The morphology and biomechanics of latissimus dorsi

Clinical Biomechanics I3 (1098) 377-385

ELSEVIER

The morphology and biomechanics of latissimus dorsi

Abstract Objective. To determine the morphology of the latissintus dorsi in order to assess its actions on the shoulder, the lumbar spine and the sacroiliac joint. Design. A dissectison study accompanied by an analysis of the force vectors of the muscle and its parts. Rack~~ound. Although recognised as a muscle of the shoulder. latissimus dorsi has been accorded a role as an extensor of the lumbar spine, and is said to brace the sacroiliac joint. Consideration of the anatomy of the latissimus dorsi suggests that the magnitude of these actions has been overstated. Methorls. The fascicular anatomy of the latissimus dorsi was determined by dissection in five adult cadavers. The size. attachments, and orientation of each fascicle were determined. By applying a force coefficient the maximum force of each fascicle was estimated from its physiological cross-sectional area. By summing the forces and moments of each fasciclc the maximum force exerted by latissimus dorsi was calculated for its actions on the shoulder, the lumbar spine, and the sacroiliac joint. Results. The latissimus dorsi was found to consist of a series of fascicles with segmental attachments to the lower six thoracic spinous processes, the Ll and L2 spinous processes, the lateral raphe of the thoracolumbar fascia, the iliac crest and the lower three ribs. These fa:,ciclcs wcrc uniform in size across a given muscle but varied from specimen to specimen. The maximum total force exerted by the latissimus dorsi on the shoulder was estimated to range between 162 and 529 N, but in view of the attachments of the muscle, only a portion of that force can be exerted on the lumbar spine. The maximum extensor moment exerted on the lumbar spine was calculated to be 6.3 N m. The maximum force exerted across the sacroiliac joint was calculated to be 30N. Conclusions. The latissimus dorsi is designed to move the upper limb or to raise the entire trunk in brachiation. Its possible contribution to extension of the lumbar spine is trivial as is its capacity to brace the sacroiliac joint.

Relevance Despite assertions and concerns to the contrary, the latissimus dorsi is of little mechanical importance region. 0 1998 Pub1 tshed by Elsevier Science Ltd. All rights reserved.

in the lumbosacral

Keywo&: Latissimus clorsi; Shoulder; Lumbar spine; Modeling: Biomechanics: Muscles

1. Introduction Latissimus dorsi is an intriguing muscle. From a relatively short, linear attachment on the humerus, this muscle covers the back of the thorax and assumes widespread, distant attachments to the thoracic, lumbar and sacral spinous proccsscs and to the ilium. Furthcrmore, some authorities have ventured to say that its aponeurosis crosses the midline to reach the contralateral posterior superior iliac spine, and that it is *Corresponding au,:hor. Present address: Professor N. Bogduk, Newcastle Bone and Joint Institute, Royal Newcastle Hospital, Newcastle, NSW 2300, Australia. 0268-0033/98/$19.00 + Cl.000 1998 Published PII: SO268-0033(98)00102-2

thereafter continuous with the aponeurosis of the contralateral gluteus maximus [l]. Mechanically, latissimus dorsi is a powerful adductor and extensor of the shoulder, but it has also been accorded an action on the lumbar spine [2-71 and, of late, on the sacroiliac joint [1,7]. It has been inlcuded as an extensor and lateral flexor of the back [2-71, and is purported to have a bracing effect on the sacroiliac joint, in concert with the gluteus maximus, through its action on the posterior layer of thoracolumbar fascia [l]. The posterior layer of thoracolumbar fascia is formed by the aponeurosis of latissimus dorsi, and

by Elsevier Science Ltd. All rights resewed.

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A! Bogduk et al./Clinical Biomechanics 13 (1998) 377-385

