Interspinales

The lumbar interspinales are short paired muscles that lie on either side of the interspinous ligament and connect the spinous processes of adjacent lumbar vertebrae (see Fig. 9.2). There are four pairs in the lumbar region.

Although disposed to produce posterior sagittal rotation of the vertebra above, the interspinales are quite small and would not contribute appreciably to the force required to move a vertebra. This paradox is similar to that which applies for the intertransversarii mediales and is discussed further in that context.

Intertransversarii mediales

The intertransversarii mediales can be considered to be true back muscles for, unlike the intertransversarii laterales, they are innervated by the lumbar dorsal rami.^3,10 The intertransversarii mediales arise from an accessory process, the adjoining mamillary process and the mamillo-accessory ligament that connects these two processes.¹¹ They insert into the superior aspect of the mamillary process of the vertebra below (see Fig. 9.2).

The intertransversarii mediales lie lateral to the axis of lateral flexion and behind the axis of sagittal rotation. However, they lie very close to these axes and are very small muscles. Therefore, it is questionable whether they could contribute any appreciable force in either lateral flexion or posterior sagittal rotation. It might perhaps be argued that larger muscles provide the bulk of the power to move the vertebrae and the intertransversarii act to ‘fine tune’ the movement. However, this suggestion is highly speculative, if not fanciful, and does not account for their small size and considerable mechanical disadvantage.

A tantalising alternative suggestion is that the intertransversarii (and perhaps also the interspinales) act as large proprioceptive transducers; their value lies not in the force they can exert but in the muscle spindles they contain. Placed close to the lumbar vertebral column, the intertransversarii could monitor the movements of the column and provide feedback that influences the action of the surrounding muscles. Such a role has been suggested for the cervical intertransversarii, which have been found to contain a high density of muscle spindles.^12–14 Indeed, all unisegmental muscles of the vertebral column have between two and six times the density of muscles spindles found in the longer polysegmental muscles, and there is growing speculation that this underscores the proprioceptive function of all short, small muscles of the body.^15–17

Multifidus

The multifidus is the largest and most medial of the lumbar back muscles. It consists of a repeating series of fascicles which stem from the laminae and spinous processes of the lumbar vertebrae and exhibit a constant pattern of attachments caudally.⁹

The shortest fascicles of the multifidus are the ‘laminar fibres’, which arise from the caudal end of the dorsal surface of each vertebral lamina and insert into the mamillary process of the vertebra two levels caudad (Fig. 9.4A). The L5 laminar fibres have no mamillary process into which they can insert, and insert instead into an area on the sacrum just above the first dorsal sacral foramen. Because of their attachments, the laminar fibres may be considered homologous to the thoracic rotatores.

Figure 9.4 The component fascicles of multifidus. (A) The laminar fibres of multifidus. (B–F) The fascicles from the L1–L5 spinous processes, respectively.

The bulk of the lumbar multifidus consists of much larger fascicles that radiate from the lumbar spinous processes. These fascicles are arranged in five overlapping groups such that each lumbar vertebra gives rise to one of these groups. At each segmental level, a fascicle arises from the base and caudolateral edge of the spinous process, and several fascicles arise, by way of a common tendon, from the caudal tip of the spinous process. This tendon is referred to hereafter as ‘the common tendon’. Although confluent with one another at their origin, the fascicles in each group diverge caudally to assume separate attachments to mamillary processes, the iliac crest and the sacrum.

The fascicle from the base of the L1 spinous process inserts into the L4 mamillary process, while those from the common tendon insert into the mamillary processes of L5, S1 and the posterior superior iliac spine (Fig. 9.4B).

The fascicle from the base of the spinous process of L2 inserts into the mamillary process of L5, while those from the common tendon insert into the S1 mamillary process, the posterior superior iliac spine, and an area on the iliac crest just caudoventral to the posterior superior iliac spine (Fig. 9.4C).

The fascicle from the base of the L3 spinous process inserts into the mamillary process of the sacrum, while those fascicles from the common tendon insert into a narrow area extending caudally from the caudal extent of the posterior superior iliac spine to the lateral edge of the third sacral segment (Fig. 9.4D). The L4 fascicles insert onto the sacrum in an area medial to the L3 area of insertion, but lateral to the dorsal sacral foramina (Fig. 9.4E), while those from the L5 vertebra insert onto an area medial to the dorsal sacral foramina (Fig. 9.4F).

It is noteworthy that while many of the fascicles of multifidus attach to mamillary processes, some of the deeper fibres of these fascicles attach to the capsules of the zygapophysial joints next to the mamillary processes (see Ch. 3).¹⁸ This attachment allows the multifidus to protect the joint capsule from being caught inside the joint during the movements executed by the multifidus.

The key feature of the morphology of the lumbar multifidus is that its fascicles are arranged segmentally. Each lumbar vertebra is endowed with a group of fascicles that radiate from its spinous process, anchoring it below to mamillary processes, the iliac crest and the sacrum. This disposition suggests that the fibres of multifidus are arranged in such a way that their principal action is focused on individual lumbar spinous processes.⁹ They are designed to act in concert on a single spinous process. This contention is supported by the pattern of innervation of the muscle. All the fascicles arising from the spinous processes of a given vertebra are innervated by the medial branch of the dorsal ramus that issues from below that vertebra (see Ch. 10).^9,10 Thus, the muscles that directly act on a particular vertebral segment are innervated by the nerve of that segment.

In a posterior view, the fascicles of multifidus are seen to have an oblique caudolateral orientation. Their line of action can therefore be resolved into two vectors: a large vertical vector and a considerably smaller horizontal vector (Fig. 9.5A).⁷

Figure 9.5 The force vectors of multifidus. (A) In a posteroanterior view, the oblique line of action of the multifidus at each level (bold arrows) can be resolved into a major vertical vector (V) and a smaller horizontal vector (H). (B) In a lateral view, the vertical vectors of the multifidus are seen to be aligned at right angles to the spinous processes.

