Back support mechanisms during manual lifting.

Back Support Mechanisms During Manual Lifting Low back pain has been reported as the leading cause of disability in those under the age of 45 years and the third major cause of disability in general, following arthritis and heart disease. [1,2] Because low back pin has been shown to be a leading cause of lost work time in occupations involving manual labor, [3] it is of interest to study the forces on the low back during activities performed in manual labor occupations. Lifting is a common activity in many of these occupations. With the onset of back schools and work-hardening programs for the treatment of low back pain, an understanding of anatomical and biomechanical factors that make lifting possible (back support mechanisms) has become increasingly important to the physical therapy profession.

Although manual lifting of loads has been studied for many years, [4-28] the back support mechanism of lifting and the forces sustained by the body are still not fully understood. [1,4,20] The force exerted on the intervertebral disks while lifting a load held in front of the body has been estimated to be large enough to bring the disks to failure. [4,7,9,19] There has also been a suspicion that the back muscles alone are not capable of producing enough force to complete even moderate lifts. [1,21] This problem has led researchers to search for a mechanism that will better explain this phenomenon.

Since the late 1950s, different methods of the calculation of forces and support mechanisms for lifting have been developed. The current literature offers apparent conflicting explanations of the lifting mechanism, [1,20] which has led to controversy regarding a "proper" technique of lifting. [1,8,20,22,23] The purposes of this article are 1) to review and compare the current theories of back support mechanisms during lifting and 2) to explore the issue of a proper lifting technique.

Mechanisms Used In Lifting

Intra-abdominal Pressure

Intra-abdominal pressure (IAP) as an aid in reducing the compressive forces on the spine during heavy lifting was first proposed in 1957 by Bartelink [7] and was later expanded by other investigators. [1,6,8,9,21,28] Bartelink found that during heavy lifting there was a rise in IAP as measured by a gastric balloon. [7] Electromyographic studies during the same lifts showed parallel activity of the transversus abdominis and internal oblique muscles when IAP increased. [7h The rectus abdominis muscle was not seen to contract during these lifts. These studies concluded that the contraction of the former muscles in the presence of a closed glottis was responsible for the increase in IAP. The role of this mechanism, it was theorized, was to spare the elements of the spine excessive compression. Bartelink used the analogy of an inflated football to describe the mechanism of load bearing through the abdominal cavity and the transfer of force through the pelvic floor. [7] The moment arm of the erector spinae muscle group is lengthened in this model because the axis of rotation for sagittal movement is shifted anteriorly into the abdomen from the intervertebral space. This lengthened moment arm would allow the erector spinae muscles to maintain the body in static equilibrium with less force, thus reducing spinal compression. Later studies show that although there is an increase in IAP with increasing loads, this is not a purely linear relationship. [1,21,25] Hemborg et al have reported that the IAP was load specific and unrelated to training of the abdominal musculature. [25h These factors suggest that other support mechanisms exist that must be accounted for.

Thoracolumbar Fascia 

The role of the thoracolumbar fascia (TLF) as a lumbar support mechanism has recently been studied. [12,29] Mathematical models [12,30] have been proposed based on the recent description of the anatomy of the TLF. [29] The TLF as a back support mechanism works well in conjunction with the IAP mechanism theory.

The thoracolumbar fascia is composed of three layers: 1) anterior, 2) middle, and 3) posterior. The posterior layer consists of two lamellae: 1) a superficial lamella and 2) a deep lamella. The superficial fibers run mediocaudally, and the deep fibers run laterocaudally from the midline. Mediocaudal fibers are formed by the aponeurosis of the latissimus dorsi muscle and attach to the iliac crests, to the lateral border of the erector spinae muscle group, and to the spinous processes of L3 through S1. The laterocaudal fibers run from the spinous processes of L4 and L5 to the iliac crests and from the L2 and L3 spinous processes to the lateral raphe. Fibers from the T12 and L1 spinous processes become membranous and attach diffusely to the surrounding tissue (Figs. 1, 2).

