Part III: Biomechanical analysis of lumbar spine exercises
The criterion employed was to examine whether postures, exercises and techniques involving stretching the soft tissues of the lumbar region assisted or hampered the lower back in functioning efficiently as a loco-motor system without undue discomfort. Electronic search engines were used to find biomechanics literature. The literature was surveyed in an attempt to link the process of stretching with the physical response of soft tissues. Evidence was found indicating specific stretching exercises involving spinal flexion and extension may be useful in alleviating or preventing back pain because of their Fig 12 influence on the intervertebral discs and adjacent tissues. Stretching, as an isolated activity, is useful in treating pathology associated with tissue shortening and reduced range of joint motion. A wider role in promoting the health and efficiency of the lower back is possible when it is combined with strength and postural training.
Keywords: Low back pain, stretching, stretching risk, biomechanics.
This paper provides a biomechanical analysis of stretching at the lumbar spine.
Texts on the biomechanics of the low back and the biomechanics of stretching soft tissues were sought from major contemporary authors. Texts were also sought describing the practices of stretching tissues of the low back used by manual therapists and fitness, yoga and pilates instructors.
Literature searches were conducted on the British Library catalogue and the internet using primarily Medline, Pubmed, Science Direct, BMJ.com and other search engines.
The available literature was then systematically reviewed.
Biomechanics: Spinal flexion from a position of an upright trunk is initiated by the abdominal and hip flexor muscles and assisted by gravity. The hip extensor muscles stabilize the pelvis during the first few degrees of flexion (Okada 1970; Portnoy and Morin 1956) fig13. Rate of descent into spinal flexion is controlled by eccentric contraction of the sacrospinalis muscles (Kapandji 1974) and Multifidus (Twomey and Taylor 1983), the back muscle most associated with spinal stability (Richardson et al 1999). The extensor muscles initially lock until the lumbar lordosis curve becomes flat but not fig 14 reversed as the spine moves into flexion (Bogduk and Twomey 1987). The back muscles and the lumbo-dorsal fascia restrict the fully flexed spine to ten degrees short of its elastic limit. The activity of the back muscles increases proportionally with increased flexion until about 40 degrees of flexion at L1 when myoelectric activity in the back muscles decreases markedly until the trunk is supported by the passive resistance of ligaments (Schultz et al 1985 in Alter p218).
The inhibition of erector spinae activity is thought to result from stimulation of the stretch receptors of the vertebral ligaments (Floyd and Silver 1951), joints proprioceptors and muscle spindles (Kippers and Parker 1984). This conserves energy because passive components do not consume energy that would be required if stabilization was reliant on muscle contraction. Patients with low back pain sometimes demonstrate less reduction of myoelectric activity in their back extensors in flexion (Floyd and Silver 1951,1955; Shirado et al. 1995; Silrvonen et al. 1991, all cited in Alter 219), suggesting a link between low back pain and disturbances of proprioception (Flor et al 1990 and 1994, cited by Waddell (1998, p15).
Rationale for spinal flexion: Williams (1937a, 1937b, 1955 – cited in Elnagger 1991; Nwuga et al 1983) postulated that vertebral deformation ensued when homo sapiens evolved from quadrupeds to bipeds. The upright position imposed more weight on the posterior part of the intervertebral disc increasing the risk of rupture. Williams advocated spinal flexion to alleviate low back pain. He argued that weak spinal flexors evident in those with a sedentary lifestyle results in the protrusion of the abdomen and a consequent exaggeration of the lordosis curve. This exerts force on the posterior lumbar and lumbar-sacral structures causing back pain. The lordosis therefore needs to be reduced by strengthening the flexors and stretching the spinal extensors to establish balance of opposing postural muscles.
