Mechanics Of Lifting
Poor manual handling technique is a significant risk factor for back pain:
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| Mechanics Of Lifting |
Epidemiological and ergonomic studies confirm that workplace activities that load the spine severely are closely associated with back pain in general and with acute disc prolapse in a particular way.
It is therefore essential to identify the best way to lift to indicate which technique or techniques minimize the loading of vulnerable tissues in the spine.
Some biomechanical analyses concentrate on one aspect of the problem, such as the shear force acting on the lumbar spine or the activity of the back muscles or leg muscles. However, lifting is performed by the whole body, and many structures can give rise to back pain including the intervertebral discs, apophyseal joints, and sacroiliac joint.
Lift with a straight back:
The need to include many anatomical structures makes the lifting problem appear very complicated. A possible solution would be to construct a detailed mathematical model to cope with all of the complexity and then choose some optimizing principle' (such as minimizing the shear or compressive force acting on L5-S1) to solve the equations and 'discover' the best way
of lifting.
Mechanics of lifting:
why heavy lifting can lead to high compressive and
bending forces acting on the lumbar spine?
why it would be useful to try to employ the lumbodorsal fascia to assist in lifting?
Back muscles must generate high forces during weightlifting
During lifting movements, the upper body pivots about centers of rotation, which lie in the nucleus pulposus of each intervertebral disc. Tensile forces in structures lying posterior to each pivot must generate an 'extensor moment' (a moment is a force multiplied by distance) that opposes the 'flexor moment' due to the weight of the upper body and the object being lifted. Because the back muscles act on shorter lever arms than these weights, the forces generated by them must be correspondingly larger. During a very slow or static lift, when accelerating ('inertial') forces can be discounted, the extensor and flexor moments must balance.
(During lifting, the spine pivots about a center
of rotation (0) near the middle of each intervertebral disc.
The forward bending moment arising from the weight lifted 0JV) and from upper body weight (w) must be balanced by the extensor moment generated by tensile structures lying posterior to O. Only two of these are shown: the lumbodorsal fascia (F) and the ligaments of the apophyseal joints (l). The ligaments act on a shorter lever arm than the fascia and so must exert a higher compressive 'penalty' on the disc. The lumbar back muscles lie between land F. The resultant compressive force acting on the spine (R) is the sum total of these tensile and compressive forces.)
The forward bending moment arising from the weight lifted 0JV) and from upper body weight (w) must be balanced by the extensor moment generated by tensile structures lying posterior to O. Only two of these are shown: the lumbodorsal fascia (F) and the ligaments of the apophyseal joints (l). The ligaments act on a shorter lever arm than the fascia and so must exert a higher compressive 'penalty' on the disc. The lumbar back muscles lie between land F. The resultant compressive force acting on the spine (R) is the sum total of these tensile and compressive forces.)
Lifting is best done by muscles and fascia that act on big lever arms:
The required extensor moment can be generated by tension in many structures, including the posterior annulus fibrosus, the ligaments of the neural arch, the erector spine, muscles, and the lumbodorsal fascia. The extensor moment generated by each structure is equal to the tensile force acting on it multiplied by the lever arm between that structure and the center of rotation in the disc.
it is apparent that some structures are better placed than others to assist in lifting: those that lie furthest from the pivot can generate a high extensor moment with only a small tensile force. This is advantageous because these tensile forces all act to pull the vertebrae closer together and so compress the intervertebral disc.
The benefits of lifting with a straight back:
The benefits of lifting with a straight back:
1- Lumbar curvature is a convenient and meaningful way of quantifying spinal 'posture'
Lumbar curvature is a convenient and meaningful measure of posture
because it affects load sharing between various
structures of the spine:
First, it determines the relative orientation of adjacent lumbar vertebral bodies and thereby affects the distribution of stress acting
in the intervertebral discs.
in the intervertebral discs.
Second, it influences the distribution of compressive loading between the discs and the neural arch. Third, lumbar curvature affects tension in the intervertebral ligaments, particularly those of the neural arch.
As discussed above, it has long been recognized that heavy weights should be lifted with a straight back, but what does a 'straight
back' actually mean? The shape of the back during erect standing actually increases the spine's curves, rather than reduces them, and standing upright involves considerable lumbar lordosis Conversely, a fully flexed toe-touching posture reverses the lumbar curvature into a kyphosis.
In between these extremes, a straight-back can be achieved by moderate lumbar flexion combined with some thoracic extension, to flatten the curves naturally present in an unloaded cadaver spine. As far as the lumbar spine is concerned, it is useful to contrast 'straight back' postures, which flatten the lumbar lordosis, and 'lordotic' (or 'erect') postures, which preserve or exaggerate it.
2- A straight back leads to even stress distributions within the intervertebral discs:
It is not surprising, therefore, that lumbar curvature has a profound effect on stress distributions within the disc. For example, lordotic postures create concentrations of vertical compressive stress within the posterior annulus particularly following sustained 'creep' loading or the following damage to an adjacent vertebra.
Repetitive loading in lordotic posture can cause 'hairpin bend' deformations of lamellae within the posterior annulus, leading to posterior bulging of the disc.
Moderate flexion of lumbar motion segments brings
the endplates are approximately parallel and equalize compressive stress across the whole disc.
Flexion right up to the elastic limit of the intervertebral ligaments can generate concentrations of compressive stress in the anterior annulus but these are rarely as high as similar concentrations in the posterior annulus and are unlikely to have significant adverse effects on what is the strongest and thickest region of most lumbar discs.
Spinal flexion and extension also influence tensile stresses within intervertebral discs. The outer posterior annulus resists flexion rather like a ligament but its proximity to the center of rotation means that it generates only a small extensor moment and applies a very high compressive penalty to the rest of the disc.
When stretched vertically, the posterior annulus fails at a stress of 9MPa and a strain of 34%. The small resting length of the posterior annulus means that the total stretch at failure is less than 3 mm, so there is little potential for energy storage. Even this potential will not normally be realized, because the intervertebral ligaments prevent the posterior annulus from being stretched to its elastic limit. In any case, the discs are not well suited to store energy because they are avascular, and therefore unable to dissipate the proportion of the stored energy which is lost as heat (hysteresis energy). Thermal damage to collagen and to cells might well be a problem in large, poorly vascularized skeletal tissues.
Flexion close to or beyond the normal physiological limit stretches and thins the posterior annulus to such an extent that the disc can prolapse posteriorly if it is subjected to a high compressive force at the same time.
Posterior disc prolapse can occur even if the flexion angle is not extremely provided that the compressive force exceeds physiological limits. For these reasons, high tensile forces in the outer posterior annulus in full flexion do little to assist in the mechanics of lifting, and they could lead to thermal or physical damage to the tissue. Evidently, full lumbar flexion should be avoided during heavy lifting.
(Disc prolapse produced in a cadaveric specimen by high loading in compression and bending)