The aponeurosis of the latissimus dorsi muscle attaches to the middle layer of the TLF where the transversus abdominis muscle originates. Medial to the 12th rib, the middle layer forms the lumbocostal ligament. The internal oblique muscle attaches to the lateral raphe of the TLF an d occasionally to the aponeurosis of the latissimus dorsi muscle. A triangular pattern emerges, which creates an extension, or antiflexion, moment through the lumber spine during contraction of the latissimus dorsi, transversus abdominis, and internal oblique muscles (Fig. 3). Gracovetsky et al also described the "hydraulic amplifier" mechanism, [12] which could add tension to the anterior layer of the TLF, thereby increasing its antiflexion moment. This mechanism is accomplished through the contraction of the erector spinae muscles, which are contained in a thick, inelastic envelope. As the muscles become engorged with blood, tension increases throughout the TLF.

The TLF can be engaged passively or actively. The passive mechanism of the TLF is engaged by flexion of the lumbar spine on the pelvis, as in bending forward. It is also engaged by a contraction of the abdominal, hamstring, and gluteal muscles, which posteriorly rotate the pelvis and place tension along the TLF. [28] The active mechanism of the TLF is engaged by contraction of the latissimus dorsi, internal oblique, and transversus abdominis muscles. [20,28] The transversus abdominis muscle was originally thought to produce an antiflexion moment through the TLF. [12] Later calculations showed that it was not strong enough to produce a significant resistance to flexion. [30] Tension along the transversus abdominis and internal oblique muscles is increased by the IAP pushing out along the course of the muscles, which adds tension to the TLF. When the IAP is low, there is little tension on the TLF, which allows a contraction of the rectus abdominis muscle to produce flexion of the lumbar spine (Fig. 4). [28]

Posterior Ligamentous System

The interspinuous and supraspinous ligaments lie further posterior than the sacrospinalis muscles. Because the lever arm is longer for the ligaments than the muscles, using the ligaments while lifting would reduce the compressive forces through the lumbar spine. The midline ligaments were shown to contribute 19% of the ligamentous resistance to flexion. [31] With a semiflexed lumbar spine, the ligamentous restraint to flexion is shared by the midline ligaments, the ligamentum flavum, the posterior annulus, [1,31] the zygapophyseal joint capsules, [31,32] and the TLF. [12] The TLF bears most of the stress in flexion. [4h The density of the tissue suggests that it is a major load-bearing structure. When lifting is performed from a flexed position, the inert structures play a vital role in maintaining equilibrium because the erector spinae muscles are electrically silent in the flexed position. [5,6,28,33-35]

Muscle Activity During Lifting

Muscle Relaxation Phenomenon

Electromyographic studies of the erector spinae muscles with regard to sagittal lumbar movement and lifting have been reported by several investigators. [5,6,33-35] These muscles contract eccentrically as the lumbar spine is lowered into flexion by gravity acting on the upper body. As the lumbar spine approaches two thirds of maximal flexion, the muscles are electrically silent. [5,6,33,34] Kippers and Parker suggest that because the zygapophyseal joints contain mechanoreceptors, there is a reflex shutdown of these muscles at the point where the joint is fully loaded. [34] Gracovetsky suggests that the body will use the most energy-efficient system for movement and that it uses the TLF and midline ligament system as flexion of the spine increases. [28]

Nachemson performed EMG studies of the vertebral portion of the psoas major muscle in several static positions on small groups of subjects. [36,37] The needle electrode was placed lateral to the third lumbar intervertebral disk with the assistance of an image intensifier. The muscle was highly active during unsupported sitting and when the subjects leaned forward at 20 degrees of trunk flexion while standing. This seemingly paradoxical action of the psoas major muscle has been attributed to its ability to add stability to the spine by countering the pull of the deep postvertebral muscles. [36,37] During lifting, the psoas major muscle acts as a stabilizer, which prevents excessive lateral bending and rotation. [1,14,20]