In excessive lordosis, the lumbar arch is abnormally increased and weight normally evenly distributed over the entire lumbar curve is concentrated at the peak of the arch. When standing this will impose a downward and forward shearing force exerting uneven pressure on the disc with extra pressure on the posterior disc. This restricts the flow of nutrients into the discs that may result in disc deterioration and shrinkage. A reduction in disc height reduces the space between the facets enabling their bony surfaces to grate, possibly producing arthritic changes. Facet joint pathology places further stress on the discs creating a vicious circle. Excessive lordosis may therefore cause disc degeneration or disc hernia, spondylolysis, spondylolisthesis, facet joint degeneration, and nerve root impingement (Corrigan & Maitland 1998). Williams (1937a, 1937b, 1955 – cited in Elnagger 1991) argued that lumbar spine extension exacerbated this and prescribed flexion exercises to reduce the lumbar lordosis. Postures in Figs 12, 13 and 14 bear similarities to exercises advocated by Williams (see Figs 2, 3 and 4 in part 2 of this series). McKenzie (1981) also advocated flexion exercises to treat excessive lordosis and to stretch scarred tissue at the posterior disc following injury. Spinal flexion can open the intervertebral foramen (Corrigan and Maitland 1998) and can release nerve root compression (Torg 1987), either at the anterior disc as the annulus fibrosis is pushed posteriorly (McNab 1977), or at the facet joints as they separate in response to flexion (Swezey 1978). fig 15
Flexion and extension generate the highest compressive stresses in the discs and enhance fluid flow within the discs. Metabolic transport by fluid flow into and within the disc is increased with an alternation of flexion and extension postures (Adams et al 2002). Flexion increases the stresses inside the discs and the stretching of the posterior disc increases its posterior surface area so expelling fluid from the disc whilst under pressure. Fluid returns to the disc when the pressure is released bringing with it metabolites that enter the posterior annulus. This is particularly beneficial as the posterior annulus is more likely to suffer degenerative changes than the anterior annulus.
Risks of spinal flexion: The vulnerability if the disc is widely discussed in the literature in the context of spinal flexion. The discs provide 38% and 29% resistance on full and half flexion respectively. Lumbar flexion compresses the anterior of the intervertebral disc and stretches its posterior annular fibres (Oliver and Middleditch 1991). Flexion pushes the nucleus towards the posterior of the disc that if excessive may result in disc hernia causing material from the nucleus to protrude and aggravate neural structures (Seimon 1983; Cailliet 1995). Habitual spinal flexion that increases disc pressure to the extent of pressing out substantial amounts of fluid can lead to low back pain (Twomey and Taylor 2000).
The literature emphasizes the risks to discs from forward flexion but the ligaments are probably initially at greater risk than the discs. Adams et al (1980), Adams and Hutton (1986) and Twomey and Taylor 1983) argue that supraspinous, interspinous and facet capsular ligaments are likely to be the initial structures to sustain injury in forced flexion. Ligaments stretched beyond their limits do not return to their original length and so may slacken and be unable to protect the discs and joints. Injuries to the ligaments are slow to heal because of their poor blood supply and will result in slackening that is not reversible compromising their role in supporting the discs and joints (Coulter 2001). Nordin and Frankel (2000) claim the posterior ligaments resist 80% of the flexion moment and restrict flexion to 80% of that likely to cause damage to the disc. Resistance by the ligaments is reduced when flexion is slow and reduces further as it is held (Nordin and Frankel (2000). However, Adams and Hutton (1986) claim the posterior ligaments offer little resistance to flexion because they are close to the fulcrum of movement. Their role is therefore likely to be the imposition of compressive forces to maintain spinal stability as described by (Aspden 1996), so stretching them may adversely affect spinal stability.
Reducing the risk in spinal flexion: Pressure on the lumbar intervertebral discs is about one third more when standing unsupported in forward flexion than in sitting unsupported in forward flexion (Nordin and Frankel 2001) so other things being equal it might be preferable to perform spinal flexion sitting rather than standing. The positioning of the upper extremities affects the loading of the lumbar spine and pressure on the discs is considerably reduced if the trunk is supported by the hands or arms (Nordin and Frankel 2001). Postures 3 and 4 are sometimes performed with the arms initially held overhead and swooped anteriorly as the trunk is taken into flexion. When descending or ascending the trunk the force or moment exerted on the spine is proportional to the weight of the upper body and its distance from the axis of the body. It therefore exerts less pressure on the discs and other structures of the lower back if the arms are placed near the body throughout this posture (Nordin and Frankel 2001). Zacharkow 1984, Tyne and Mitchell 1983, and J Alter 1983 (all cited in Alter 1996, p216) argue that postures involving spinal flexion are contraindicated for persons with a herniated disc because of the stress imposed on the discs, the posterior ligaments and the sciatic nerve.