The small horizontal vector suggests that the multifidus could pull the spinous processes sideways, and therefore produce horizontal rotation. However, horizontal rotation of lumbar vertebrae is impeded by the impaction of the contralateral zygapophysial joints. Horizontal rotation occurs after impaction of the joints only if an appropriate shear force is applied to the intervertebral discs (see Ch. 7) but the horizontal vector of multifidus is so small that it is unlikely that the multifidus would be capable of exerting such a shear force on the disc by acting on the spinous process. Indeed, electromyographic studies reveal that the multifidus is inconsistently active in derotation and that, paradoxically, it is active in both ipsilateral and contralateral rotation.¹⁹ Rotation, therefore, cannot be inferred to be a primary action of the multifidus. In this context, the multifidus has been said to act only as a ‘stabiliser’ in rotation,^18,19 but the aberrant movements, which it is supposed to stabilise, have not been defined (although see below).

The principal action of the multifidus is expressed by its vertical vector, and further insight is gained when this vector is viewed in a lateral projection (Fig. 9.5B). Each fascicle of multifidus, at every level, acts virtually at right angles to its spinous process of origin.⁷ Thus, using the spinous process as a lever, every fascicle is ideally disposed to produce posterior sagittal rotation of its vertebra. The right-angle orientation, however, precludes any action as a posterior horizontal translator. Therefore, the multifidus can only exert the ‘rocking’ component of extension of the lumbar spine or control this component during flexion.

Having established that the multifidus is primarily a posterior sagittal rotator of the lumbar spine, it is possible to resolve the paradox about its activity during horizontal rotation of the trunk.⁷ In the first instance, it should be realised that rotation of the lumbar spine is an indirect action. Active rotation of the lumbar spine occurs only if the thorax is first rotated, and is therefore secondary to thoracic rotation. Secondly, it must be realised that a muscle with two vectors of action cannot use these vectors independently. If the muscle contracts, then both vectors are exerted. Thus, the multifidus cannot exert axial rotation without simultaneously exerting a much larger posterior sagittal rotation.

The principal muscles that produce rotation of the thorax are the oblique abdominal muscles. The horizontal component of their orientation is able to turn the thoracic cage in the horizontal plane and thereby impart axial rotation to the lumbar spine. However, the oblique abdominal muscles also have a vertical component to their orientation. Therefore, if they contract to produce rotation they will also simultaneously cause flexion of the trunk, and therefore of the lumbar spine. To counteract this flexion, and maintain pure axial rotation, extensors of the lumbar spine must be recruited, and this is how the multifidus becomes involved in rotation.

The role of the multifidus in rotation is not to produce rotation but to oppose the flexion effect of the abdominal muscles as they produce rotation. The aberrant motion ‘stabilised’ by the multifidus during rotation is, therefore, the unwanted flexion unavoidably produced by the abdominal muscles.⁷

Apart from its action on individual lumbar vertebrae, the multifidus, because of its polysegmental nature, can also exert indirect effects on any interposed vertebrae. Since the line of action of any long fascicle of multifidus lies behind the lordotic curve of the lumbar spine, such fascicles can act like bowstrings on those segments of the curve that intervene between the attachments of the fascicle. The bowstring effect would tend to accentuate the lumbar lordosis, resulting in compression of intervertebral discs posteriorly, and strain of the discs and longitudinal ligament anteriorly. Thus, a secondary effect of the action of the multifidus is to increase the lumbar lordosis and the compressive and tensile loads on any vertebrae and intervertebral discs interposed between its attachments.

Lumbar erector spinae

The lumbar erector spinae lies lateral to the multifidus and forms the prominent dorsolateral contour of the back muscles in the lumbar region. It consists of two muscles: the longissimus thoracis and the iliocostalis lumborum. Furthermore, each of these muscles has two components: a lumbar part, consisting of fascicles arising from lumbar vertebrae, and a thoracic part, consisting of fascicles arising from thoracic vertebrae or ribs.^6,8 These four parts may be referred to, respectively, as longissimus thoracis pars lumborum, iliocostalis lumborum pars lumborum, longissimus thoracis pars thoracis and iliocostalis lumborum pars thoracis.⁸

In the lumbar region, the longissimus and iliocostalis are separated from each other by the lumbar intermuscular aponeurosis, an anteroposterior continuation of the erector spinae aponeurosis.^6,8 It appears as a flat sheet of collagen fibres, which extend rostrally from the medial aspect of the posterior superior iliac spine for 6–8 cm. It is formed mainly by the caudal tendons of the rostral four fascicles of the lumbar component of longissimus (Fig. 9.6).

Figure 9.6 The lumbar fibres of longissimus (longissimus thoracis pars lumborum). On the left, the five fascicles of the intact muscle are drawn. The formation of the lumbar intermuscular aponeurosis (LIA) by the lumbar fascicles of longissimus is depicted. On the right, the lines indicate the attachments and span of the fascicles.

Longissimus thoracis pars lumborum

The longissimus thoracis pars lumborum is composed of five fascicles, each arising from the accessory process and the adjacent medial end of the dorsal surface of the transverse process of a lumbar vertebra (see Fig. 9.6).

The fascicle from the L5 vertebra is the deepest and shortest. Its fibres insert directly into the medial aspect of the posterior superior iliac spine. The fascicle from L4 also lies deeply, but lateral to that from L5. Succeeding fascicles lie progressively more dorsally, so that the L3 fascicle covers those from L4 and L5 but is itself covered by the L2 fascicle, while the L1 fascicle lies most superficially.

The L1–L4 fascicles all form tendons at their caudal ends. These converge to form the lumbar intermuscular aponeurosis, which eventually attaches to a narrow area on the ilium immediately lateral to the insertion of the L5 fascicle. The lumbar intermuscular aponeurosis thus represents a common tendon of insertion, or the aponeurosis, of the bulk of the lumbar fibres of longissimus.

