The natural shape of the spinal column, when we are standing, is an S-shaped curve when viewed it occurs when the head, hips and feet are vertically aligned and symmetrical. When the spine is positioned this way, the passive structures (the vertebrae, ligaments and discs) are at their strongest alignment and the body is in its absolute strongest condition (from a posture perspective) to take on physical loading. Posture strongly influences the spinal loading and disc compression, in terms of loading on the lumbar (lower) spine, the difference between standing and sitting is significant.According to Kroemer and Grandjean (1997 p. 73), if we normalize the loading from standing in an erect, relaxed position with the naturally occurring S-curve to 100% compressive loading, then relativeto tha pressure in the lower back:
• Sitting down with a straight back corresponds to 140% loading.
• Sitting down in a slouch or leaning forward corresponds to up to 190% loading.
• On the other hand, lying down brings down the compressive loading to 24%.
This difference in compressive loading provides a clue to why it is so important to get sufficient amounts of sleep – not only does the body need to recuperate and the mind work subconsciously with processing information, the discs in our back also need a chance to return to their uncompressed, round form in order to be ready for another day’s work as flexibility enablers, loading relief and shock absorbers.
3.1. The components of physical loading
As you learned in Chapter 2, Basic Anatomy and Physiology, the body’s tissues work together to withstand many. Different types of biomechanical loading. Exceeding the body’s physical ability to handle these loads results in pain and physical injury, which can be either sudden or chronic. But if we regard the problem from an engineering perspective, we need concepts and methods to identify what exactly makes physical loading a risk. To make this possible, we adopt the view that:
Physical Loading = posture × forces × time
Body posture demands that the body’s muscles actively work to maintain a position, which is a form of internal loading. The posture aspect includes how internal forces are distributed across the different parts of the body (for example, lifting something off the ground with a straightened back engages mostly the leg muscles which are large and strong, while lifting the same object with a bent back loads the upper torso which has smaller, weaker muscles).
External loading occurs as a result of handling weights, e.g. by pushing, pulling, lifting, pressingor dragging something. Generally, when force is counted as a component of loading, we are mainly referring to external loading. In some biomechanical analyses, the weights of the human’s own body parts are sometimes also considered a load, especially if gravity influences the chosen posture. Finally, time factors describe how long, how often or how frequently the body’s structures are loaded.Since you now know that the muscles and tissues can work for a limited time until they are fatigued and need to rest, the level of risk depends on whether the exposure is suitable for strength- or endurance-type body structures. The time component most frequently focuses on repetitiveness, which is considereda major health risk because the body’s structures are not allowed enough recovery between loadings.