consists of two laminae a superficial lamina derived from the ipsilateral latissimus dorsi, and a deep lamina ostensibly derived from the contralateral muscle [8]. The two laminae endow the fascia with a criss-cross structure. Lateral to the erector spinae, the posterior fascia is fused with the middle layer of thoracolumbar fascia along what is known as the lateral raphe, which runs parallel to the erector spinae over much of the lumbar region [8]. Latissimus dorsi could have an action on the lumbar spine in either or both of two ways. Those of its fibres that span the lumbar region posteriorly could have a direct extensor action, like that of the erector spinae. In addition, since latissimus dorsi attaches laterally to the posterior layer of thoracolumbar fascia it could draw this fascia laterally and thereby exert an extensor moment on the lumbar spine through the criss-cross arrangement of its fibres. This tension mechanism has been studied with respect to the action of abdominal muscles on the thoracolumbar fascia [9], but not with respect to latissimus dorsi. However, anatomy alone is not a sufficient basis upon which to ascribe an action of a muscle. Morphology is required to determine the magnitude of any putative action. Although a muscle might be suitably disposed geometrically to exert an action, the magnitude of any force exerted by the muscle is determined by its size and the extent to which it is neurally driven. Its size, in terms of physiological cross-sectional area, determines the maximum forcecapacity of the muscle. Neural drive determines how much of that maximum force might be delivered in a given circumstance. Any consideration of the possible actions of muscle must, therefore, first involve a consideration of its maximum force capacity. Actions that seem plausible upon inspection of a muscle may become inconsequential if the maximum forces involved are trivial. Accordingly, the present study was undertaken to explore the anatomy of the latissimus dorsi, to determine which of its parts might act on the shoulder and on the lumbar spine, to determine the size of those parts, and thereby determine an estimate of its force capacity for the various actions that it has been accorded. 2. Methods The latissimus dorsi was studied by gross dissection in five embalmed, elderly human adult cadavers. One cadaver was studied on one side only; all others were dissected bilaterally. Once the overlying skin, fascia and trapezius were removed, the latissimus dorsi was systematically stripped fascicle by fascicle from above downwards. A fascicle was defined as a bundle of

muscle fibres that assumed a distinctive, discrete medial attachment, typically to a given spinous process or interspinous space or equivalent site. Each fascicle was detached from its medial attachment and then peeled from the remaining mass of intact fibres as far as it reached the tendon of insertion into the humerus. There it was cut from the body of the tendon. Once each fascicle had been isolated, the length of its muscle fibres was measured with a ruler to the nearest 0.5 cm, and its volume was determined by immersing the muscle fibres in a volumetric cylinder filled with water and noting the displacement of water to the nearest millilitre. By dividing the volume of each fascicle by its length the physiological cross-sectional area (PCSA) of each fascicle was determined. Representative mean sizes of each fascicle were determined by summing the PCSA of all corresponding fascicles and dividing by the total number of fascicles. A representative PCSA of the entire latissimus dorsi was determined by summing the mean areas of its constituent fascicles. To determine the uniformity of size of the latissimus dorsi, the sizes of its fascicles were expressed in terms of equivalents of the average thoracic fascicles. For each muscle, the mean PCSA of its thoracic fascicles was calculated and then, the size of each of its fascicles was expressed as a ratio of this average size, calculated by dividing the PCSA of each fascicle by the PCSA of an average fascicle. Subsequently, the means and standard deviations of the ratios for each fascicle were calculated in order to generate a representative profile of the relative sizes of fascicles across all specimens. The maximum possible force exerted by each of the fascicles of latissimus dorsi was determined by multiplying the physiological cross-sectional area of the fascicle by a force co-efficient of 49 N cm ‘, which is the co-efficient obtained for the lumbar back muscles [lo]. Each force was resolved into longitudinal and transverse vectors with respect to the sagittal plane, according to the orientation of the fascicle in question. The extensor moment exerted by each fascicle on the lumbar spine was determined by multiplying its sagittal force-vector by a moment arm. For fibres acting across the back of the erector spinae the moment arm was taken to be 7 cm, which is the mean distance between the axes of rotation of the lumbar vertebrae and the tips of the lumbar spinous processes [ll]. For fibres acting on the lateral raphe and ilium, the moment arm was taken to be 4 cm which is the anteroposterior distance between the centre of the latissimus dorsi and the centre of the L2-3 disc, as measured on magnetic resonance scans [ll]. For a given lumbar segment, the total extensor moment was determined by summing the moments