Each fascicle of the lumbar longissimus has both a dorsoventral and a rostrocaudal orientation.⁸ Therefore, the action of each fascicle can be resolved into a vertical vector and a horizontal vector, the relative sizes of which differ from L1 to L5 (Fig. 9.7A). Consequently, the relative actions of the longissimus differ at each segmental level. Furthermore, the action of the longissimus, as a whole, will differ according to whether the muscle contracts unilaterally or bilaterally.

Figure 9.7 The force vectors of the longissimus thoracis pars lumborum. (A) In a lateral view, the oblique line of action of each fascicle of longissimus can be resolved into a vertical (V) and a horizontal (H) vector. The horizontal vectors of lower lumbar fascicles are larger. (B) In a posteroanterior view, the line of action of the fascicles can be resolved into a major vertical vector and a much smaller horizontal vector.

The large vertical vector of each fascicle lies lateral to the axis of lateral flexion and behind the axis of sagittal rotation of each vertebra. Thus, contracting the longissimus unilaterally can laterally flex the vertebral column, but acting bilaterally the various fascicles can act, like the multifidus, to produce posterior sagittal rotation of their vertebra of origin. However, their attachments to the accessory and transverse processes lie close to the axes of sagittal rotation, and therefore their capacity to produce posterior sagittal rotation is less efficient than that of the multifidus, which acts through the long levers of the spinous processes.⁸

The horizontal vectors of the longissimus are directed backwards. Therefore, when contracting bilaterally the longissimus is capable of drawing the lumbar vertebrae backwards. This action of posterior translation can restore the anterior translation of the lumbar vertebrae that occurs during flexion of the lumbar column (see Ch. 7). The capacity for posterior translation is greatest at lower lumbar levels, where the fascicles of longissimus assume a greater dorsoventral orientation (Fig. 9.7B).

Reviewing the horizontal and vertical actions of longissimus together, it can be seen that longissimus expresses a continuum of combined actions along the length of the lumbar vertebral column. From below upwards, its capacity as a posterior sagittal rotator increases, while, conversely, from above downwards, the fascicles are better designed to resist or restore anterior translation. It is emphasised that the longissimus cannot exert its horizontal and vertical vectors independently. Thus, whatever horizontal translation it exerts must occur simultaneously with posterior sagittal rotation. The resolution into vectors simply reveals the relative amounts of simultaneous translation and sagittal rotation exerted at different segmental levels.

It might be deduced that because of the horizontal vector of longissimus, this muscle acting unilaterally could draw the accessory and transverse processes backwards and therefore produce axial rotation. However, in this regard, the fascicles of longissimus are orientated almost directly towards the axis of axial rotation and so are at a marked mechanical disadvantage to produce axial rotation.

Iliocostalis lumborum pars lumborum

The lumbar component of the iliocostalis lumborum consists of four overlying fascicles arising from the L1 through to the L4 vertebrae. Rostrally, each fascicle attaches to the tip of the transverse process and to an area extending 2–3 cm laterally onto the middle layer of the thoracolumbar fascia (Fig. 9.8).

Figure 9.8 The lumbar fibres of iliocostalis (iliocostalis lumborum pars lumborum). On the left, the four lumbar fascicles of iliocostalis are shown. On the right, their span and attachments are indicated by the lines.

The fascicle from L4 is the deepest, and caudally it is attached directly to the iliac crest just lateral to the posterior superior iliac spine. This fascicle is covered by the fascicle from L3 that has a similar but more dorsolaterally located attachment on the iliac crest. In sequence, L2 covers L3 and L1 covers L2, with insertions on the iliac crest becoming successively more dorsal and lateral. The most lateral fascicles attach to the iliac crest just medial to the attachment of the ‘lateral raphe’ of the thoracolumbar fascia (see below). The most medial fibres of iliocostalis contribute to the lumbar intermuscular aponeurosis but only to a minor extent.

Although an L5 fascicle of iliocostalis lumborum is not described in the literature, it is represented in the iliolumbar ‘ligament’. In neonates and children this ‘ligament’ is said to be completely muscular in structure (see Ch. 4).²⁰ By the third decade of life, the muscle fibres are entirely replaced by collagen, giving rise to the familiar iliolumbar ligament.²⁰ On the basis of sites of attachment and relative orientation, the posterior band of the iliolumbar ligament would appear to be derived from the L5 fascicle of iliocostalis, while the anterior band of the ligament is a derivative of the quadratus lumborum.

The disposition of the lumbar fascicles of iliocostalis is similar to that of the lumbar longissimus, except that the fascicles are situated more laterally. Like that of the lumbar longissimus, their action can be resolved into horizontal and vertical vectors (Fig. 9.9A).

Figure 9.9 The force vectors of the iliocostalis lumborum pars lumborum. (A) In a lateral view, the line of action of the fascicles can be resolved into vertical (V) and horizontal (H) vectors. The horizontal vectors are larger at lower lumbar levels. (B) In a posteroanterior view, the line of action is resolved into a vertical vector and a very small horizontal vector.

The vertical vector is still predominant, and therefore the lumbar fascicles of iliocostalis contracting bilaterally can act as posterior sagittal rotators (Fig. 9.9B), but because of the horizontal vector a posterior translation will be exerted simultaneously, principally at lower lumbar levels where the fascicles of iliocostalis have a greater forward orientation. Contracting unilaterally, the lumbar fascicles of iliocostalis can act as lateral flexors of the lumbar vertebrae, for which action the transverse processes provide very substantial levers.

Contracting unilaterally, the fibres of iliocostalis are better suited to exert axial rotation than the fascicles of lumbar longissimus, for their attachment to the tips of the transverse processes displaces them from the axis of horizontal rotation and provides them with substantial levers for this action. Because of this leverage, the lower fascicles of iliocostalis are the only intrinsic muscles of the lumbar spine reasonably disposed to produce horizontal rotation. Their effectiveness as rotators, however, is dwarfed by the oblique abdominal muscles that act on the ribs and produce lumbar rotation indirectly by rotating the thoracic cage. However, because the iliocostalis cannot exert axial rotation without simultaneously exerting posterior sagittal rotation, the muscle is well suited to cooperate with the multifidus to oppose the flexion effect of the abdominal muscles when they act to rotate the trunk.

