This informal CPD article ‘Vector Analysis of the Vertebral Column in the Frontal Plane – Part 2’, 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.
Pattern B of the Latissimus Dorsi, Vertebral Variants and Biomechanical Interpretation of Scoliosis
In the first part of this article, the principles of vector analysis applied to the vertebral column in the frontal plane were introduced, and Patterns A and B were described as expressions of different mechanical resultants generated by the latissimus dorsi and its functional unit [1–5].
In this second part, these principles are extended to more articulated configurations, while maintaining the same interpretative criterion: vertebral deviations are interpreted as predictable outcomes of direct muscular tractions or of indirect mechanical resultants produced by the displacement of other skeletal segments [2–6].
9. Pattern B of the Latissimus Dorsi: the scapular elevation pattern
In the first part of the article, the principles of vector analysis applied to the vertebral column in the frontal plane were introduced, and Patterns A and B were described as expressions of different mechanical resultants of the latissimus dorsi and its functional unit [1,3,4].
In this section, Pattern B—more frequently observed in clinical practice—is examined in greater detail, analysing its mechanical sequence, main variants, and diagnostic implications [3–7].
9.1 Initial mechanical sequence of Pattern B
The first phase of Pattern B is represented by elevation of the shoulder girdle, determined by the dominance of the upper fibres of the trapezius and the levator scapulae [8–11]. Elevation of the scapula is associated with an ascending clavicle and constitutes the initial mechanical event from which the pattern originates [8,9,12].
9.2 Onset of upper thoracic lateral convexity
Elevation of the shoulder girdle initiates lateral convexity of the thorax at the level of the upper thoracic vertebrae (T4–T7) [3,5,6,13]. At this stage, thoracic convexity represents a direct consequence of scapular elevation and of the traction exerted on the upper vertebral segments [3,6,13].
9.3 Role of the thoraco-humeral fibres of the latissimus dorsi
With the establishment of thoracic convexity, the thoraco-humeral fibres of the latissimus dorsi assume an oblique orientation [1,4,14]. The combination of a convex thorax and oblique fibres of the latissimus dorsi stabilises the scapula in an elevated position, thereby fixing the pattern [4–6,14]. The thoraco-humeral fibres also contribute to lateral expansion of the thorax, amplifying thoracic asymmetry [6,14,15].
9.4 Pattern B: the elevation pattern
Pattern B is characterised by the combined action of:
- upper fibres of the trapezius,
- levator scapulae,
- rhomboids,
- upper portions of the latissimus dorsi [3,8–11,14].
The overall resultant determines:
- elevation and adduction of the scapula,
- elevation of the clavicle,
- ipsilateral lateral thoracic convexity (T4–T12),
- elevation of the hemipelvis due to the action of the inferior fibres of the latissimus dorsi [3–6,14,16].
This results in an associated pattern composed of:
- elevated shoulder,
- lateral thoracic convexity,
- elevated hemipelvis [3–6,16].
9.5 Mechanical consequences of scapular elevation in Pattern B
The rhomboids and middle fibres of the trapezius, by elevating and adducting the scapula, produce an ipsilateral thoracic vertebral convexity through direct traction on the vertebrae [3,6,9,17].
The serratus anterior increases its tension in an attempt to oppose scapular adduction and elevation [18,19]. Being functionally subordinate, the scapula becomes its fixed point, while the ribs constitute the mobile segment [18,19].
Traction exerted by the serratus anterior therefore manifests as lateral expansion of the thorax, further contributing to upper thoracic convexity [18–20].
9.6 Variants of Pattern B: double curve and single curve
Double-curve variant
If, at the lumbar level, the mechanical resultant generated by elevation of the hemipelvis prevails, a double vertebral curve is observed [3,5,6,16]:
- lumbar concavity as a mechanical effect of pelvic elevation,
- thoracic convexity directly determined by traction of the upper fibres of the latissimus dorsi and the scapular adductors [3–6,14].
Single-curve variant
If the fibres of the quadratus lumborum, the diaphragmatic pillars, and the psoas are shortened together with the latissimus dorsi, they oppose the mechanical resultant of hemipelvic elevation [5,16,21–23].
In this case, the lumbar spine may remain aligned or present ipsilateral convexity, despite elevation of the hemipelvis [5,16,21]. This results in a broad dorso-lumbar curve, with direct convexity determined by muscular traction [3,5,21–23].