N. Bogduk

et al.lClinic~al

Biomechunics

on that segment exerted by all fascicles that crossed the segment. The effect of the transverse vector of fibres acting on the lateral ra;phe was calculated in the same way that the action of transversus abdominis was calculated in a previous study [9]. From the lateral raphe, fibres of the deep and superficial laminae of the posterior layer of thoracolumbar fascia pass upwards and downwards, in a triangular fashion, to the L2 to L5 spinous processes. Lateral tension on the apex of these triangles has the capacity to draw their basal corners together, effectively exerting an extensor force on the L2 versus L4 and L3 versus L5 spinous processes (Figure 1). The extensor force exerted on the spinous processes can be shown to be F tanz, where z is the obliquity of the laminae of the posterior layer of thoracolumbar fascia, and F is the transverse force acting on the lateral raphe [9] (Figure 1). In a previous study, the force studied was that of the transvlersus abdominis [9]. For the present study, this force was replaced by the transverse vector of the fibres of latissimus dorsi acting on the lateral raphe. Such a simple biomechanical analysis ignores factors such as the moulding of fibres around the curvature of the erector spinae, and assumes that all the force from a given fascicle exerted at a given point on the thoracolumbar fascia is transmitted exclusively into the tendon fibres attached at that point, instead of dissipating through the fascia in a less selective manner. To include such factors in an analysis would render the analysis more complex, but would not enhance the actions of the muscle. If

Fig. 1. The arrangement of tibres in the posterior layer of thoracolumbar fascia. From the lateral raphe (Ir) fibres pass downwards and medially in the superficial lamina at an angle of 30” below horizontal, and fibres pass upwards and medially in the deep lamina at a similar angle. Lateral tension applied to the lateral raphe is distributed in a triangular fashion through these fibres resulting in an extension force between sub-consecutive spinous processes.

13 (1998)

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anything it would reduce the magnitude of possible actions by introducing a larger number of directions (or vectors) in which the longitudinal and transverse vectors of the muscle are exerted. Therefore, the simple approach is appropriate given that the objective of the present study was to determine, in the first instance, the maximum possible forces exerted by the various parts of the latissimus dorsi. 3. Results 3.1. Anutomy

On inspection, the latissimus dorsi is a large fan-shaped muscle covering the posterolateral aspect of the thorax. Rostrally, all of its fibres converge on a thick, flat tendon that twists under the teres major to insert into the floor of the intertubercular sulcus of the humerus. Medially and caudally its fibres become aponeurotic before assuming a diversity of attachments. The transition from muscular to aponeurotic fibres occurs along a line that has a relatively constant disposition and shape (Figure 2 and Figure 3). The line is sigmoid in shape with an upper arm that runs parallel, but some 4 cm lateral, to the lower thoracic spinous processes. At the thoracolumbar junction this line curves laterally across the back of

Fig. 2. A dissection of the latissimus dorsi bilaterally. LR: lateral raphe, Th: thoracic fihres, Tr: transitional fibres, R: raphe fibres, IL: iliac fibres, PS: posterior superior iliac spine. Costal fibres are not evident in this posterior view for they lie deep to the iliac fibres. The numbers mark the spinous processes.

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Biomechanics 13 (1998) 377-385

the erector spinae and then descends parallel and immediately lateral to the erector spinae, forming the lateral raphe. This line allows different parts of the latissimus dorsi to be defined. From above downwards these are the thoracic fibres, the transitional fibres, the raphe fibres, the iliac fibres, and the costal fibres (Figure 2 and Figure 3). Irregular fibres include a small bundle attached to the inferior angle of the scapula. The thoracic fibres of latissimus dorsi are those that attach to the lower six thoracic spinous processes and intervening supraspinous ligament. From the tendon of the muscle these fibres approach the midline obliquely, the upper fibres at an angle of about 60” and the lower fibres at about 50” from the sagittal plane. As they become aponeurotic this orientation remains unchanged. The transitional fibres are characterised by becoming aponeurotic along an oblique line that crosses the back of the erector spinae at thoracolumbar levels. They descend across the back of the thorax at an angle of about 40” from the sagittal plane. Their tendons form the upper part of the posterior layer of thoracolumbar fascia and pass towards the Ll and L2 spinous processes and supraspinous ligaments at these levels.