Longissimus thoracis pars thoracis

The thoracic fibres of longissimus thoracis typically consist of 11 or 12 pairs of small fascicles arising from the ribs and transverse processes of T1 or T2 down to T12 (Fig. 9.10). At each level, two tendons can usually be recognised, a medial one from the tip of the transverse process and a lateral one from the rib, although in the upper three or four levels, the latter may merge medially with the fascicle from the transverse process. Each rostral tendon extends 3–4 cm before forming a small muscle belly measuring 7–8 cm in length. The muscle bellies from the higher levels overlap those from lower levels. Each muscle belly eventually forms a caudal tendon that extends into the lumbar region. The tendons run in parallel, with those from higher levels being most medial. The fascicles from the T2 level attach to the L3 spinous process, while the fascicles from the remaining levels insert into spinous processes at progressively lower levels. For example, those from T5 attach to L5, and those from T7 to S2 or S3. Those from T8 to T12 diverge from the midline to find attachment to the sacrum along a line extending from the S3 spinous process to the caudal extent of the posterior superior iliac spine.⁸ The lateral edge of the caudal tendon of T12 lies alongside the dorsal edge of the lumbar intermuscular aponeurosis formed by the caudal tendon of the L1 longissimus bundle.

Figure 9.10 The thoracic fibres of longissimus (longissimus thoracis pars thoracis). The intact fascicles are shown on the left. The darkened areas represent the short muscle bellies of each fascicle. Note the short rostral tendons of each fascicle and the long caudal tendons, which collectively constitute most of the erector spinae aponeurosis (ESA). The span of the individual fascicles is indicated on the right.

The side-to-side aggregation of the caudal tendons of longissimus thoracis pars thoracis forms much of what is termed the erector spinae aponeurosis, which covers the lumbar fibres of longissimus and iliocostalis but affords no attachment to them.

The longissimus thoracis pars thoracis is designed to act on thoracic vertebrae and ribs. Nonetheless, when contracting bilaterally it acts indirectly on the lumbar vertebral column and uses the erector spinae aponeurosis to produce an increase in the lumbar lordosis. However, not all of the fascicles of longissimus thoracis span the entire lumbar vertebral column. Those from the second rib and T2 reach only as far as L3, and only those fascicles arising between the T6 or T7 and the T12 levels actually span the entire lumbar region. Consequently, only a portion of the whole thoracic longissimus acts on all the lumbar vertebrae.

The oblique orientation of the longissimus thoracis pars thoracis also permits it to flex the thoracic vertebral column laterally and thereby to indirectly flex the lumbar vertebral column laterally.

Iliocostalis lumborum pars thoracis

The iliocostalis lumborum pars thoracis consists of fascicles from the lower seven or eight ribs that attach caudally to the ilium and sacrum (Fig. 9.11). These fascicles represent the thoracic component of iliocostalis lumborum and should not be confused with the iliocostalis thoracis, which is restricted to the thoracic region between the upper six and lower six ribs.

Figure 9.11 The thoracic fibres of iliocostalis lumborum (iliocostalis lumborum pars thoracis). The intact fascicles are shown on the left, and their span is shown on the right. The caudal tendons of the fascicles collectively form the lateral parts of the erector spinae aponeurosis (ESA).

Each fascicle of the iliocostalis lumborum pars thoracis arises from the angle of the rib via a ribbon-like tendon 9–10 cm long. It then forms a muscle belly 8–10 cm long. Thereafter, each fascicle continues as a tendon, contributing to the erector spinae aponeurosis and ultimately attaching to the posterior superior iliac spine. The most medial tendons, from the more rostral fascicles, often attach more medially to the dorsal surface of the sacrum, caudal to the insertion of multifidus.

The thoracic fascicles of iliocostalis lumborum have no attachment to lumbar vertebrae. They attach to the iliac crest and thereby span the lumbar region. Consequently, by acting bilaterally, it is possible for them to exert an indirect ‘bowstring’ effect on the vertebral column, causing an increase in the lordosis of the lumbar spine. Acting unilaterally, the iliocostalis lumborum pars thoracis can use the leverage afforded by the ribs to laterally flex the thoracic cage and thereby laterally flex the lumbar vertebral column indirectly. The distance between the ribs and the ilium does not shorten greatly during rotation of the trunk, and therefore the iliocostalis lumborum pars thoracis can have little action as an axial rotator. However, contralateral rotation greatly increases this distance, and the iliocostalis lumborum pars thoracis can serve to de-rotate the thoracic cage and, therefore, the lumbar spine.

Minor active movements

In the upright position, the lumbar back muscles play a minor, or no active, role in executing movement, for gravity provides the necessary force. During extension, the back muscles contribute to the initial tilt, drawing the line of gravity backwards^25,26 but are unnecessary for further extension. Muscle activity is recruited when the movement is forced or resisted²⁷ but is restricted to muscles acting on the thorax. The lumbar multifidus, for example, shows little or no involvement.²⁸

The lateral flexors can bend the lumbar spine sideways, but once the centre of gravity of the trunk is displaced lateral flexion can continue under the influence of gravity. However, the ipsilateral lateral flexors are used to direct the movement, and the contralateral muscles are required to balance the action of gravity and control the rate and extent of movement. Consequently, lateral flexion is accompanied by bilateral activity of the lumbar back muscles, but the contralateral muscles are relatively more active as they are the ones that must balance the load of the laterally flexing spine.^{25,26,29–32} If a weight is held in the hand on the side to which the spine is laterally flexed, a greater load is applied to the spine, and the contralateral back muscles show greater activity to balance this load.^29,31

Maintenance of posture

The upright vertebral column is well stabilised by its joints and ligaments but it is still liable to displacement by gravity or when subject to asymmetrical weight-bearing. The back muscles serve to correct such displacements and, depending on the direction of any displacement, the appropriate back muscles will be recruited.

