This informal CPD article ‘Vector Analysis of the Vertebral Column in the Frontal Plane – Part 1’, was provided by Dr. Mauro Lastrico, Physiotherapist at AIFiMM Formazione, an organisation recognised by the Italian Ministry of Health as an authorised CME provider. They offer organised training courses in the Mézières Method, a rehabilitative and postural approach.
Interpretative foundations, cervical and cervico-thoracic segments, introduction to the latissimus dorsi
This article is part of a series of contributions dedicated to the application of physical principles to the musculoskeletal system, with the aim of providing a coherent biomechanical interpretation of the main joint and vertebral alterations [3–6].
In the first article, Clinical Assessment of Muscle Shortening [1], the physical model of muscle shortening was introduced as a viscoelastic deformation of the connective tissue components, distinguishing the mechanical behaviour of the contractile component from that of the passive elastic component [1,8,9].
The theoretical framework was subsequently completed through the introduction of vector analysis as an interpretative tool for reading musculoskeletal alterations. In particular, Vector Analysis in Musculoskeletal Biomechanics – Part 1 [3] presented the fundamental principles of force decomposition and mechanical resultants, while Part 2 [4] illustrated their clinical applications [3,4,6].
The contributions dedicated to the vertebral column in the frontal plane apply these principles to the analysis of lateral deviations and vertebral rotations, showing how specific muscular dominances produce predictable configurations through mechanical resultants and geometric constraints [3–5].
The analysis is conducted in a descriptive and diagnostic form, with the aim of providing a coherent biomechanical key for interpreting the patterns observable in the frontal plane. For reasons of explanatory clarity, the topic has been divided into two complementary parts.
1. From two-dimensional observation to three-dimensional interpretation
Deviations of the vertebral column observable in the frontal plane never represent a purely two-dimensional phenomenon [3,5,6]. Every lateral deviation is necessarily associated with a rotational component of the vertebral bodies and with an asymmetric redistribution of compressive loads [5,10].
A correct interpretation of the frontal plane requires the adoption of a three-dimensional model, in which the shape of the spine is interpreted as the expression of mechanical resultants produced by specific muscular dominances [3,4,6].
In this context, lateral deviation represents the geometric consequence of the decomposition of muscular forces and of the constraints imposed by the skeletal architecture of the spine [3,5].
2. General principles of vector analysis in the frontal plane
In the frontal plane, the analysis of vertebral deviations requires the distinction and correlation of three fundamental parameters: rotation, lateral translation, and inclination [3,4,5,6].
2.1 Relationship between associated vertebral movements
In vertebral mechanics, rotation of the vertebral bodies, lateral translation with respect to the midline (convexity), and inclination represent kinematically correlated phenomena.
In most observable configurations, rotation is associated with a contralateral convexity and with an inclination ipsilateral to the concavity [3,4,6].
This relationship is not invariant, however: in certain types of structured thoracic scoliosis, rotation and convexity may occur on the same side, indicating an alteration of physiological vertebral mechanics [13].
2.2 Scope of the analysis
In the present analysis, attention is focused on vertebral deviations induced by asymmetric shortening of symmetrical muscles, in the absence of primary structural deformities of the spine.
This allows the examination of configurations in which the observable deviations represent the predictable expression of muscular dominances and of the mechanical resultants generated by them [3–5].
2.3 Choice of the guiding parameter
For reasons of interpretative clarity, the analysis prioritises the parameter of lateral translation, clinically expressed as vertebral convexity, assuming as implicit the presence of the corresponding rotational and inclination components, oriented in the opposite direction [3,4].
This choice allows a more direct identification of the side of muscular dominance responsible for the observed configuration.
2.4 Convexity and concavity: a causal criterion
In the adopted model, vertebral convexity and concavity are differentiated according to the mechanism that produces them.
Convexity is considered the expression of a direct action of muscles with vertebral insertion, which generate a localised traction on the vertebral body [3–5].
Concavity, by contrast, represents the outcome of the convergence of two skeletal elements, as occurs in configurations determined by the approximation between scapula and hemipelvis, and therefore constitutes an indirect mechanical consequence [3,5].