Tr R IL

The raphe fibres are those that attach to the lateral raphe of the thoracolumbar fascia. They descend from the humerus at an angle of about 30” from the sagittal plane, and are fused strongly with the lateral raphe along its entire length. Nevertheless, individual tendons can be seen to continue through the raphe into the posterior layer of thoracolumbar fascia. However, these tendons undergo a deflection in the raphe, and cross the back of the erector spinae at an angle of about 60 from the sagittal plane. Some of these tendons reach the L3, L4 and L5 spinous processes. Others, directed towards the lower lumbar interspinous spaces,interlace with corresponding fibres from the opposite side, and thereby replace the supraspinous ligament at these levels. The iliac fibres are those that attach to the iliac crest. They descend from the humerus at an angle of about 15” from the sagittal plane and become aponeurotic at variable distances short of the iliac crest. They assume a linear attachment to the iliac crest for some 2-5 cm lateral to the erector spinae. The costal fibres attach to the lower 1-3 ribs. They descend almost vertically to reach the distal end of the 12th rib and adjacent portions of the 11th and 10th ribs. These fibres are inconstant; those to the 12th rib being most often represented, those to the 10th rib most often absent. The scapula fibres, when present, stem from the inferior angle of the scapula and join the upper thoracic fibres to pass to the humeral tendon. Topographically, most of the muscle fibres of the latissimus dorsi are located in the thoracic region. Opposite the L4-5 level the iliac fibres may be fully present or some of them may be aponeurotic. Opposite L3-4 more of the iliac fibres are completely present as muscle fibres. The raphe fibres are fully evident only above L3; below that level they become progressively more aponeurotic as they join the lateral raphe. The transitional fibres are fully evident only above Ll. These topographical considerations are important with respect to using CT scans or MRI scans of the lumbar region to determine the size of latissimus dorsi. Scans at L3 or L4 will, at best, incur only the iliac fibres and will not include the bulk of the raphe fibres. 3.2. Morphometry

Fig. 3. A sketch of the dissection of latissimus dorsi shown in Figure 2. Note the line of transition (j) between muscle fibres and their aponeurosis (a). The dashed lines show how the various parts of latissimus dorsi can be discerned. The iliac hbres (IL) are directed to the ilium. The raphe fibres (R) are directed to the lateral raphe (Ir) before entering the posterior layer of thoracolumbar fascia (p). The transitional fibres are those that become aponeurotic at the thoracolumbar junction where the line of transition between muscle fibres and aponeurosis deflects laterally to the upper end of the lateral raphe.

In the specimens studied, the latissimus dorsi was large in two, small in two, and intermediate in size in one, with total cross-sectional areas that ranged from 3.3 to 10.8 cm2 (Table 1). On average, each of the thoracic fascicles exhibited a cross-sectional area of 0.4-0.6 cm’, and constituted between 7 and 9% of the total cross-sectional area of the muscle (Table 1). The transitional fibres and raphe fibres were twice as

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381

Table 1 Morphometric features of the fascicles of latissimus dorsi. The PCSA is the physiological cross sectional area of each fascicle. The proportion of total refers the proportion of the entire muscle that each fascicle constitutes. Costa1 fascicles were not represented in every specimen Fasicle

N

Length (cm) mean

Scapula Thoracic: T7 T8 T9 TlO Tll T12 Transitional Raphe Iliac Costal: Rib 12 Rib 11 Rib 10 Total

3

8.5

range

Volume (ml) mean

PCSA (cm’)