Forward flexion of the spine occurs in a number of postures using a variety of techniques that will influence the degree of force imposed on the soft tissues. The spinal flexion postures illustrated also flex the hip joint and when combined with knee extension stretch the hamstring muscles. However, the body takes the path of least resistance, so the flexible components will be stretched more than the inflexible components (Kendall et al 1971). This applied both within the interrelationships between hips, hamstring muscles and spine and within respective vertebral components. It may therefore be more effective to eliminate particularly tight structures from a posture and to stretch them independently. Postures 1 and 2 on page 21, for example, eliminate the stretching of the hamstrings allowing the hips and spine to move into flexion more easily. Defenders of postures involving trunk flexion argue that the risk of strain to the lower back is reduced if flexion is initiated with an adequate anterior tilt of the pelvis. However, the degree of hip flexion involved is considered by some therapists to be excessive and likely to result in hip joint instability.
The role of the abdominal muscles in supporting the spine is widely recognized by most physical exercise disciplines and physical therapists. The abdominal muscles protect the lumbar vertebrae from the anterior aspect and can do so more effectively when in contraction. Contraction of the abdominal muscles, together with the respiratory and pelvic diaphragms, creates a taut unit to protect the discs from the anterior aspect (fig 5, page 41). Abdominal contraction is central to Pilates practice (Robinson, Fisher, Knox et al 2000) and is combined with pelvic floor muscle contraction in yoga practices (Iyengar 1966; Coulter 2001) and postures (Bender Birch 1995).
Biomechanics: Lumbar spine extension involves posterior saggital rotation and a small degree of posterior translation of the vertebral bodies whereby the inferior articular facet glides downwards on the superior articular facet. The average range of spinal extension at the lumbar is 30 degrees (Kapandji 1974) with most movement occurring at L4 and S1 (Yamamoto et al 1989). Resistance to extension is initially by muscular‑intra‑abdominal pressure generated by the respiratory and pelvic diaphragms and the abdominal muscles. A function of these muscles is to maintain spinal stability. The degree to which they may be stretched without compromising this role is not documented
Rationale for spinal extension: Lumbar spine extension exercises advocated by McKenzie (1981) (see figs 8 to 11 in part 2 of this series) obtained the most support from trials investigating the effects of stretching exercises on the relief of chronic mechanical back pain (Elnagger et al 1991; Waddell 1998) although, it was conceded, more research was needed to clarify their efficacy (Faas 1996: Koes et al 1991). McKenzie (1981) argues that modern lifestyle is conducive to poor posture leading to mechanical deformations whereby the spine is devoid of the natural lumbar lordosis. A flattened lordosis reduces the ability of the spine to absorb longitudinal shock. A ‘normal’ lordosis renders less pressure in the disc than a straight or kyphotic lumbar. The lumbar lordosis curvature when standing in the anatomical position is 49 to 61 degrees and in unsupported sitting is 22 to 34 degrees A ‘neutral’ lordosis imposes minimal compression on facet joints but this increases substantially with small degrees of swayback (Adams et al 2002). Nachemson 1975 (cited in Nordin and Frankel, 2001) demonstrated reduced pressure on the disc as the spine is moved into lordosis.
McKenzie (1981) reported that patients with a reduced lordosis suffering low back pain were relieved of pain when the lordosis was restored. Pain often resumed after prolonged bending or sitting whereby the lumbar spine tissue became deranged. Lumbar spine flexion tended to produce derangement by inducing posterior migration of the nucleus pulpous within the disc. and overstretching ligaments and other soft tissue around the spine. Derangement produced low back pain that lumbar spine extension exercises could alleviate. They did so by re-establishing the lumbar lordosis that reduced stress on the posterior disc and ligaments and moved the nucleus anteriorly to a central position within the disc (McKenzie 1981).
Risks in spinal extension: fig 17
Damage to the facet joints is the most common injury caused by forced hyperextension of the lumbar (Hartley 1995; Bogduk & Twomey 1987). Spinal extension pushes the nucleus of the disc anteriorly stretching and possibly weakening the anterior annular fibres. The discs resist extension more effectively than they resist flexion but limitation is primarily by impact of either the spinal processes or the bony impaction of the inferior facets onto the lamina of the vertebra below (Oliver and Middleditch 1991). Impact of the spinal process is accentuated and compression increased when the back extensor muscles actively draw the inferior articular process downwards (Bogduk 1997). This is likely to occur in postures 10, 11 and 12 on page 26 where the back extensors are particularly active.