3. Assessment in standing and supine positions: informational differences
Assessment of the vertebral column in the frontal plane provides different information depending on whether it is performed in the standing position or in the supine position [2,3,5].
The two conditions are not interchangeable and may provide coincident or partially discordant indications.
In the standing position, the observable lateral deviations represent the result of the sum of:
- shortening of the connective tissue components of the involved muscles;
- activation of the contractile components required to maintain balance [2,3,12].
In the supine position, the organism is in a condition of stable equilibrium, with a marked reduction in contractile activation. The observed lateral deviations therefore represent more directly the expression of shortening of the connective tissue components alone [2,3,5].
Comparison between the two assessments allows the distinction between stable structural configurations and patterns predominantly sustained by muscular activation, clarifying why certain parameters may coincide or appear reversed [3,5].
Assessment of the vertebral column
4. Investigation of vertebral deviations in the supine position: palpatory and visual criteria
In the supine position, analysis of lateral deviations of the vertebral column is based on palpatory and visual criteria that vary according to the segment considered. The choice of the reference landmark depends on the anatomical and functional characteristics of the different vertebral segments and decisively influences the correct interpretation of the muscular dominances responsible for the observed configurations [3,6,16].
Upper cervical segment (C2–C5)
In the upper cervical segment, assessment is predominantly palpatory and focuses on the orientation of the transverse processes, which allows identification of vertebral body rotation. From rotation, it is possible to infer the associated lateral translation (convexity) and inclination of the segment.
The C1 vertebra is not assessed directly; however, its configuration can be deduced from the rotation of C2 and C3. From a muscular perspective, the cervical vertebrae function as a group unit, generally involving at least three consecutive segments.
In this context, C1 is laterally translated (convex) in the same direction as C2 and C3.
Cervico-thoracic segment (C6–T3)
In the cervico-thoracic segment, assessment becomes primarily palpatory again, using the orientation of the spinous processes with respect to the midline as a reference. Deviation of the spinous process allows identification of vertebral convexity, bearing in mind that the observed orientation is the expression of a contralateral rotation of the vertebral body.
Mid and lower thoracic segment (T4–T12)
In the mid and lower thoracic segment, analysis is predominantly visual and palpatory.
In this region, the vertebrae are functionally integrated with the rib cage; consequently, lateral translation of the thorax represents a reliable indicator of the corresponding vertebral convexity.
Assessment of thoracic convexity therefore allows identification of the underlying vertebral configuration without the need for direct palpation of the spinous processes.
Lumbar segment (L1–L5)
In the lumbar segment, assessment again becomes palpatory, based on the orientation of the spinous processes with respect to the midline. In this case as well, deviation of the spinous process indicates vertebral convexity, associated with a contralateral rotation of the vertebral body.
5. Upper cervical segment (C2–C5): combined action of scalenes and levator scapulae
In the upper cervical segment (C2–C5), deviations in the frontal plane are predominantly generated by muscles with direct vertebral insertion, particularly the scalenes and the levator scapulae [3,4,8,16]. In this region, vertebral alignment results from the combined and asymmetric action of these muscles rather than from the isolated intervention of individual components.
The scalenes, through their insertions on the cervical transverse processes and the first ribs, introduce oblique vectors that generate direct traction on the cervical vertebrae [16]. The levator scapulae adds a vertical and lateral component to this action, establishing a mechanical continuity between the cervical spine and the scapular girdle [15,16].
In the absence of extreme patterns, such as torticollis with marked cranial approximation of the scapula, the causal problem in the C2–C5 segment is almost always attributable to the side of vertebral convexity, which expresses the asymmetric traction exerted by the scalene–levator scapulae functional unit.
In this segment, lateral convexity represents the primary causal sign and is produced directly by the action of muscles with vertebral insertion [3–5]. Rotation of the vertebral bodies is a mechanical consequence of this traction and occurs contralaterally with respect to the convexity [3,4,16]. Inclination, ipsilateral to the concavity, is the parameter most responsible for the concentration of compressive discal and radicular stresses [5,10,11].
From a clinical perspective, rotational and convexity configurations of the upper cervical segment frequently constitute the basis of symptomatology commonly defined as “cervical pain.”
