Safe Lifting and Carrying for Susceptible Backs
Although not readily apparent, back strain associated with incorrect lifting is often the result of how leverage works.
When lifting or carrying a heavy object, the actual load on any given part of the spine is multiplied by the distance between the object and that part of the spine. The bigger the distance, the bigger the load. This is called the “leverage effect”
In ancient Greece, Archimedes pointed out the mechanical advantage of the power gained from leverage.
The approximate calculation for the load experienced on the spine at the level of L5 (the first lumbar vertebra just above the sacrum) is depicted in the figure to the left. For the height of the individual photographed, the horizontal distance from L5 to the weight to be lifted is about eight times the distance from L5 to the fulcrum (indicated by “d” in the figure.) This means that if the object is lifted as shown in the figure and its weight is, for example, 20kg, the effect of leverage will result in a compressive load at L5 of up to 160 kg. (8 X 20kg) when the object is lifted from a place at such a long distance from the fulcrum.
In calculating the exact load to the lumbar spine we must also include the weight of the body, from the head to the point of load, even straightening the trunk by itself without lifting an object will cause stress to an already-compromised back. That is the reason for a common instruction given in yoga classes to bend the knees when returning to standing from a forward bend.
Explaining the principles behind the sequence depicted above may help establish healthier habits and minimize back pain recurrences.