During standing at ease, the back muscles may show slight continuous activity,^{19,25–27,30,32–42} intermittent activity^{25,27,32,42,43} or no activity,^36,39–42 and the amount of activity can be influenced by changing the position of the head or allowing the trunk to sway.²⁵

The explanation for these differences probably lies in the location of the line of gravity in relation to the lumbar spine in different individuals.^{27,36,41,42,44} In about 75% of individuals the line of gravity passes in front of the centre of the L4 vertebra, and therefore essentially in front of the lumbar spine.^36,41 Consequently, gravity will exert a constant tendency to pull the thorax and lumbar spine into flexion. To preserve an upright posture, a constant level of activity in the posterior sagittal rotators of the lumbar spine will be needed to oppose the tendency to flexion. Conversely, when the line of gravity passes behind the lumbar spine, gravity tends to extend it, and back muscle activity is not required. Instead, abdominal muscle activity is recruited to prevent the spine extending under gravity.^36,41

Activities that displace the centre of gravity of the trunk sideways will tend to cause lateral flexion. To prevent undesired lateral flexion, the contralateral lateral flexors will contract. This occurs when weights are carried in one hand.^25,39 Carrying equal weights in both hands does not displace the line of gravity, and back muscle activity is not increased substantially on either side of the body.^25,39

During sitting, the activity of the back muscles is similar to that during standing^34,35,45,46 but in supported sitting, as with the elbows resting on the knees, there is no activity in the lumbar back muscles,^25,32 and with arms resting on a desk, back muscle activity is substantially decreased.^34,35,45 In reclined sitting, the back rest supports the weight of the thorax, lessening the need for muscular support. Consequently, increasing the declination of the back rest of a seat decreases lumbar back muscle activity.^{34,35,45,47,48}

Major active movements

Forward flexion and extension of the spine from the flexed position are movements during which the back muscles have their most important function. As the spine bends forwards, there is an increase in the activity of the back muscles;^{19,25,26,28–30,32,33,43,49–52} this increase is proportional to the angle of flexion and the size of any load carried.^29,31,53,54 The movement of forward flexion is produced by gravity, but the extent and the rate at which it proceeds is controlled by the eccentric contraction of the back muscles. Movement of the thorax on the lumbar spine is controlled by the long thoracic fibres of longissimus and iliocostalis. The long tendons of insertion allow these muscles to act around the convexity of the increasing thoracic kyphosis and anchor the thorax to the ilium and sacrum. In the lumbar region, the multifidus and the lumbar fascicles of longissimus and iliocostalis act to control the anterior sagittal rotation of the lumbar vertebrae. At the same time the lumbar fascicles of longissimus and iliocostalis also act to control the associated anterior translation of the lumbar vertebrae.

At a certain point during forward flexion, the activity in the back muscles ceases, and the vertebral column is braced by the locking of the zygapophysial joints and tension in its posterior ligaments (see Ch. 7). This phenomenon is known as ‘critical point’.^26,43,44,55 However, critical point does not occur in all individuals or in all muscles.^19,25,32,42 When it does occur, it does so when the spine has reached about 90% maximum flexion, even though at this stage the hip flexion that occurs in forward bending is still only 60% complete.^44,55 Carrying weights during flexion causes the critical point to occur later in the range of vertebral flexion.^44,55

The physiological basis for critical point is still obscure. It may be due to reflex inhibition initiated by proprioceptors in the lumbar joints and ligaments, or in muscle stretch and length receptors.⁵⁵ Whatever the mechanism, the significance of critical point is that it marks the transition of spinal load-bearing from muscles to the ligamentous system.

Extension of the trunk from the flexed position is characterised by high levels of back muscle activity.^{19,25,26,43,52} In the thoracic region, the iliocostalis and longissimus, acting around the thoracic kyphosis, lift the thorax by rotating it backwards. The lumbar vertebrae are rotated backwards principally by the lumbar multifidus, causing their superior surfaces to be progressively tilted upwards to support the rising thorax.

Strength of the back muscles

The strength of the back muscles has been determined in experiments on normal volunteers.⁶⁶ Two measures of strength are available: the absolute maximum force of contraction in the upright posture and the moment generated on the lumbar spine. The absolute maximum strength of the back muscles as a whole is about 4000 N. Acting on the short moment arms provided by the spinous processes and pedicles of the lumbar vertebrae, this force converts to an extensor moment of 200 Nm. These figures apply to average males under the age of 30; young females exhibit about 60% of this strength, while individuals over the age of 30 are about 10–30% weaker.⁶⁶

Easy standing involves some 2–5% of maximum isometric strength; manual handling of heavy loads involves between 75% and 100%; sitting involves between 3% and 15% of maximum activity.⁶⁷

Detailed dissection studies have allowed the strength of contraction to be apportioned to individual components of the back muscles.⁶⁸ Of the total extensor moment, the thoracic fibres of iliocostalis and longissimus account for some 50%. Thus, half of the extensor moment on the lumbar spine is exerted through the erector spinae aponeurosis. The other half is exerted by the muscles that act directly on the lumbar vertebrae, with the multifidus providing half of that 50% and the longissimus thoracis pars lumborum and iliocostalis lumborum pars lumborum providing the remainder. The compression loads exerted by the lumbar back muscles differ from segment to segment because of the different spans and attachments of the various muscles. However, at L5–S1 the thoracic fibres of the lumbar erector spinae exert about 42% of the total compression load, the lumbar fibres of this muscle contribute 36% and the multifidus contributes 22%.⁶⁸ At higher lumbar levels, relatively more of the total compression load on the segment is exerted by the thoracic fibres of the lumbar erector spinae.

With respect to shear forces, in the upright position the various lumbar back muscles exert forces that differ in magnitude and in direction at different levels.⁶⁸ This arises because of the different orientation of particular fascicles of the various muscles and because of the different orientation of particular vertebrae in the lumbar lordosis. As a result, the multifidus exerts mainly anterior shear forces at upper lumbar levels, but either anterior or posterior shear forces at lower levels; the lumbar fibres of erector spinae exert posterior shear forces on the vertebrae to which they are attached, but anterior shear forces on vertebrae below these; the thoracic fibres of lumbar erector spinae exert posterior shear forces on upper lumbar segments, but anterior shear forces on L4 and L5.⁶⁸ The net effect is that the back muscles exert posterior shear forces on upper lumbar segments in the upright spine but, paradoxically, they exert a net anterior shear force on L5.