The psoas major muscle originates on the intervertebral disks, the lateral vertebral bodies, and the transverse processes of the lumbar vertebrae. It courses over the anterior pelvic brim and inserts on the lesser trochanter. [38] Figure 5 is taken from tracings of normal bending roentgenograms. The calculated instantaneous axis of rotation (IAR) was estimated from the work of White and Panjabi. [39] When the spine is in the flexed position, the muscle fibers cross anterior to or through the IAR at each lumbar level. When the spine is in a neutral lordosis, the muscle fibers run on either side of the IAR, counterbalancing each other and giving a stabilizing force to the spine. When the lumbar spine is extended, most of the muscle fibers run posterior to the IAR and provide an extension moment. Depending on the starting position of the spine, therefore, the vertebral portion of the psoas major muscle can act as a flexor, a stabilizer, or an extensor of the lumbar spine. Regardless of the position of the lumbar spine during lifting, the psoas major muscle is active in stabilization of the lumbar spine. This analysis has been supported by a recently completed investigation (N Bogduk, M Pearcy; unpublished data; November 1987).

Isotonic Lifting Investigations

In recent years, isotonic (dynamic) lifting studies have observed compressive forces within the spine and ground reaction forces with different lifting speeds. [4,11,15,19,20,26] McGill and Norman performed a detailed study of EMG recordings of erector spinae, abdominal, and latissimus dorsi muscles; computed tomography scan measurement of muscle mass; computerized assessment of lines of action for muscles and ligaments; IAR; cinematographic analysis; and equations to assess the isotonic loading characteristics of muscle. [29] The authors concluded the muscle forces produced by the erector spinae muscle, with the assistance of the latissimus dorsi muscles and the TLF, were great enough to lift loads weighing up to 91 kg. Because of the orientation of the muscular and ligamentous constraint, the shear forces on the intervertebral disks were found to be less than those predicted by other investigators. [4,8] Compressive forces on the disks were calculated using a composite moment arm of 6.14 cm to represent the ligaments, muscles, and TLF. Previously, the moment arm was calculated at 5 cm. [8,11] This lengthening of the moment arm reduces disk compressive forces to well within the limits of vertebral body failure. [16 McGill and Norman concluded that the contraction of the latissimus dorsi and abdominal muscles created tension within the TLF. [20] The abdominal muscles were tensed primarily because of a pressurized abdominal cavity. The lift studied by McGill and Norman was performed with a straight back and began with the subjects' hands 32 cm above the floor.

Stoop Lift Versus Squat Lift

Many investigators have examined stoop lifting (back bent) in comparison with squat lifting (knees bent). [6,15,18,21,23] The squat lift is superior to a stoop lift for several reasons. It allows the subject to lift an object from between the legs, which shortens the horizontal distance of the load from the body and reduces compressive forces in the intervertebral disks. [5,15,18] The squat lift also enables greater force production than the stoop lift. [15,16] The lower center of gravity in a squat lift improves balance and the ability to redirect force rapidly if necessary. the hamstring muscles probably operate closer to their resting length in squat lifting as compared with stoop lifting, which could improve their efficiency as stabilizers of the pelvis. [40]

The squat lift has a greater energy expenditure than the stoop lift because of the greater amount of body weight displaced vertically. [15,23] Because of this high energy expenditure, endurance training for the quadriceps and gluteal muscles should be incorporated into a program that teaches squat lifting.

Flexion Versus Extension

Recent studies have focused on the effects of lifting with the spine flexed and with the spine extended. [24,27] Hart et al examined squat lifting in lordotic, straight, kyphotic, and subject-preferred postures. [24] They reported the lowest compressive forces and the highest EMG activity of the erector spinae muscle in the lordotic lift. Other studies have reported higher compressive forces with higher EMG activity of the erector spinae muscle. [1,5,6,14,28] This contraction could be explained by the fact that Hart et al [24] did not account for a difference in lever arm length for the ligamentous system (kyphotic posture), the muscular system (lordotic posture), or a combined ligamentous and muscular system (straight posture). [20] The midline ligaments and TLF have longer lever arms than the muscles. [28] If the distance of the load from the front of the body and the forward inclination of the trunk remain the same, and only the lumbar posture is manipulated, the flexed posture offers the least compression. [28]