Any radicular manifestations typically occur on the concave side, while the primary muscular cause should be sought on the convex side, which represents the site of tensile dominance.
In the C2–C5 segment, supine assessment is predominantly based on palpation of the transverse processes, which represent the most reliable landmark for identifying vertebral deviations and rotations [3,6,16].
6. Cervico-thoracic segment (C6–T3): action of the rhomboids and middle fibres of the trapezius
The transition from the lower cervical segment to the upper thoracic segment (C6–T3) represents a biomechanical transition zone, in which the resultants generated by the scapular girdle and by muscles with direct vertebral insertion begin to significantly influence the configuration of the thoracic spine [3,6,15].
In this segment, the spinous processes constitute a more reliable palpatory reference than the transverse processes, due to the anatomical characteristics of the vertebrae involved [3,6]. In this region, a central role is played by the rhomboids and the middle fibres of the trapezius, both muscles with direct vertebral insertion [3,6,15].
The action of these muscles determines a mechanical convergence between the scapula and the vertebral column, in which simultaneous traction on the thoracic vertebrae and on the medial border of the scapula produces an associated pattern characterised by vertebral convexity and scapular adduction [3,5,6,15].
In this configuration, vertebral convexity represents the direct expression of muscular traction on the upper thoracic segments [3–5], while scapular adduction constitutes the corresponding adaptation of the scapular girdle to the same vector resultant [3,6,15].
The dominance of this functional unit determines configurations characterised by ipsilateral vertebral convexities and contralateral rotations of the vertebral bodies, consistent with the direction of the tractions exerted at vertebral and scapular levels [3,6,15].
Continuity of the deviation with respect to the cervical segment suggests a single mechanical resultant involving the cervical and thoracic spine in an integrated manner. Conversely, a discontinuity in the direction of the curve indicates the intervention of different vectors and distinct muscular dominances [3,6].
From a clinical perspective, configurations of the C6–T3 segment may be responsible not only for local cervico-dorsal symptoms, but also for referred pain patterns. In particular, compressions involving segments T1–T3 may constitute the biomechanical basis of pain referred to the elbow, such as epicondylitis and epitrochleitis, in the absence of evident local pathology [5,10,11].
Latissimus dorsi represents largest muscle
7. Introduction to the latissimus dorsi: geometric complexity and lines of force
The latissimus dorsi represents the largest muscle in the human body and exhibits a biomechanical complexity proportional to its size. Although it does not insert directly onto the lumbar vertebral bodies, it exerts a profound influence on spinal configuration through distant mechanical resultants generated by the convergence of its multiple lines of force [5,6,11].
Its architecture allows selective activation of different fascicular components, which may combine with one another in variable ways. This characteristic makes a non-unitary interpretation of the muscle necessary, as the predominance of specific lines of force determines different biomechanical and clinical patterns [6,11].
Associated with this intrinsic complexity is that of the muscles participating in the functional unit of the latissimus dorsi. The concept of a functional unit, which will be further developed in the systemic section, identifies anatomically distinct muscles that, by virtue of their arrangement and biomechanical connections, functionally behave as a single muscular system [3,5,6].
The functional unit of the latissimus dorsi includes:
In the lower quadrant
- quadratus lumborum
- transversus abdominis
- abdominal oblique muscles
In the upper quadrant
- subscapularis
- teres major
Interaction between the latissimus dorsi and these muscles contributes to the generation of complex vector resultants that simultaneously involve the spine, pelvis, thorax, and upper limb [5,6,11].
7.1 Main lines of force of the latissimus dorsi
From a biomechanical perspective, the latissimus dorsi may be described through five main lines of force, whose predominance determines different configurations of the spine and associated skeletal segments [6,11]:
- line of force from the iliac crest to the humerus
- line of force from the iliac crest to the lumbar vertebrae, through the associated action of the quadratus lumborum
- line of force from the iliac crest to the lower thoracic vertebrae (T7–T12)
- line of force from the lower thoracic vertebrae (T7–T12) to the humerus
- line of force from the last four ribs to the humerus
The predominance of one or more of these lines of force, in relation to selective shortening of the involved muscular components, allows the latissimus dorsi to produce different biomechanical effects, both at the vertebral level and in the relationships between thorax, pelvis, and upper limb [5,6,11].