range

mean

sd

8-9

5

5-6

0.6

0.6-0.7

Proportion range

mean

sd

range

7 7 7 9 9 8 14 17 12

3.1 3.1 2.4 1.6 3.6 1.7 3.0 6.1 4.2

4-12 4-12 4-10 6-11 6-16 5-10 10-18 9-25 3-18

9 9 9 9 Y 9 9 9 9

19 19 20 22 23 24 25 28 30

12-28 14-22 15-23 15-2s 17-27 20-28 22-29 24-35 24-35

7 7 8 12 13 11 21 30 20

4-12 3-13 3-20 4-19 4-22 4-22 10-43 10-59 IO-37

0.4 0.4 0.4 0.6 0.6 0.5 0.9 1.1 0.7

0.15 0.16 0.27 0.26 0.29 0.20 0.42 0.65 0.39

0.2-0.6 0.2-0.7 0.2-1.0 0.3-0.9 0.2-1.2 0.2-0.8 0.4- 1.5 0.4-2.1 0.3- 1.s

5 4 2

25 25 16

18-31 23-28 13-l

12 13 Y

3-19 S-24 7

0.4 0.6 4-9 6.2

0.2-0.7 0.2-0.9 0.4 2.7

7 8 0.3-0.5 3.3- 10.8

large as a single thoracic fascicle and, on average, constituted 14 and 17% of the muscle, respectively. The iliac fibres were 1.6 x the average size of a thoracic fascicle, and constituted some 12% of the muscle. The costal fibres, when present, were each about the same size as a thoracic fascicle. The raw data in Table 1 depict considerable variance in size. However, much of this variance stems from the variation in total size of the muscle from different specimens. Some also stems from difficulties encountered in defining fascicles exactly. Since the latissimus dorsi lacks any intrinsic features, such as cleavage planes, by which fascicles could be defined, their upper and lower boundaries had to be extrapolated from their midline attachments. Errors in determining exactly where one spinous process ends and the next begins can result in one fascicle being accorded a greater size at the expense of its neighbour, and vice versa. However, this variance is reduced when fascicles are normalised for relative size. Within a given muscle the muscle fibres were uniformly distributed across the breadth of the muscle. Figure 4 depicts the profile of latissimus dorsi expressed in equivalents of the average physiological cross-sectional area of a thoracic fascicle. Upper thoracic fascicles tended to be slightly smaller than average, lower fascicles slightly larger. Given that the transitional fibres are directed to two lumbar segments, each receives the equivalent of one thoracic fascicle in cross-sectional area. If the raphe fibres are assigned to the three lower lumbar segments, each segment receives slightly less than the equivalent of one thoracic fascicle, on average. When

(%) Total

4-14 s-12 6

s-7

normalised in this way, the sizes of fascicles clearly centre on approximately one thoracic fascicle-equivalent per segment, with little variance from this idealised pattern. Only the iliac fibres tend to exceed the representative average size. 3.3. Biomechanics Calculation of the maximum force of each fascicle revealed that the thoracic fascicles were reasonably strong, capable of delivering between 20 and 30 N, on average, with a range of lo-60 N (Table 2). The transitional, raphe and iliac fibres were stronger in proportion to their greater size. Based on total crosssectional area, the total force of the latissimus dorsi ranged from 162 to 529 N, with a representative average of 304 N. With respect to the shoulder, latissimus dorsi is a two-joint muscle, crossing both the glenohumeral and scapula-thoracic joints and so, all of its force would

T7

8

9

10

11

T horacic

12

Ll

2 Tr

3

4 5 Raphe

IL

RI2 Rll

Fig. 4. The profile of latissimus dorsi. The graph shows the relative size of individual fascicles of latissimus dorsi. arranged serially by spinal segment. The ordinate depicts a unit of I, being the average size of a thoracic fascicle in a given muscle. The areas of the rectangles are directly proportional to the size of the corresponding fascicle. The bars indicate one standard deviation of the relative size of each fascicle. Tr: transitional fibres, IL: iliac fibrcs, R12. Rll: costal tibres from ribs 12 and 1 I.

382 Table

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Biomechanics 13 (1998) 377-385

2

The mean values and ranges of the maximum possible forces exerted unilaterally by fascicles of the latissimus dorsi, determined by multiplying their physiological cross-sectional area by 49 N cm-’ Fascicle

F max (N)

T

F sag (N)

F trans (N)

Moment Arm (cm)

Tl T8 T9 TlO Tll T12 Transitional Raphe

20 (10-30) 20 (10-35) 20 (10-50) 30 (15-45) 30 (10-60) 25 (10-40) 44 (20-73) 54 (20-103)

60 60 60 60 50 50 40 30

IO@-15) 10 (5-18) 10 (5-25) 15 (8-23) 19 (6-38) 16 (6-24) 34 (15-57) 47 (17-90)

17 (9-26) I7 (9-30) 17 (9-43) 26 (13-39) 23 (8-46) 19 (8-30) 28 (12-47)

na na na na na

Iliac

34 (15-73)

15

33 (14-71)

na

7 4 7 4

27 (10-52) 9 (4-19)

F sag and F trans are the vectors of each force in the sagittal and transverse planes respectively. (is the orientation of the fascicle with respect

to the sagittal plane.

be available for action on the humerus, with an additional effect on the scapula. Its fibres do not pass in a straight line to the humerus. Rather, they wrap around the posterior chest wall, and their orientation is governed largely by the position of the arm. Their line of action is not, for practical purposes, defined by the caudal attachments. Rather, it would be a line passing from the humerus tangential to the chest wall. To model the latissimus dorsi, therefore, would require specification of the position of the humerus, and a knowledge of the size and curvature of the chest wall. These issues were not addressed in the present study, although they have been examined in detail by Hogfors et al. 1121.However, valid estimation of the maximum glenohumeral moment produced by latissimus dorsi remains possible. Its point of attachment to the humerus gives it a maximum potential moment arm of around 60 mm but, since the line of action of the tendon will be greatly influenced by wrapping round the chest wall, the maximum moment will only be achieved in one particular position - flexed and abducted, at which the line of action lies at right angles to the long axis of the humerus. This maximum moment will be approximately 18 N m. Table 3 The moments exerted

by

individual components of the latissimus dorsi on segments of the lumbar spine Moment (Nm)