Lifting with a posterior pelvic tilt initiated by a contraction of the hip extensor and abdominal muscles has been advocated by Gracovetsky and Farfan [1h and Gracovetsky. [28] Gracovetsky et al demonstrated that individuals will choose a posture that is unique for each individual and that shares the load between the TLF, the posterior ligaments, and posterior annulus. [14] This unique posture, which always reduces the lordosis, was calculated to be the most energy-efficient method for lifting and produced the lowest compressive loads on the intervertebral disks. [28] This method uses inert structures in the early part of the lift, then distributes the load among all structures as it gradually switches to muscle contraction to complete the lift. The property of creep in the TLF and ligamentous system requires that lifts be performed at faster speeds. [28,41] Isometric (static) lifting tests and lifting slowly while in a flexed posture are potentially dangerous because the elongation under load increases with time. [28]

Discussion

The TLF provides a major support mechanism for lifting, regardless of the lumbar posture adopted. [1,20,26-28] With its attachments to muscles and its role as a ligament, it can be engaged passively or actively. The TLF has the best mechanical advantage of all lumbar tissues that provide an antiflexion moment. It, therefore, produces the least amount of intervertebral compression. [1,4,28] The IAP serves to increase the tension along the TLF, assisting in the antiflexion moment. [20,28,30]

Understanding the TLF support mechanism and the role of the IAP is essential to proper care of patients with low back pain and to proper instruction for industrial workers. Maximizing effectiveness of the TLF can be accomplished by training muscles attached to it and by utilization of its passive mechanism.

Finding evidence to support one technique of lifting over another is difficult because some investigators support the use of a lordosis while lifting, [24,27] others support the use of a flexed spine, [1,28] and others advise patients to lift however they like. [2,17] There are a few clear indications for preferring lifting in either flexion or extension for the uninjured worker. For the patient with low back pain, choosing one lumbar posture over another should be dependent on the type of injury and the response of the injury to mechanical stress.

Hart et al [24] and Delitto et al [27] have recommended lifting with a lordosis, primarily to prevent injury to the ligamentous system. Low back pain is a self-limiting problem in which 90% of all cases resolve within six weeks. [42] Because ligament injuries take between six weeks and six months to fully heal, [1] it follows that most painful episodes of low back pain do not involve the ligaments.

Poor endurance of the back extensor muscles is a good predictor of future low back pain, [43] and the lumbar multifidus was found to be generally deconditioned in a group of sedentary individuals. [44] Instructions for lifting with a lordosis, which puts demands on the extensor muscles, must be augmented with strenth and endurance training for these muscles. Failure to do so may result in future muscle injuries to the low back.

Instructing uninjured workers or patients to lift an object without handles from the floor while maintaining a straight back or a lordosis may be unreasonable. Although a lordotic lift has been recommended by some investigators, [24,27] none have demonstrated how maintaining a lordosis is possible when performing a floor lift. Some degree of flexion is required at the spine for most individuals to complete such a task (Fig. 6). When lifting an object from the floor, the TLF and posterior ligamentous mechanisms are best applied because the erector spinae muscles are inactive. These mechanisms offer the only antiflexion support while lifting in a flexed posture. It is interesting to note that the IAP, which assists in activating the TLF mechanism, is greatest in the fully flexed postures. [21,25] When lifting loads greater than 68.4 kg, flexion of the lumbar spine has been noted in the initial part of the lift [21,23,28] when inertia must be overcome and ground reaction forces are greatest. [11]

Advice for Uninjured Workers

Some general rules for lifting are widely agreed upon, such as keeping the load close to the body [1,4,5,10,11,18] and squat lifting. [1,23-27,30] Twisting while lifting must be avoided because torsion combined with loading can damage the facet joints [40,44] and the intervertebral disks. [43] Planning for a lift should include sizing up the load and determining whether help is needed and pathways are clear.

