Thoracic kyphosis, lumbar lordosis, and spinal straightening: vector analysis in the sagittal plane

Thoracic hyperkyphosis, flat back, lumbar hyperlordosis, and lumbar straightening are biomechanical patterns predictable through vector analysis of the muscles acting on the spine in the sagittal plane. This article analyses the thoracic segment T4–T6, the thoraco-lumbo-sacral segment T7–S1, and the pelvis, identifying muscular dominances, systemic compensation mechanisms, and the resulting disc compressions.

The attached PDF document, available for free download, develops the complete vector analysis with images and bibliographic references. This article is the direct continuation of the analysis of the cranio-cervico-thoracic segment.

Thoracic kyphosis T4–T6: the posterior junction

The physiological thoracic kyphosis is the posterior junction between the two functional lordoses. It extends from T4 to T6, with its apex at T5. When it follows a physiological course, the apex of the T5 spinous process is aligned with the medial border of the scapulae, and the scapulae lie at the sides of the rib cage.

The muscles acting directly on this segment are all posterior: paravertebral muscles, rhomboids, middle and lower trapezius fibres. All reduce the physiological kyphosis.

Scapular adductor dominance

Thoracic hypokyphosis is produced by the scapular adductors — rhomboids, middle and lower trapezius fibres — through scapular adduction. The balancing force is the serratus anterior, which has lower vector potential and is subdominant. Calculation using the parallelogram rule shows that the adductor vector is more than twice that of the serratus anterior.

The result: it is not the scapulae that "move outward" from the spine, but the spine that "moves inward" between the scapulae, compressing the intervertebral discs. The serratus anterior, attempting to balance adduction with the scapula as its fixed point and the ribs as its mobile point, increases the transverse diameter of the thorax and reduces the anteroposterior diameter. The thorax loses its physiological roundness and becomes ovalized.

Hyperkyphosis: an only apparent contradiction

In hyperkyphotic patterns, an apparently contradictory phenomenon occurs: T5 is still sunken inward due to the action of the scapular adductors, but the kyphotic apex has shifted caudally between T7 and T12 due to the action of the thoraco-lumbar muscles — particularly the latissimus dorsi, which with its insertions from T7 to T12 projects the vertebrae posteriorly and downward.

Hyperkyphosis is therefore only apparent. The true anatomical kyphosis has its apex at T5, and in that segment vector dominance is always toward reduction. When the apex lies below T5, it is more correct to speak of curve inversion.

Thoraco-lumbo-sacral lordosis T7–S1: all co-agonists

At the lumbar level, all muscles with direct spinal insertion are co-agonists in increasing lordosis: paravertebral muscles, quadratus lumborum, and latissimus dorsi posteriorly; diaphragm crura and psoas anteriorly. The iliacus contributes through anterior pelvic tilt.

The only antagonists are the rectus abdominis muscles, which have no direct spinal insertion and are vectorially subdominant. Their vertical force line produces only minor horizontal components. Their ability to restrain lumbar lordosis depends on their capacity to stiffen the abdominal wall.

Equilibrium is highly unstable: even modest shortening of muscles with direct spinal insertion produces modification of the thoraco-lumbar curve.

Paradoxical lumbar straightening

In some radiographic patterns, the lumbar spine appears straightened. Since locally all muscles increase lordosis, straightening is the result of the exacerbation of the very forces that produce it.

The mechanism: latissimus dorsi, paravertebral muscles, and iliacus anteriorly tilt the pelvis. If anterior tilt is further increased by the combined traction of latissimus dorsi and iliacus, the sacrum horizontalises. T7, for maintenance of upright standing, functions as a fixed point. The lumbo-sacral curve is transformed into two straight segments with an angular apex at L4–L5. Lumbar straightening is the product of the exacerbation of the forces that increase lordosis. With horizontalisation of the sacrum, a force moment is created between the sacrum and the fifth lumbar vertebra that, by projecting L5 anteriorly, may generate a listhesis.

Disc compressions

Bilateral shortening of symmetrical muscles — anterior or posterior — modifies the physiological curves. The vertical components of their forces sum together, generating mechanical compression on the intervertebral discs, distributed in different ways according to the direction of the acting vectors.

In hyperlordosis, the G and R forces applied to each vertebral body produce, through their g and r components, compressions on the discs. The forces may fail to meet on a disc, and if their junction occurs on the articular facets, mechanical compressions may develop with potentially degenerative consequences.

Pelvis: anterior and posterior tilt

The anteroposterior stability of the pelvis in upright standing is determined by two antagonist groups. Anterior tilt is produced by latissimus dorsi, paravertebral muscles, iliacus, and rectus femoris. Posterior tilt by hamstrings and rectus abdominis. The dominant vector force favours anterior tilt, mainly because of the great pulling force of the latissimus dorsi.

The most frequent patterns are: excess thoraco-lumbar lordosis with anterior pelvic tilt, and straightening of the lumbar segment associated with sacral horizontalisation and an angular fulcrum at L4–L5. Patterns with posterior tilt dominance are less frequent.

Physical foundations of the model.
This article applies the AIFIMM biomechanical model.
Its physical foundations are developed in three sequential articles, best read in order:
1. How muscle shortening generates joint conflict — why muscles shorten and the Resistant Force / Working Force model
2. Do antigravity muscles really oppose gravity? — how segmental malalignment raises Resistant Force
3. Why joint conflict develops: vector analysis of muscular forces — how the responsible forces are identified and predicted

This topic is part of the online course Systemic and Segmental MSK Biomechanics.