Segmental Level

Transitional fibres Ll L2 L3 L4 L5

With respect to the lumbar spine, the actions of latissimus dorsi are restricted to those fibres that cross the lumbar spine. The thoracic and costal fascicles do not do so and, therefore, have no action on the lumbar spine. That capacity is restricted to the transitional, raphe and iliac fibres. Each of these latter fascicles, however, differs with respect to how they might act on the lumbar spine, and the extent to which they might do so. The iliac fibres have the most direct and most extensive action. They span the entire lumbar spine and, therefore, act on every segment. Since they are orientated steeply, most of their action is expressed as a sagittal vector (Table 2). Endowed with a moment arm of 4 cm, these fibres might provide an average extensor moment of 1.3 N m, with a range of 0.5-2.8 N m (Table 3). The transitional fibres might act on the lumbar spine, but only on the upper two segments. However, because of their obliquity, only a portion of their force is exerted longitudinally (Table 2) but they are endowed with a large moment arm because they cross the back of the erector spinae. All these fibres would act across the Ll segment but only about half reach L2. Accordingly, they contribute an extensor moment of

2.4 (1.1-4.1) 1.1 (0.6-1.8)

Source of Moment ~~ ~~~ Iliac fibres Raphe fibres 1.8 (0.6-3.5) 1.8 (0.6-3.5) 1.8 (0.6-3.5) 1.8 (0.6-3.5) 1.8 (0.6-3.5)

1.3 (0.5-2.8) 1.3 (0.5-2.8) 1.3 (0.5-2.8) 1.3 (0.5-2.8) 1.3 (0.5-2.8)

Total Fascia 0.6 (0.3-1.1) 0.6 (0.3-1.1)

5.5 (2.2-10.4) 3.7 (1.4-7.4) 3.7 (1.4-7.4) 3.1 (1.1-6.3) 3.1 (1.1-6.3)

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2.4 N m across L:l and 1.1 N m across L2 (Table 3) but no moment across low lumbar segments. The action of the raphe fibres is complicated. The tendons of these fibres are firmly attached to the lateral raphe as they change direction to enter the posterior layer of thoracolumbar fascia. As a result, the longitudinal vector of the raphe fibres will be directed longitudinally through the raphe and resisted by the ilium (Figure 5). This accords the raphe fibres with an action parallel and similar to that of the adjacent iliac fibres. The sagittal force of the raphe fibres averages 47 N, with a range of 17-90 N (Table 2), and would exert an extensor moment of 0.6-3.5 N m (Table 3). The lateral vector of the raphe fibres would tense the posterior layer of thoracolumbar fascia laterally. Because of the criss-cross arrangement of fibres in this fascia an extensor moment can be generated between the L2--L4 and L3-L5 spinous processes (Figure 1 and Figure 5). However, not all the raphe fibres act laterally on the entire length of the raphe. The upper fibres would act on its upper half and the lower fibres on its lower half. Therefore, in deter-

383

mining the extensor moment, half of the transverse vector of the raphe fibres (27 N) should be accorded between L2 and L4 and half between L3 and L5. Resolution of the various vectors in the posterior layer (Figure 1 and Figure 5), and applying a moment arm of 7 cm yields an extensor moment at each level of between 0.3 and 1.1 N m (Table 3). Summing all the moments acting across the lumbar spine reveals that the latissimus dorsi could potentially exert some 3.1 N m on lower lumbar segments, with a range of 1.1-6.3 N m, and up to about 10 N m at higher lumbar levels. With respect to the sacroiliac joint, the only components of latissimus dorsi that might conceivably have an action over that joint are the lower raphe fibres. To act on the joint the tendons of these fibres would have to cross the midline to reach the contralateral ilium. Assuming that they do so [l] and are free to exert tension across the midline, the maximum tension that they might exert is the tension along the lower fibres of the posterior layer of thoracolumbar fascia, exerted by the transverse vector of the raphe fibres of latissimus dorsi (Figure 5). This vector amounts to 27 N. Dividing by 2~0~30yields a force of no more than 15.5 N, with a possible range of 6-30 N. 4. Discussion

Fig. 5. The biomechanics of the lateral raphe and posterior layer of thoracolumbar fascia. On the left are shown the orientation of the tendons of the raphe tibres (R) and the transitional fibres (T), and the course of fibres (S) from the opposite side that cross the sacroiliac joint. On the right is shown the dispersal of the force (F) exerted by the raphe fibres. Their sagittal vector (FS) is resisted by the lateral raphe and its attachment to the ilium. Their transverse vector (FT) tenses the fascia laterally (see Figure 1). The tension (T) exerted along the fibrer, of the deep and superficial laminae of the thoracolumbar fascia c.an be resolved into transverse components (Tt) and sagittal components (Ts), and related to the force of the raphe fibres, as shown.