Intriguingly, flexion of the lumbar spine does not compromise the strength of the back muscles.⁶⁹ The moment arms of some fascicles are reduced by flexion but those of others are increased, resulting in no significant change in the total capacity to generate moments. All fascicles, however, are elongated, but although this reduces their maximum force on active contraction, it increases the passive tension in the muscles, resulting in no reduction in total tension. Consequently, upon flexion, the total extensor moment of the back muscles and the compression load that they exert change little from those in the upright position. However, the shear forces change appreciably. The posterior shear forces on upper lumbar segments are reduced by flexion but the shear force on L5 reverses from an anterior shear force in the upright position to a posterior shear force in full flexion.⁶⁹

With respect to axial rotation, although the back muscles have reasonable moment arms, they are compromised by their longitudinal orientation.⁷⁰ Only their horizontal vectors can exert axial rotation but these are very small components of the action of any of the muscles. As a result, the total maximal possible torque exerted by all the back muscles is next to trivial, and that exerted by any one muscle is negligible.⁷⁰ Consequently, the back muscles afford no stability to the lumbar spine in axial rotation. For that, the lumbar spine is reliant on the abdominal muscles.⁷⁰

Histochemistry

As postural muscles, the back muscles are dominated by slow-twitch fibres. Furthermore, the density of slow-twitch and fast-twitch fibres differs from muscle to muscle.

Slow-twitch fibres constitute some 70% of the fibres of longissimus.⁶⁷ They constitute about 55% of the iliocostalis and multifidus. Conversely, fast-twitch type A fibres constitute 20% of the fibres of multifidus, iliocostalis and longissimus, and fast-twitch type B fibres constitute 25% of the fibres of multifidus and iliocostalis but only 11% of longissimus.⁶⁷

These histochemical profiles seem to correlate with the fatigue resistance and endurance times of the back muscles, which are larger than most human muscles.⁶⁷ However, individuals exhibit a large variance in fatigue resistance.⁶⁷ The possibilities arise that endurance may be a direct function of the density of slow-twitch fibres in the back muscles, that lack of resistance to fatigue is a risk factor for back injury, and that conditioning can change the histochemical profile of an individual to overcome this risk. These possibilities, however, remain to be explored.

Some practitioners believe that muscle weakness, or muscle fatiguability, is the basis for back pain in some, if not many, patients. To them, data on muscle fibre types are attractive for they would seem to tally with the fact that patients with weak and fatiguable back muscles would show changes in fibre type towards type II fibres. This notion, however, has been dispelled.

A study has shown that there is no correlation between pain and either fatigue or fibre type.⁷¹ Patients with back pain my exhibit less strength than asymptomatic individuals but not because of histochemical differences in their muscles.

Lifting

In biomechanical terms, the act of lifting constitutes a problem in balancing moments. When an individual bends forwards to execute a lift, flexion occurs at the hip joint and in the lumbar spine. Indeed, most of the forward movement seen during trunk flexion occurs at the hip joint.⁵⁵ The flexion forces are generated by gravity acting on the mass of the object to be lifted and on the mass of the trunk above the level of the hip joint and lumbar spine (Fig. 9.15). These forces exert flexion moments on both the hip joint and lumbar spine. In each case, the moment will be the product of the force and its perpendicular distance from the joint in question. The total flexion moment acting on each joint will be the sum of the moments exerted by the mass to be lifted and the mass of the trunk. For a lift to be executed, these flexion moments have to be overcome by a moment acting in the opposite direction. This could be exerted by longitudinal forces acting downwards behind the hip joint and vertebral column or by forces acting upwards in front of the joints, pushing the trunk upwards.

Figure 9.15 The flexion moments exerted on a flexed trunk. Forces generated by the weight of the trunk and the load to be lifted act vertically in front of the lumbar spine and hip joint. The moments they exert on each joint are proportional to the distance between the line of action of each force and the joint in question. The mass of the trunk (m₁) exerts a force (W₁) that acts at a measurable distance in front of the lumbar spine (d₁) and the hip joint (d₃). The mass to be lifted (m₂) exerts a force (W₂) that acts at a measurable distance from the lumbar spine (d₂) and the hip joint (d₄). The respective moments acting on the lumbar spine will be W₁d₁ and W₂d₂; those on the hip joint will be W₁d₃ and W₂d₄.

There are no doubts as to the capacity of the hip extensors to generate large moments and overcome the flexion moments exerted on the hip joint, even by the heaviest of loads that might be lifted.^72,73 However, the hip extensors are only able to rotate the pelvis backwards on the femurs; they do not act on the lumbar spine. Thus, regardless of what happens at the hip joint, the lumbar spine still remains subject to a flexion moment that must be overcome in some other way. Without an appropriate mechanism, the lumbar spine would stay flexed as the hips extended; indeed, as the pelvis rotated backwards, flexion of the lumbar spine would be accentuated as its bottom end was pulled backwards with the pelvis while its top end remained stationary under the load of the flexion moment. A mechanism is required to allow the lumbar spine to resist this deformation or to cause it to extend in unison with the hip joint.

Despite much investigation and debate, the exact nature of this mechanism remains unresolved. In various ways, the back muscles, intra-abdominal pressure, the thoracolumbar fascia and the posterior ligamentous system have been believed to participate.

For light lifts, the flexion moments generated are relatively small. In the case of a 70 kg man lifting a 10 kg mass in a fully stooped position, the upper trunk weighs about 40 kg and acts about 30 cm in front of the lumbar spine, while the arms holding the mass to be lifted lie about 45 cm in front of the lumbar spine. The respective flexion moments are, therefore, 40 × 9.8 × 0.30 = 117.6 Nm, and 10 × 9.8 × 0.45 = 44.1 Nm, a total of 161.7 Nm. This load is well within the capacity of the back muscles (200 Nm, see above). Thus, as the hips extend, the lumbar back muscles are capable of resisting further flexion of the lumbar spine, and indeed could even actively extend it, and the weight would be lifted.