9.7 Diagnostic principles of Pattern B
From a diagnostic perspective, muscles with direct action on the vertebrae are dominant in determining vertebral deviations, compared with deviations produced by indirect mechanical resultants [2–6].
Accordingly, the following definitions apply:
- convexity: deviations produced by direct muscular traction;
- concavity: deviations resulting from mechanical resultants consequent to displacement of other skeletal segments [3–6].
Apparently incongruent patterns, particularly at the lumbar level, can be explained by the presence of active muscular forces opposing the expected mechanical resultants, with consequent stiffness and increased discal compression [5,16,21,24].
9.8 Clinical frequency of the two patterns
In clinical practice, Pattern B is significantly more frequent than Pattern A [3–6]. In Pattern A, a single dominant line of force prevails, oriented from the iliac crest to the humerus, requiring specific biomechanical conditions to emerge as a primary shortening [1,4,14].
In Pattern B, multiple dominant lines of force coexist:
- vertebrae–iliac crest,
- vertebrae–humerus,
- ribs–humerus [3–6,14].
The presence of multiple active resultants, distributed across different skeletal segments, makes Pattern B biomechanically more stable and clinically more frequent than Pattern A [3–6,24].
10 Diagnostic considerations for the lumbar segment
Lumbar concavity generally represents the mechanical effect of elevation and/or rotation of the hemipelvis, both determined by the combined action of the latissimus dorsi and the quadratus lumborum [5,16,21].
Lumbar convexity, by contrast, expresses direct muscular traction exerted mainly by:
- quadratus lumborum,
- diaphragmatic pillars,
- psoas [21–23].
Vertebral convexity
11 Diagnostic synthesis of the four independent curves in the frontal plane
The deviations observable in the frontal plane can be traced back to four independent curves, each determined by specific muscular dominances and distinct causal mechanisms [3–6,13].
Diagnostic synthesis of vertebral curves in the frontal plane
C1–C5 segment
At the upper cervical level, vertebral convexity is primarily determined by the levator scapulae. Shortening of this muscle produces ipsilateral deviation of the upper cervical vertebrae, typically associated with elevation and adduction of the scapula. Concavity at this level may be influenced by the scalenes, whose action contributes to convexity of the upper ribs and participates in cervical alignment.
C6–T3 segment
In the cervico-thoracic junction, the rhomboids and the middle fibres of the trapezius exert direct traction on the vertebrae, producing ipsilateral convexity. This configuration is commonly associated with elevation and adduction of the scapula.
T4–T12 segment – Pattern B of the latissimus dorsi
In the thoracic segment, Pattern B of the latissimus dorsi generates ipsilateral lateral convexity.
This deviation is associated with elevation of the scapula and elevation of the hemipelvis, forming a mechanically coherent pattern.
T4–T12 segment – Pattern A of the latissimus dorsi
In Pattern A, thoracic concavity represents the mechanical resultant of latissimus dorsi traction, combined with a descending clavicle and elevation of the hemipelvis.
L1–L5 segment
At the lumbar level, vertebral configuration reflects the combined effects of hemipelvic elevation and rotation, involving both Pattern A and Pattern B of the latissimus dorsi.
When direct muscular traction predominates, lumbar convexity may be produced by the quadratus lumborum, diaphragmatic pillars and psoas.
12. Scoliosis: application of physical principles
12.1 Aetiological and biomechanical premises
The aetiology of idiopathic scoliosis remains substantially unknown despite decades of research [25–27]. Numerous hypotheses—genetic, neurological, biomechanical, metabolic, hormonal—have been proposed, yet none currently provides an exhaustive explanation for the onset and progression of the condition [25–28]. Clinically, curve progression is observed to be more rapid during pubertal growth and tends to stabilise, though not necessarily cease, after skeletal maturity [26–28].
The biomechanical analysis that follows does not aim to explain the causes of scoliosis, but rather to describe the vector mechanisms through which vertebral deviations organise themselves and can be interpreted from a muscular perspective, using the principles presented in the preceding sections [3–6].