7.2 The principle of opposing resultants and classification into two patterns
Vector analysis of the lines of force of the latissimus dorsi highlights how different components of the muscle may generate mechanical resultants oriented in opposite directions, depending on which portions are predominantly shortened [3,6,11].
Selective predominance of specific lines of force determines different skeletal configurations, which do not represent random variations, but rather predictable outcomes of the biomechanical organisation of the muscle and its functional unit [3–5].
On this basis, it has been possible to identify two main patterns, termed Pattern A and Pattern B, which represent the two fundamental ways in which the resultants of the latissimus dorsi are expressed at the level of the pelvis, vertebral column, and scapular girdle [6,11].
The proposed classification should not be interpreted rigidly. In clinical practice, the two patterns do not always present in a pure form, and intermediate or mixed configurations may be observed, depending on interaction with other muscles belonging to the same functional unit [3,5,6].
However, because the functional unit of the latissimus dorsi establishes biomechanical continuity between the pelvis, spine, scapula, and humerus, distinction between these two patterns proves particularly useful. It allows simplification of the diagnostic interpretation of complex configurations by relating them to dominant vector logics, and guides coherent clinical interpretation of the observed patterns [3–6].
8. Pattern A: mechanical resultant of the latissimus dorsi and associated vertebral configuration
In pure Pattern A, configuration of the vertebral column in the frontal plane is dominated by the main line of force of the latissimus dorsi from the iliac crest to the humerus [6,11]. The predominant mechanical resultant is represented by approximation between the ipsilateral scapula and hemipelvis, expressing selective shortening of this muscular component [5,6,11].
In this configuration, the vertebral column follows the mechanical resultant of the scapula–hemipelvis unit, adapting geometrically to their convergence [3,5,6]. This results in a vertebral concavity ipsilateral to the adducted scapula and elevated hemipelvis.
From a descriptive perspective, the convexity observable on the opposite side – as would typically be reported in radiographic contexts – does not represent the primary cause of deformation, but rather the geometric consequence of the concavity produced by convergence of the two skeletal poles [3–5].
In other words, in pure Pattern A:
- the primary biomechanical cause is the concavity produced by scapula–hemipelvis approximation [3,5,6];
- the contralateral convexity constitutes the adaptive outcome of the spine to the dominant mechanical resultant [3–6].
8.1 Double-curve variant: lumbar opposition to the latissimus dorsi resultant
In some cases, the mechanical resultant of the latissimus dorsi is opposed by the action of muscles with direct vertebral insertion in the lumbar region, particularly the quadratus lumborum, the diaphragmatic pillars, and the psoas muscle [5,10,11].
In this variant, elevation of the hemipelvis induced by the latissimus dorsi is partially counteracted by the action of these muscles, which exert direct traction on the lumbar vertebrae, producing an ipsilateral lumbar vertebral convexity [3,5,11].
The result is a double-curve configuration, in which:
- the lumbar segment presents an ipsilateral vertebral convexity, expressing the direct action of lumbar muscles [3,5,10];
- the thoracic segment maintains an ipsilateral concavity, consistent with the mechanical resultant of the scapula–hemipelvis unit dominated by the latissimus dorsi.
Conclusions and continuity of the analysis
In the present contribution, Pattern A has been analysed as the expression of the dominant mechanical resultant of the latissimus dorsi and of the associated vertebral configurations in the frontal plane.
The analysis has shown how interpretation of lateral deviations and vertebral curvatures requires a clear distinction between concavities produced by convergence of skeletal elements and convexities determined by direct muscular tractions, according to a coherent vector logic.
Pattern B, characterised by a predominance of opposing resultants and by different configurations of the vertebral column, will be addressed in the second part of the article, together with the related variants and the diagnostic implications that derive from them.
This subdivision allows maintenance of a progressive and systematic treatment of the biomechanical patterns generated by the latissimus dorsi, preserving interpretative clarity and coherence of the proposed model.
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