Professor Delmas was the first to clarify the functional value of certain vertebrae :
- the wedge-shaped character of L5 to be used as transition between a sacrum more or less horizontal and a rachis vertical.

- the role of knee-cap of D12, hinge vertebra on which the muscles of gutter have little insertion.

- but it is especially L3 having a posterior arc very developped which is useful as a point of support with ear-thorn-bush in its action of rachis erecting as soon as L3 is fixed to pelvis by the intermediary of iliac beams of longissimus.

Located at the node of lumbar lordosis with plates parallel between them, L3 is the first really mobile vertebra of the lumbar rachis. L4 and L5 strongly moored to the pelvis forming more a static transition than a dynamic one.

The use ( we will see it further on ) of L4 and L5 for dynamic purposes causes lumbagos by constraint on the posterior articulars, wear of the intervertebral discs or slipped discs.

For better understanding of the operation of the muscular system characterizing the dynamic attitude,we will initially describe the system being used for the rachis erection starting from L3 , then the system being used for the mooring of the rachis below L3.

A) Above L3 :

1. Ear-thorn-bush : muscle starting from the thorny of L3 to finish on the thorny of D2.Muscle to plates fitting on all the thornies with one particular force starting from D10 supporting the first rachis erection.
2. Longissimus : lons spindle-shaped mass located apart the previous fitting on the transverse processes of the lumbar and dorsal vertebrae until D2 and the second coasts.The higher beams of this muscle finish on the thorny and the body of L3 and sacral and iliac beams starting from the processes of L3.
3. Posterior and lower notched : which leaves from the three first lumbar vertebra thornies and from the two last dorsal ones to finish on the three or four last coasts.

B) Below L3 :

1. We already spoke about sacral and iliac beams starting from L3 processes.
2. The sacral and lombo-iliac ligaments which moor the L4 and L5 processes to iliac bone and sacrum.These ligaments limit especially the side inflection.

These two elements though significant are insufficient to explain low lumbar kyphosis necessary to the dynamics of the human body. We should integrate the concept of basin retroversion.

PELVIC RETROVERSION : there are two kinds of retroversion but are both characterized by a sacral verticalisation.

I find tempting the terminology of certain sport trainers or certain teachers in deportment.They distinguish the bascule with soft and low buttocks and the bascule with high and hard buttocks.

• Bascule with a soft and low buttocks :
  It is characterized by a total axial rotation around coxo-femoral article, with sacral lowering and total lumbar kyphosis.
  It makes take part the ischio-leg muscles (long head femoral biceps, semimembranous, semi tendinous ) which draw the ischium downwards and often, unfortunately, the right of the abdomen which draws the pubis upwards.

In this precise case the distance sternum-pubis is decreased with for corollary a diaphragmatic blocking and a discomfort for the trunk erection, factor of pubalgies, contractures or muscle strains (abdominal,ischio-leg ).
This adynamic bascule preserves the dynamics of the lower limbs when one prevents rectifying oneself and especially when one avoids serving of abdominals muscles ( skier taking the "egg position"or gardener working hunched back a long time for example ).

• Bascule with hard and high buttocks : It is essential for the dynamic attitude.The numerous pelvic-trocanteric muscles producing external rotation, support the opening of iliac wings, the sacrum verticalisation which via the setting in tension of the sacral and lumbo-iliac ligaments, raises the sacrum and tracts back L4 and L5.
  So the low lumbar kyphosis is carried out, necessary to erect the rachis from the thorny of L3.
  The last two lumbar vertebrae , source of lumbagos and slipped discs will not move any more for peoples comfort.
  Notice that, during the rachis erection , the abdominal muscles will be put in passive tension and that breathing will be free.

Notice too that the rise of the sacrum will relieve the constraints brought by the weight of the body at the coxo-femoral level.
Notice too that when the chest is erected, low lumbar kyphosis will be maintained by the trunk erecting muscles without necessiting a pelvic-trocanteric contraction.

Moving your body will be easier.