Williams (1937a; 1937b; 1955 in Elnagger 1991) and Macnab (1977) argue against lumbar spine extension exercises because of the potential for compressing posterior annular fibres of the disc and compressing possibly arthritic facet joints. Facet joints become swollen when strained and can put pressure against nearby nerves causing pain. The compressive forces on the facet joint are worse where there is loss of intervertebral disc height perpetuating a vicious circle as the facet joints lose their ability to protect the intervertebral discs from shear forces from rotation and extension (Hartley 1995). Once the facets are impacted, further force will cause the opposite inferior facet to swing backwards causing its joint capsule to become tense and, with enough force, to rupture (Yang and King 1984 cited in Oliver and Middleditch 1991). Hyperextension can also damage the ligamentum flavum and anterior longitudinal ligament and buckle the interspinous ligament if trapped between impacting spinous processes. The spinous process and vertebral arches in turn can fracture under compression and spondilolisthesis can result (Cailliet and Gross 1987). Spinal extension exercises can make a swayback condition worse, particularly if performed with weak abdominal muscles (Flint 1964 in Alter p221).
Reducing the risks in spinal extension: Modifications are necessary to reduce the degree of posterior concavity in the lumbar with its associated compression of the posterior spinal components. The Cobra posture (see fig 9 in part 2) is suitable for therapeutic treatment (McKenzie 1981) but is considered an extreme stretch for fitness or yoga classes where the ‘elbow dog’ or ‘sphinx’ posture (fig 8 in part 2) could be used as a safer alternative (Smith 1994). Placing props beside the ankles to put the hands on can modify the ‘Camel’ posture (fig 18).
Putting the feet on a step can modify the back arch in the ‘Bridge’ posture (fig 19). Posterior pelvic rotation will help minimise compression on the posterior spine in hyperextension.
A supported back extension can be obtained by lying over an arched structure (fig 20)
Biomechanics: Lateral flexion is usually combined with flexion and rotation and is rarely performed as an isolated movement (Adams 2002). Axial rotation in the lumbar is always accompanied by some lateral flexion and vice versa. This occurs contra-laterally at L1 to L4 and ipso-laterally at L5 – S1. Lateral flexion, in turn, is also always accompanied by some extension (Bogduk and Twomey, 1987). The available range of lateral flexion is about fifteen degrees to each side. Most of the available range of movement is at L2-L3 and the least is at L1-L2 (Yamamoto et al 1989). In lateral flexion, mostly the contra-lateral transverse ligament, assisted by ligament flavum and the capsular ligaments, takes the strain. The lumbosacral fascia and the iliolumbar and iliofemorals ligaments, latissimus dorsi, quadratus lumborum, the deep spinal muscles and the lateral fibres of the external and internal oblique muscles also limit lateral flexion.
In lateral flexion, the body of the upper vertebra tilts and the nucleus of the disc migrates towards the opposite side (Kapandji 1974). The side-to-side diameter of the lumbar disc is about 50% more than its anterior posterior diameter, so a movement in the frontal plane will induce 50% more deformation of the disc than the same degree of movement on the saggital plane. Stresses in the disc are therefore more sensitive to lateral flexion than they are to flexion or extension. On the side of lateral flexion, there is a downward movement of the inferior facet joint onto the superior facet of the vertebra (Twomey and Taylor 2000). Lateral flexion can therefore generate high compressive stresses in the ipsilateral facet joint and high tensile stresses in the counter-lateral facet joint. Spinal lateral flexion combined with forward flexion produces excessive force on the facet joints and the ligaments with the iliolumbar
ligament particularly at risk. Exaggerating the lordosis curve in lateral flexion will increase shearing forces through the discs and the facet joints (Hartley 1995).
In axial rotation, the articular facets restrict rotation at the upper lumbar joints to about one degree per vertebrae. Movement is limited by impact of one of the inferior facet joints on the upper vertebra with the opposing superior facet on the vertebra below. Further rotation once the facet joints have impacted will occur around the axis of the impacted joint. This entails a lateral and posterior shift of the vertebral body causing a lateral shearing force on the disc in addition to the torsion forces on the disc caused by rotation. The intervertebral discs resist torsion whereby the annular fibres orientated in the direction of rotation are strained, although those orientated in the opposite direction are relaxed.