Increasing the load to be lifted to over 30 kg increases the flexion moment to 132.2 Nm, which when added to the flexion moment of the upper trunk exceeds the capacity of the back muscles. To remain within the capacity of the back muscles such loads must be carried closer to the lumbar spine, i.e. they must be borne with a much shorter moment arm. Even so, decreasing the moment arm to about 15 cm limits the load to be carried to about 90 kg. The back muscles are simply not strong enough to raise greater loads. Such realisations have generated concepts of several additional mechanisms that serve to aid the back muscles in overcoming large flexion moments.

In 1957, Bartelink⁷⁴ raised the proposition that intra-abdominal pressure could aid the lumbar spine in resisting flexion by acting upwards on the diaphragm: the so-called intra-abdominal balloon mechanism. Bartelink himself was circumspect and reserved in raising this conjecture but the concept was rapidly popularised, particularly among physiotherapists. Even though it was never validated, the concept seemed to be treated as proven fact. It received early endorsement in orthopaedic circles,²⁸ and intra-abdominal pressure was adopted by ergonomists and others as a measure of spinal stress and safe-lifting standards.^75–82 In more contemporary studies, intra-abdominal pressure has been monitored during various spinal movements and lifting tasks.^29,64,83

Reservations about the validity of the abdominal balloon mechanism have arisen from several quarters. Studies of lifting tasks reveal that, unlike myoelectric activity, intra-abdominal pressure does not correlate well with the size of the load being lifted or the applied stress on the vertebral column as measured by intradiscal pressure.^56,57,84 Indeed, deliberately increasing intra-abdominal pressure by a Valsalva manoeuvre does not relieve the load on the lumbar spine but actually increases it.⁸⁵ Clinical studies have shown that although abdominal muscles are weaker than normal in patients with back pain, intra-abdominal pressure is not different.⁸⁶ Furthermore, strengthening the abdominal muscles both in normal individuals⁸⁷ and in patients with back pain⁸⁸ does not influence intra-abdominal pressure during lifting.

The most strident criticism of the intra-abdominal balloon theory comes from bioengineers and others who maintain that:

These reservations inspired an alternative explanation of the role of the abdominal muscles during lifting. Farfan, Gracovetsky and colleagues^23,72,91,93 noted the criss-cross arrangement of the fibres in the posterior layer of thoracolumbar fascia and surmised that, if lateral tension was applied to this fascia, it would result in an extension moment being exerted on the lumbar spinous processes. Such tension could be exerted by the abdominal muscles that arise from the thoracolumbar fascia, and the trigonometry of the fibres in the thoracolumbar fascia was such that they could convert lateral tension into an appreciable extension moment: the so-called ‘gain’ of the thoracolumbar fascia.⁹¹ The role of the abdominal muscles during lifting was thus to brace, if not actually extend, the lumbar spine by pulling on the thoracolumbar fascia. Any rises in intra-abdominal pressure were thereby only coincidental, occurring because of the contraction of the abdominal muscles acting on the thoracolumbar fascia.

Subsequent anatomic studies revealed several liabilities of this model.²¹ First, the posterior layer of thoracolumbar fascia is well developed only in the lower lumbar region, but nevertheless its fibres are appropriately orientated to enable lateral tension exerted on the fascia to produce extension moments at least on the L2–L5 spinous processes (Fig. 9.16). However, dissection reveals that of the abdominal muscles the internal oblique offers only a few fibres that irregularly attach to the thoracolumbar fascia; the transversus abdominis is the only muscle that consistently attaches to the thoracolumbar fascia, but only its very middle fibres do this. The size of these fibres is such that, even upon maximum contraction, the force they exert is very small. Calculations revealed that the extensor moment they could exert on the lumbar spine amounted to less than 6 Nm.⁹⁴ Thus, the contribution that abdominal muscles might make to anti-flexion moments is trivial, a conclusion also borne out by subsequent independent modelling studies.⁸³

Figure 9.16 The mechanics of the thoracolumbar fascia. From any point in the lateral raphe (LR), lateral tension in the posterior layer of thoracolumbar fascia is transmitted upwards through the deep lamina of the posterior layer, and downwards through the superficial layer. Because of the obliquity of these lines of tension, a small downward vector is generated at the midline attachment of the deep lamina, and a small upward vector is generated at the midline attachment of the superficial lamina. These mutually opposite vectors tend to approximate or oppose the separation of the L2 and L4, and L3 and L5 spinous processes. Lateral tension on the fascia can be exerted by the transversus abdominis (TA) and to a lesser extent by the few fibres of the internal oblique when they attach to the lateral raphe.

A totally different model of lifting was elaborated by Farfan and Gracovetsky.^23,72,91 Noting the weakness of the back muscles, these authors proposed that extension of the lumbar spine was not required to lift heavy loads or loads with long moment arms. They proposed that the lumbar spine should remain fully flexed in order to engage, i.e. maximally stretch, what they referred to as the ‘posterior ligamentous system’, namely the capsules of the zygapophysial joints, the interspinous and supraspinous ligaments, and the posterior layer of thoracolumbar fascia, the latter acting passively to transmit tension between the lumbar spinous processes and the ilium.

Under such conditions the active energy for a lift was provided by the powerful hip extensor muscles. These rotated the pelvis backwards. Meanwhile, the external load acting on the upper trunk kept the lumbar spine flexed. Tension would develop in the posterior ligamentous system which bridged the thorax and pelvis. With the posterior ligamentous system so engaged, as the pelvis rotated backwards the lumbar spine would be passively raised while remaining in a fully flexed position. In essence, the posterior sagittal rotation of the pelvis would be transmitted through the posterior ligaments first to the L5 vertebra, then to L4 and so on, up through the lumbar spine into the thorax. All that was required was that the posterior ligamentous system be sufficiently strong to withstand the passive tension generated in it by the movement of the pelvis at one end and the weight of the trunk and external load at the other. The lumbar spine would thereby be raised like a long rigid arm rotating on the pelvis and raising the external load with it.

