Posture
The distinctive characteristics that differentiate man from the apes in its history of evolution, was bipedalism, or standing on two feet. It was confirmed with the discovery in 1925 by Raymond Dart, of a primitive child’s skull known today as the Taung Child. This skull had a distinctive feature that differentiated it from apes’ skulls, the foramen magnum through which the spinal cord travels, was further forward than in an ape, indicating that the head was erect, and the creature stood upright. This was known as the discovery of Australopithecus africanus (Simons, 1989).
Morphological features are intimately related to biomechanics, “in particular by causal morphogenesis (‘Wolff’s law’) and by the interplay of mutations and selection (Darwin’s ‘survival of the fittest’)”. Every skeletal component must endure the loads to which it is subjected. The most demanding ones occur during postural and locomotion tasks, to counteract the forces of body weight. In the skull, however, the greatest loads arise during biting and chewing (Preuschoft, 2004). In a study of young adults, a variation of the magnitude of the bite-force resultants ranged from 246.9 to 2091.9 N, with a median of 776.7N (Hattori et al., 2009).
A bipedal posture does not represent a stable biomechanical posture. The challenge to the balance control system is that two-thirds of the body mass and two-thirds of body height are located at the top of the legs well above the ground. This promotes instability that requires constant adjustments of the postural system to maintain a stance (Winter, 1995).
The definition of posture is:
The definition of stability is:
A greater stability is generally associated with smaller amplitudes and velocities of the COG displacement (Danis, Krebs, Gill-Body, & Sahrmann, 1998).
Autoregulation reflexes provide the balance control from sensory inputs coming from the eyes, ears, muscles, and joints relative to the environment. The processing of this inputted information depends on the brain and the brainstem, whereby the majority of neuromusculoskeletal disorders arise from degeneration in the balance control system. The adaptability of the Central Nervous System (CNS) may mask a pathology until the compensation system ceases to make up for a loss of function (Byl, 1992).
The CNS operates balance control through the synergetic activation of muscles at specific joints, based on its organization of the sensory afferences from visual, somatosensory and vestibular inputs. Noteworthy, is that the response latency to a balance perturbation is different and slower than from visual cues (200 msc), when compared to the faster somatosensory response (80 - 100 msc) (Samuel, Solomon, & Mohan, 2015).
The maintenance of the body stance is under multisensory control. When the eyes are closed, this control recruits kinesthetic and vestibular information relative to gravity, in order to maintain the vertical orientation of the body (Hlavačka, 2003).
Two main modes of postural control by the CNS have been described (Deliagina, Orlovsky, Zelenin, & Beloozerova, 2006):
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The Feedback or Reflex Mode: When the CNS corrects the posture in response to a perturbation of orientation.
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The Anticipatory Mode: When the CNS changes posture in advance to counteract some expected external influences, or in response to a voluntary limb movement.
The motor systems in the Central Nervous System (CNS) involves the cerebral cortex, the brainstem, the spinal cord, the cerebellum, and the basal ganglia. The cerebral cortex contains the motor areas which efferently travel from the brainstem and spinal cord to activate muscles. Descending motor pathways from the brainstem modulate, within the spinal cord, all motor activity. The brainstem integrates the information from the cortex and cranial nerves to maintain posture, dependent upon the myostatic stretch reflex, which is a monostatic reflex. The basal ganglia receives input and integrates this information for all cortical levels before communicating with the premotor and motor areas of the cortex to generate outputs (Stack & Sims, 2009).
Studies of subjects with cerebellar damage provide insight into the understanding of the cerebellum’s role in postural control. Typically, cerebellar damage leads to an increase in postural sway reflective of increased postural instability. The medial cerebellar regions play a primary regulatory role in extensor muscle tone, upright stance maintenance, control over dynamic balance, and modulation of flexor and extensor muscles in the locomotor pattern. The intermediate cerebellar regions have little importance for stance and posture, but instead they direct limb placement and regulate pair control over agonist-antagonist muscle pairs that control limb movements, notably when precision is required. The lateral cerebellum plays a role in making adjustments during novel contexts, or when visual guidance is required in locomotor patterns (Morton & Bastian, 2004).