The choice of using either a flexed or extended spine is a trade-off between lower disk compression forces in the flexed posture or better muscular control and sparing of the ligaments in the extended posture. Choosing flexion or extension should depend on whether the task must be performed slowly and whether the load must be lifted from the floor. Slower lifts favor an extended lumbar spine to reduce creeping of the ligamentous tissue. [28] Lifting from the floor favors the flexed posture because it is necessary to flex somewhat during this task.

Using a straight-back position as described by McGill and Norman [20] encompasses the combination of muscular control and suport of the TLF and IAP mechanisms. This lift will produce the greatest support while reducing compressive forces of an extended spine and ligamentous stress of a flexed spine. [20] When circumstances permit, a straight back should be used during a squat lift. Pretraining for strenth and endurance of the extensor muscles is advisable when preparing workers to lift with a straight spine, especially for repeated lifts. Understanding the precautions for both the flexed and extended positions will help the physical therapist to prepare workers for circumstances that are not optimal. when flexion is required, the lift should be performed quickly. When the lift is performed in extension, the individual should be aware of higher disk compression and the fatigue factor of the muscle

Advice for Patients with low

Back Pain

When soft tissue has been injured, it requires time to heal and should not be stressed unduly. Avoiding all lifting in the early stages of recovry is reasonable, but as the individual starts to become more active and returns to work, strategies for lifting should be applied. Knowing which tissues have actually been injured is often difficult to determine which low back pain.[45-48] This difficulty has led to systems that rely on the patient's report of pain with different postures and movements to direct treatment.[48] Psychophysical tests for lifting have recently been advocated.[49,50] These tests rely on the feedback of the patient to determine the appropriate posture[49] or the appropriate load.[50] Among other criteria for safe lifting, Blankenship advocates finding a lifting posture for the patient that is most comfortable through psychophysical tests and applying some general rules when instructing workers in lifting.[49] Under appropriate conditions, lifting can be performed in either flexion or extension.[1,24,27]

Stressing injured tissue will increase pain and direct the therapist to alternative postures for lifting. Known injuries to the ligamentous system, including the posterior annulus of the disk, should use the muscular system (extended posture) for lifting. Conversely, using the ligamentous system (flexed posture) will spare the erector spinae and multifidus muscles excessive stress if they have been injured. These postures should be tested and only used if they do not increase the patient's symptoms. Lifting any object from the floor should be avoided with an injury to the ligamentous system because it will surely stress the injured tissue.

Lifting should be avoided in the presence of a herniated intervertebral disk. A lift in flexion stresses the posterior annulus, and a lift in extension increases intradiskal pressure. Either posture could potentially worsen the injury. Further study is needed to determine force production while squat lifting in flexion and extension, preference for one lift over another based on anthropometric measurements, and the role of the psoas major muscle throughout lifting.

Conclusions

The TLF provides a major support mechanism for lifting, regardless of the lumbar posture adopted. The IAP serves to increase the tension along the TLF, assisting in the antiflexion moment. Choosing a flexed or extended lumbar spine for lifting is debatable. I believe that lifting in flexion should be performed quickly and the lifting in extension may require muscle training to be safe and effective. Whenever possible, lifting should be accomplished with a straight lumbar spine. When lifting an object from the floor, some degree of lumbar flexion may be required. Instruction in safe lifting after an injury should be aimed at avoiding stress of the injured tissue. Using flexion for muscle injuries and extension for ligamentous injuries should be weighed against the patient's report of pain through psychophysical testing. Instruction in safe lifting techniques should include the following: 1) plan the lift, 2) keep the load close to the body, 3) avoid twisting while lifting, and 4) bend from the knees.

Acknowledgment

I thank Lance Twomey, PhD, for his advice and encouragement.

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MS Sullivan, MS, PT, is Spine Rehabilitation Coordinator, Departemtn of Orthopaedics, University Hospital, State University of New York at Stony Brook, Stony Brook, NY 11794-7525 (USA). He was Associate Director of Physical Therapy, University Hospital, State University of New York at Stony Brook, when this article was written.

This paper was presented at the New York State Convention for Physical Therapy, New York, NY, April 25, 1987.

This article was submitted October 19, 1987: was with the author for revision for 28 weeks; and was accepted August 15, 1988.