Risks of lateral flexion and rotation exercises: On axial rotation to the right, the spinous process will swing to the left inducing tension in the supraspinous and interspinous ligaments. Rotational forces can injure the lumbro-sacral joint, the sacroiliac joint and the capsular ligaments, but the greatest risk is injury to the discs through resulting compression (Bogduk and Twomey, 1987). Excessive rotation at the lumbar can split the annulus fibrosis of the disc with most risk to the posterior aspect. The collagen fibres in the disc permit about three degrees of movement beyond which micro-trauma is likely to occur (Bogduk and Twomey 1987).
fig 20 fig 21
Reducing the risks in lateral flexion and rotation exercises: Relatively pure and safe spinal lateral flexion and rotation are respectively exhibited in figs 20 and 21. Spinal lateral flexion is extreme in fig 22(below left) but may be modified by not advancing so far into the stretch.
Similar spinal movement often occurs in postures 24 and 25 (below left and right respectively) although the intention is to confine the movements to hip flexion and spinal rotation. These postures may therefore pose a risk unless they can be performed without significant lateral and forward flexion of the spine.
The posture may be modified with the use of props as illustrated in figs 25 (right) and 26 (below).
Biomechanics: Traction is a mobilisation method used in manual therapy (Maitland et al 2001) whereby a distraction force is employed to stretch tissues and separate joint surfaces including vertebral bodies or facet joints (Brukner and Khan 2000).
Rationale for using traction: Distraction occurs naturally in joints (Maitland et al 2001) and its therapeutic use may facilitate normal gliding of facet joints, reduce pain due to muscle spasm, stretch fibrotic tissue and break down tissue adhesions (Alter 1996). It has been employed to relieve nerve root compression caused by the protrusion of herniated disc material or narrowing of the intervertebral foramen (Brukner and Khan 2000). It may also restore blood and lymph circulation, dissipate edema and straighten spinal curves (Alter 1996). It can be used on the lumbar spine where symptoms have appeared without trauma over a period of days. It may be used to treat backache including that associated with bony changes induced by degeneration or postural deformities (Maitland et al 2001). Spinal traction occurs in posture 29 (below) when external force is applied by a rope around the anterior pelvic rim (not shown) whilst mild traction may occur without external force. Stronger traction ensues with the trunk suspended in inversion..
Risks of using traction: Traction may aggravate symptoms caused by spinal segmental instability (Brukner and Khan 2000). Contraindications specific to traction include local tumor or infection (Brukner and Khan 2000), spinal cord compression, osteoporosis, rheumatoid arthritis, acute inflammation and acute trauma (Hinterbuchner 1980; Jaskoviak and Scafer 1986; Kisner and Colby 1990: all
cited in Alter 1996, p193.).
Risk reduction: Monitor pain response and reduce force accordingly (Maitland et al 2001).
In the final analysis, those administering stretching techniques need to know how stretching exercises can be performed safely and to good effect. Observing predetermined ranges of joint movement considered safe is likely to be impractical and of limited value as ranges of movement differ with age, lifestyle and the frequency of their employment. Optimal levels of flexibility are not clinically defined and functionality is likely to be the best criterion. More extreme flexibility required for specialist pursuits should be developed in conjunction with adequate strength in order to protect joints. There is a lack of published evidence of injury resulting from performing postures considered ‘controversial’ but this should not encourage complacency. A suitably cautious approach would include lengthening muscles without trying to loosen bony stops or cartilaginous restrains, joint capsules, tendons or ligaments. Injury is likely to be avoided if stretches are performed from a basis of adequate flexibility, strength, correct technique, intelligent sequencing and appropriate use of props (Alter 1996, Coulter 2001). Recent work highlighting the spinal stabilising role of multifidus, transversus abdominis, the pelvic floor muscles and the respiratory diaphragm suggest attention to the function and development of these muscles could be significant in the rehabilitation of mechanical chronic back pain. The safety and therapeutic value of stretching techniques that affect the low back would be enhanced if strengthening techniques for the spinal stabilising muscles could be incorporated into their protocol.