Cervical hyperlordosis, straightening, and forward head: biomechanical causes and disc compressions

Cervical hyperlordosis, cervical spine straightening, and forward head projection are predictable biomechanical patterns generated by shortening of the muscles acting on the cranio-cervico-thoracic segment. Vector analysis identifies which muscles are responsible for each pattern, how anterior neck muscles undergo action reversal, and which disc compressions result.

The attached PDF document, available for free download, develops the complete vector analysis with images and bibliographic references.

The myofunctional subdivision of the spine

Muscular insertions do not respect anatomical boundaries. Observing the arrangement of muscles acting on the cranium, spine, and pelvis, a functional subdivision emerges that differs from the classical anatomical classification:

The cranio-cervico-thoracic lordosis extends from the cranium to the spinous process of T3. The thoracic kyphosis runs from T4 to T6. The thoraco-lumbo-sacral lordosis runs from T7 to the sacrum.

The extension of cervical lordosis to T3 is determined by the posterior paravertebral muscles, levator scapulae, and upper trapezius fibres posteriorly, and the anterior neck muscles, scalenes, and sternocleidomastoids anteriorly. The superomedial angle of the scapula is located at the level of T3, creating a continuous functional unit.

This subdivision explains why cervical problems may present with symptoms involving the upper thoracic region as well.

What the posterior muscles produce

Increased basal tone and subsequent shortening of the posterior muscles produce two effects: posterior flexion of the cranium and increased cervical lordosis. The paravertebral muscles and upper trapezius fibres act directly, the levator scapulae indirectly.

Action reversal of the anterior muscles

This is the key mechanism of the cervical segment. The action of the anterior muscles changes according to the position of the cranium and the existing cervical curve.

With physiological lordosis and the occiput on the same vertical as the thoracic kyphosis, the sternocleidomastoids flex the cranium anteriorly and reduce lordosis. The anterior neck muscles — rectus capitis anterior, longus capitis, longus colli — with their force line anterior to the sagittal midline of the vertebrae, straighten the cervical segment.

With pre-existing hyperlordosis, the force line of the sternocleidomastoids passes behind the mastoid and their action reverses: they posteriorly flex the cranium, contributing to the increase in lordosis together with the scalenes. Similarly, the anterior neck muscles, with the force line shifted posterior to the sagittal midline, increase lordosis instead of reducing it.

All muscles of the segment — anterior and posterior — become co-agonists in increasing lordosis. The system loses its capacity for self-regulation.

The forward head mechanism

When the muscles that increase lordosis are dominant, they also produce posterior flexion of the cranium, with the gaze directed upward. To restore horizontal vision, the sternocleidomastoids, scalenes, and anterior neck muscles enter into synergy and project the cranium and first cervical vertebrae forward, straightening the cervical spine obliquely.

Horizontal vision is restored, but the cranial centre of mass is no longer aligned with the vertebral body of T3. A moment of force is generated: with a cranium weighing approximately 5 kg and a distance of 3 cm between the cranial centre of mass and the point of vertebral counterforce, the required extensor moment is 15 kg·cm. The posterior muscles must further increase tension to balance it. A self-reinforcing loop is established.

Disc compressions

In both hyperlordosis and straightening with anterior head projection, the G forces and R reactions generate force moments on the vertebral segments and compressions with their g and r components on the intervertebral discs. The vertical components of the oblique muscles sum bilaterally, producing disc compression.

Diagnostic test: cranial traction

Cranial traction in the supine position differentiates muscular dominance. If traction tends to normalise the spine, posterior muscle dominance may be hypothesised and the manoeuvre can become a therapeutic tool. If traction projects the spine forward — even by a single vertebra — anterior muscle dominance is indicated and repeating the manoeuvre in therapy would aggravate the condition.

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.