The problem of balancing

Cone Of Economy. Balancierung des Körpers

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The upright gait requires balancing a relatively large amount of body mass on a rather unstable base (2 legs) with as little effort as possible. However, this “posture” is a highly dynamic process and requires constant readjustment of the weight distribution. From a technical point of view one can observe this at Segways, where constant motor action is required for rebalancing to keep the rider upright.

For the limb joints, this means a dynamic interplay between agonists and antagonists. We refer to the most energy-efficient of these dynamics as the locked state (e.g., fully extended knee joint). However, if the balance is disturbed (e.g., lifting the arm), muscle activity is again required (e.g., peroneal group).

In the torso area, there is a continuous shift of the center of mass around a center – this is also described as Cone Of Economy (COE) in the Anglo-Saxon world. The amplitude between left and right is usually greater than between front and back. The COE is individually different and a chaotic system in that the movements cannot be predicted linearly. This has the advantage that randomly occurring disturbance events (sudden turn to a stimulus, stumble, etc.) can be reacted to very effectively. The response to the disruptive event is always from a different initial state and the body is forced to generate a dynamic response. This skill is learned early in childhood – the successful learning outcome can be observed in the individuality of the COE.

individual cone of economy
The individual Cone Of Economy (COE, blue circles) is larger to the left/right than to the back/front

The COE is challenged in the presence of disease-related disruption of the perpendicular posture. This becomes clear in almost all neurodegenerative diseases (e.g. Parkinson’s disease, ataxias, dementia). Characteristically, the head plumb bob is not centered over the center of the body, but falls in front of the feet. Thus, the individual COE is abandoned and upright posture becomes an energetically costly endeavor. The first symptoms of this are back pain, myogelosis in the back muscles, etc. In advanced stages, it then also becomes a problem for patients to cover longer distances, as even the upright gait is no longer economical.

From the orthopedic field, we are familiar with such postural disorders in contractures of the lower extremities and, of course, in almost all spinal disorders. We are talking here about increasing kyphosis, by:

  • Disc herniation/degeneration. Due to the loss of height of the intervertebral disc and constant height of the dorsal elements (facet joints), kyphosis inevitably occurs.
  • Spinal stenosis. Foraminal and spinal constrictions force kyphosis to access intraspinal reserve spaces. Otherwise, there would be a risk of compression of the spinal nerves (pain, loss of function).
  • Vertebral body fracture. Analogous to disc degeneration, kyphosis occurs due to the loss of height of the ventral structure (vertebral bodies) relative to the dorsal elements (facet joints).
  • M. Ankylosing spondylitis. Inflammatory enthesiopathy of the longitudinal ligaments always leads to length reduction ventrally and thus to kyphosis.

Scolioses occupy a special position. Idiopathic scoliosis is a growth disorder of the adolescent. Pathogenetically, in most cases, longitudinal growth of the ventral column predominates over the dorsal structures (spinal cord, vertebral arches, ligaments), so that lordosis tends to occur here. In adult de novo scoliosis, on the other hand, the pathogenesis is based on the degenerative processes described above, so that kyphoses are also found here.

violation of cone of economy
If the Cone Of Economy is breached due to illness (red), balancing is no longer successful. Standing and moving becomes an exhausting endeavor for the muscles.

Kyphosis can remain balanced, meaning the patient succeeds in keeping the center of mass of the trunk perpendicular, or sagittal decompensation occurs. In addition, scolioses (idiopathic and adult) are characterized by the possible presence of an additional coronal imbalance (right/left). It has been demonstrated that such spinal deformities are always associated with increased muscle activity (erector spinae, gluteus) and increased amplitude of the COE (Haddas et al. “A method to quantify the cone of economy” Eur Spin J 2018). Symptomatic (painful) balance decompensations are an expression of a vicious circle of muscular overuse with collapsed homeostasis of the COE and usually require corrective treatment.

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