Contraction of the back muscles was not required if the ligaments could take the load. Indeed, muscle contraction was distinctly undesirable, for any active extension of the lumbar spine would disengage the posterior ligaments and preclude them from transmitting tension. The back muscles could be recruited only when the trunk had been raised sufficiently to shorten the moment arm of the external load, reducing its flexion moment to within the capacity of the back muscles.

The attraction of this model was that it overcame the problem of the relative weakness of the back muscles by dispensing with their need to act, which in turn was consistent with the myoelectric silence of the back muscles at full flexion of the trunk and the recruitment of muscle activity only once the trunk had been elevated and the flexion moment arm had been reduced. Support for the model also came from surgical studies which reported that if the midline ligaments and thoracolumbar fascia were conscientiously reconstructed after multilevel laminectomies, the postoperative recovery and rehabilitation of patients were enhanced.⁹⁵

However, while attractive in a qualitative sense, the mechanism of the posterior ligamentous system was not validated quantitatively. The model requires that the ligaments be strong enough to sustain the loads applied. In this regard, data on the strength of the posterior ligaments are scant and irregular, but sufficient data are available to permit an initial appraisal of the feasibility of the posterior ligament model.

The strength of spinal ligaments varies considerably but average values can be calculated. Table 9.1 summarises some of the available data. It is evident that the strongest posterior ‘ligaments’ of the lumbar spine are the zygapophysial joint capsules and the thoracolumbar fascia forming the midline ‘supraspinous ligament’. However, when the relatively short moment arms over which these ligaments act are considered, it transpires that the maximum moment they can sustain is relatively small. Even the sum total of all their moments is considerably less than that required for heavy lifting and is some four times less than the maximum strength of the back muscles. Of course, it is possible that the data quoted may not be representative of the true mean values of the strength of these ligaments but it does not seem likely that the literature quoted underestimated their strength by a factor of four or more. Under these conditions, it is evident that the posterior ligamentous system alone is not strong enough to perform the role required of it in heavy lifting. The posterior ligamentous system is not strong enough to replace the back muscles as a mechanism to prevent flexion of the lumbar spine during lifting. Some other mechanism must operate.

One such mechanism is that of the hydraulic amplifier effect.⁹³ It was originally proposed by Gracovetsky et al.⁹³ that because the thoracolumbar fascia surrounded the back muscles as a retinaculum it could serve to brace these muscles and enhance their power. The engineering basis for this effect is complicated, and the concept remained unexplored until very recently. A mathematical proof has been published which suggests that by investing the back muscles the thoracolumbar fascia enhances the strength of the back muscles by some 30%.⁹⁸ This is an appreciable increase and an attractive mechanism for enhancing the antiflexion capacity of the back muscles. However, the validity of this proof is still being questioned on the grounds that the principles used, while applicable to the behaviour of solids, may not be applicable to muscles; and the concept of the hydraulic amplifier mechanism still remains under scrutiny.

Quite a contrasting model has been proposed to explain the mechanics of the lumbar spine in lifting. It is based on arch theory and maintains that the behaviour, stability and strength of the lumbar spine during lifting can be explained by viewing the lumbar spine as an arch braced by intra-abdominal pressure.^99,100 This intriguing concept, however, has not met with any degree of acceptance and indeed, has been challenged from some quarters.¹⁰¹

In summary, despite much effort over recent years, the exact mechanism of heavy lifting still remains unexplained. The back muscles are too weak to extend the lumbar spine against large flexion moments, the intra-abdominal balloon has been refuted, the abdominal mechanism and thoracolumbar fascia have been refuted, and the posterior ligamentous system appears too weak to replace the back muscles. Engineering models of the hydraulic amplifier effect and arch model are still subject to debate.

What remains to be explained is what provides the missing force to sustain heavy loads, and why intra-abdominal pressure is so consistently generated during lifts if it is neither to brace the thoracolumbar fascia nor to provide an intra-abdominal balloon. At present these questions can only be addressed by conjecture but certain concepts appear worthy of consideration.

With regard to intra-abdominal pressure, one concept that has been overlooked in studies of lifting is the role of the abdominal muscles in controlling axial rotation of the trunk. Investigators have focused their attention on movements in the sagittal plane during lifting and have ignored the fact that when bent forward to address an object to be lifted, the trunk is liable to axial rotation. Unless the external load is perfectly balanced and lies exactly in the midline, it will cause the trunk to twist to one side. Thus, to keep the weight in the midline and in the sagittal plane, the lifter must control any twisting effect. The oblique abdominal muscles are the principal rotators of the trunk and would be responsible for this bracing. In contracting to control axial rotation, the abdominal muscles would secondarily raise intra-abdominal pressure. This pressure rise is therefore an epiphenomenon and would reflect not the size of any external load but its tendency to twist the flexed trunk.

With regard to loads in the sagittal plane, the passive strength of the back muscles has been neglected in discussions of lifting. From the behaviour of isolate muscle fibres, it is known that as a muscle elongates, its maximum contractile force diminishes but its passive elastic tension rises, so much so that in an elongated muscle the total passive and active tension generated is at least equal to the maximum contractile capacity of the muscle at resting length. Thus, although they become electrically silent at full flexion, the back muscles are still capable of providing passive tension equal to their maximum contractile strength. This would allow the silent muscles to supplement the engaged posterior ligamentous system. With the back muscles providing some 200 Nm and the ligaments some 50 Nm or more, the total antiflexion capacity of the lumbar spine rises to about 250 Nm which would allow some 30 kg to be safely lifted at 90° trunk flexion. Larger loads could be sustained by proportionally shortening the moment arm. Consequently, the mechanism of lifting may well be essentially as proposed by Farfan and Gracovetsky^22,72,93 except that the passive tension in the back muscles constitutes the major component of the ‘posterior ligamentous system’.