The present study is compromised in one respect. Perforce the cadavers studied came from elderly donors. Therefore, the mean sizes recorded of the fascicles of latissimus dorsi may well be underestimates of what they could be in younger, more athletic individuals. However, this does not compromise the cardinal findings of the present study: that the latissimus dorsi consists of several discrete parts, but that across its breadth the physiological cross-sectional areas of its parts are uniformly distributed; and that whereas all parts can act on the humerus, only certain parts have actions on the lumbar spine. Teleologically, the latissimus dorsi is designed to be a brachiating muscle. The hand having engaged the branch of a tree, the humerus becomes relatively fixed, and the latissimus dorsi can then pull the entire trunk upwards into the tree. It is in these terms that the widespread attachments of the latissimus dorsi can be understood. However, as humans have elected no longer to brachiate, the functions of latissimus dorsi have concentrated on movements of the dependent upper limb and on actions such as pulling. Nevertheless, elements of brachiation still remain in the performance of transfers, particularly in individuals with impaired function of the lower limbs. Latissimus dorsi exerts a large moment on the shoulder but not when the upper limb is at rest.

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Latissimus dorsi is engaged by flexing the arm whereupon, the muscle can act as a powerful extensor either to pull a load backwards or to draw the body towards a fixed object. Evidence of the strength of latissimus dorsi is provided by the study of Fraulin et al. [13] who demonstrated significant reduction in the maximum available moments in patients in patients who had excision of latissimus dorsi. As an adductor of the shoulder, latissimus dorsi serves not so much to draw the arms in but to lift the trunk against braced upper limbs. Such activity occurs in wheelchair transfers [14] and in crutch locomotion, which requires extension as well as adduction [15]. Crosbie and Nicol [16] found that an extension moment of 0.06 N m/kg was required for reciprocal crutch walking, and Noreau et al. [17] found a moment of 0.1 N m/kg is to be expected at the end of stance phase of swing-through locomotion. Crosbie [18] demonstrated that latissimus dorsi was always active during the stance phase of crutch walking, but he was not able to correlate this activity with other biomechanical measures. In addition to providing a moment at the glenohumeral joint, latissimus dorsi also provides a force which, particularly at high elevations of the arm, acts almost parallel to the long axis of the humerus. This would have the effect of reducing the shear forces at the glenoid during transfer tasks or during brachiation. It is interesting to note that with the arm elevated in abduction, the latissimus dorsi is no longer twisted around teres major; its fibres assume the appearance more of a uniform flat sheet. The reason for the twist in this muscle and its mechanical advantages have still to be elaborated. If it is accepted that the fundamental purpose of latissimus dorsi is brachiation, the reason for its lumbar and pelvic attachments is clearly to act on the entire trunk. By the same token, latissimus dorsi is not designed as an extensor of the back. This is evident in its lack of strength in this action. The transitional fibres of latissimus dorsi contribute an extensor moment to the upper two lumbar segments; the iliac fibres have a direct action across the entire lumbar spine; and the raphe fibres have two possible actions - a direct extensor action through the lateral raphe, and an indirect extensor effect through the posterior layer of thoracolumbar fascia. Previous investigators have differed as to the forces and moments accorded to these fibres. Without specifying the cross-sectional area, Goel et al. [4] assigned to latissimus dorsi a force of 123 N across the lumbar spine. This value is consistent with both the mean value and ranges determined in the present study for the sagittal vectors combined of the raphe and iliac fibres of latissimus dorsi. In contrast, Schultz and Andersson [3], accorded latissimus dorsi

