, and hysteresis when we speak of the intervertebral disc. The disc also acts a hydraulic system as it involves a centrally located viscous fluid surrounded by an outer ring. So, what happens to the risk of back injuries if we increase shear at the disc while we simultaneously increase the length of the lever arm? How does that play into the risk for back injury? What about repetitive compressive loading and unloading?
What we are really seeing is the spine is the primary initiator of body motion and support during tasks. If you look at the same soup can, the stress was increased at the shoulder girdle, not just the shoulder itself. The reason for this distinction is that the shoulder cannot function against gravity without a base of support (a pulley). That base is the coordination of muscle contraction and joint stability through the neck, shoulder joint structures, upper back/rib cage, and the involved muscles. To take this one step further, the lat muscle (latissmus dorsi) is a strong muscle with broad spinal attachment originating at the pelvis and inserting into the upper arm.
We use these same properties during our daily tasks. When we work with a computer mouse, how extended is our arm and for what length of time? Is our head rotated to one side? Are we sitting at the desk for prolonged periods of time during the day? All of these factors work to determine soft tissue flexibility. Now what happens over the weekend when we are more active and engage in tasks that require bending and reaching and lifting? Is it any wonder that most patients report acute back pain without significant trauma?