12.2 An interpretative criterion: the rotation/deviation relationship
In biomechanical analysis of scoliosis, observation of the relationship between vertebral body rotation and lateral deviation can provide indications regarding curve behaviour [5,6,29]. As described in the previous sections, physiologically vertebral body rotation is contralateral to lateral deviation from the midline [5,6,29]. In some scolioses, this relationship is reversed: rotation and convexity become ipsilateral [25,29].
This clinical observation suggests a possible interpretative criterion:
Hypothesis 1
When vertebral body rotation remains opposite to lateral deviation (physiological pattern), some degree of modifiability through work on the muscular system may exist.
Hypothesis 2
When vertebral body rotation is ipsilateral to lateral deviation (non-physiological pattern), the curve is likely to be structured and poorly responsive to direct muscular intervention.
This criterion represents an empirical observation and does not replace orthopaedic radiological classifications (Lenke, King–Moe), but may serve as an additional functional indicator.
12.3 Interaction between frontal deviations and sagittal alterations
When asymmetric forces are exerted by anatomically symmetrical muscles, lateral deviations of the vertebral column may be produced:
- directly, through muscular traction on vertebrae with direct insertion;
- indirectly, as mechanical resultants of approximation or displacement of skeletal segments [3–6].
In both cases, contralateral muscles do not remain neutral [21,24]. They increase their tension in an attempt to oppose lateral deviation, consequently undergoing shortening of their connective tissue components [21,24].
From a vector perspective, this condition causes the vertical components of the muscles on both sides of the spine not to cancel each other out, but to summate [5,21,24]. The summation of vertical components alters the physiological sinusoidal profile of the spine in the sagittal plane as well, producing stiffness and modifications of normal curvature [5,24,30].
12.4 Systematic classification of muscular actions
Direct and indirect muscular actions on vertebral configuration
Upper trapezius fibres
The upper fibres of the trapezius produce ipsilateral head inclination and shoulder elevation as direct actions. Indirectly, they may generate contralateral cervico-thoracic vertebral convexity.
Sternocleidomastoid
This muscle directly contributes to ipsilateral shoulder and clavicle elevation, ipsilateral head inclination and contralateral head rotation. Its indirect action manifests as contralateral rotation of the cervical vertebral bodies.
Scalenes
The scalenes directly induce contralateral rotation of vertebral bodies from C1 to C7 and ipsilateral lateral convexity of the cervical spine.
Levator scapulae
Direct traction of the levator scapulae produces ipsilateral convexity of the upper cervical vertebrae (C1–C4).
Rhomboids
The rhomboids exert direct traction producing ipsilateral convexity of the vertebrae from C6 to T4, associated with contralateral vertebral body rotation.
Middle fibres of the trapezius
These fibres produce ipsilateral convexity of the vertebrae from C7 to T3, with associated contralateral vertebral body rotation.
Lower fibres of the trapezius
The lower trapezius fibres contribute to ipsilateral convexity of the thoracic vertebrae from T2 to T12, accompanied by contralateral vertebral body rotation.
Latissimus dorsi
The thoracic portion (T7–T12) produces ipsilateral convexity of the thoracic vertebrae and contralateral vertebral body rotation. The humerus–iliac crest portion directly elevates the hemipelvis and depresses the shoulder, indirectly generating ipsilateral concavity of the thoraco-lumbar vertebrae. The lumbar portion (L1–L5) produces ipsilateral convexity of the lumbar vertebrae with contralateral vertebral body rotation.
Quadratus lumborum
This muscle produces ipsilateral convexity of the lumbar vertebrae (L1–L4) with associated contralateral vertebral body rotation.
Diaphragm
The diaphragm contributes to contralateral rotation of the vertebral bodies from L1 to L4 and ipsilateral lumbar convexity.
Psoas
The psoas produces contralateral rotation of the vertebral bodies from T12 to L5, associated with ipsilateral lumbar convexity.
13 Summary of the articles
Vector analysis applied to the frontal plane allows vertebral deviations to be interpreted as expressions of mechanical resultants produced by specific muscular dominances [3–6].
The distinction between convexity, determined by direct muscular traction on the vertebrae, and concavity, as a mechanical resultant of displacement of other skeletal segments, represents a fundamental causal criterion [3–6].
In scoliosis, the relationship between vertebral body rotation and lateral deviation provides functional information regarding curve behaviour [25,29]. The sagittal alterations observed can be interpreted as consequences of the summation of vertical components of bilaterally shortened muscles [5,21,24,30].
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