a force of 240 N which is well in excess of the upper limit determined in the present study of the force exerted by the raphe and iliac fibres (Table 3), but might be understood if Schultz and Andersson [3] were reporting bilateral forces. Based on lumbar MRI scans, Guzik et al. [2] reported a cross-sectional area of 3.62 cm2 for latissimus dorsi in the lumbar region, but without specifying the segmental level, whereas Tracy et al. [ll] reported a range of values from 2.6 to 6.6 cm2 at the L2-3 level. Such scans would incur the iliac fibres and some of the raphe fibres. Collectively these fibres in the present study exhibited a physiological crosssectional area of 0.7-3.6 cm’. McGill [7] accorded the latissimus dorsi a force of 337 N bilaterally at L5, which amounts to 168 N unilaterally which is just above the range of values determined in the present study. Others accorded bilateral forces of 100 N6 and 112 N5, which when halved fall into the range determined in the present study. These studies, however, used moment arms of 5.3 cm [5,7] or 7 cm’ which are somewhat greater than those of the present study. Perhaps they can be explained by the larger size of individuals used in these other modeling studies. A moment arm of 9 cm, as used in one study [19], is well in excess of the upper limit reported by Tracey et al. [II], and would conceivably apply only to very large individuals. Notwithstanding these specific differences, the moments reported in these studies [5-71 are reasonably consistent with the range and maximum possible values determined in the present study. The biomechanical significance of the posterior layer of thoracolumbar fascia has been a focus of contention for over 10 years. Gracovetsky et al. [20,21] originally proposed that lateral tension exerted by the abdominal muscles could exert an extensor moment on the lumbar spinous processes. Fairbank and O’Brien [22] reported demonstrating this effect in cadavers. However, Macintosh et al. [9] subsequently showed that the magnitude of this effect could be no greater than 5 N m. Their calculations, however, addressed only the abdominal muscles; they did not consider latissimus dorsi. The present study completes that consideration. On anatomical grounds, the only component of latissimus dorsi that could tense the posterior layer of thoracolumbar fascia is the lateral vector of the raphe fibres. The present calculations place the maximum extensor moment generated in this way at 1.1 N m. For an individual with a muscle twice the maximum size encountered in the present study, this moment would not exceed 2.2 N m. This finding differs quantitatively, although not qualitatively, from that of McGill and Norman [19] who calculated the moment exerted by the thoraco-

N Bogduk rt al.lClinical Biomrchunics 13 (1998) 377-38.~

lumbar fascia to fall between 4.6 and 17.6 N m. These generous values arise because McGill and Norman [19] treated the thoracolumbar fascia as an isotropic sheet in which tension delivered to its superolateral border would be distributed uniformly throughout the sheet. The anatomy of the posterior layer is inconsistent with this treatment. The transitional fibres of latissimus dorsi act on the superolateral corner of the fascia but their tendons pass through the superficial lamina directly lo the Ll and L2 spinous processes. Any action of these fibres, therefore, would be exerted predominantly, if not exclusively on those bones and not at lower levels. The raphe fibres of latissimus dorsi c:annot pull upwards on the fascia, for that action is diverted through the lateral raphe to the ilium. Only the lateral vector of the raphe fibres can possibly tense the fascia. but that effect is next to trivial. Consequently, the analysis of McGill and Norman [19] overstates the magnitude of the force exerted by latis,simus dorsi on the thoracolumbar fascia. Nonetheless, their conclusion is reinforced by the present stucly, that through the thoracolumbar fascia, latissimus dorsi contributes no appreciable moment to back extension. A similar conclusion pertains to the putative action of latissimus dorisi on the sacroiliac joint. Anatomically, only the lowest fibres of the thoracolumbar fascia cross the contralateral sacroiliac joint. The only fibres of latissimus dorsi that might tense these fibres are the lowest raphe fibres. Any other tension is dissipated at higher levels not associated with the sacroiliac joint. The present study shows that the maximum force that might be generated across the sacroiliac joint is 6-30 N. Moreover, this estimate presupposes a large muscle and one that is maximally contracted. Under average circumstances a much lesser force would be encountered. Thus, even though it might appear that latissjmus dorsi could brace the sacroiliac joint, the magnitude of the forces involved is trivial. In essence, what the biomechanical components of the present study reveal is that although the latissimus dorsi has been purported to have various actions on the lumbosacral region the magnitude of the possible forces is small. Even allowing the maximum values determined in the present study, and even doubling them in deference to young individuals having larger muscles, the maximum possible extensor moment exerted on the lumbar spine by latissimus dorsi is unlikely to exceed 12 N m, which is less than 5% of the moment required for a moderately heavy lift. For that reason it must be concluded that for biomechanists interested in the lumbar spine, any preoccupation with latissimus dorsi is barely justified. Clearly, the latissimus dorsi belongs to the shoulder, and should be modeled there, and not in the lumbar spine.

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