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Course : PG Pathshala-Biophysics
Paper 13 : Physiological Biophysics
Module 05 : Posture and Equilibrium
Principal Investigator: Dr. Moganty R. Rajeswari, Professor
AIIMS, New Delhi
Co-Principal Investigator: Dr. T. P. Singh, Professor
AIIMS, New Delhi
Paper Coordinator: Dr. K. K. Deepak, Prof. & HOD, Physiology
AIIMS, New Delhi
Content Writer: Dr. Chitturi Vinay, Sr. Resident
AIIMS, New Delhi
Content Reviewer: Dr. Renuka Sharma, Professor
VMMC & SJH, New Delhi
Learning Objectives
Postural Equilibrium and Postural Orientation
Biomechanics of posture
Postural Responses
Sensorimotor integration in postural control
Neural centres involved in postural control.
Postural abnormalities
Introduction
A system is said to be at equilibrium if the competing forces across it are balanced. A body is said to be at physical equilibrium when all the forces acting on it are in balance. Postural equilibrium involves active balancing of the external forces and torques acting on the body. Postural orientation describes the orientation and alignment of body parts with respect to each other as well as the environment. Bipedal stance, locomotion, gait and volitional movements present significant postural challenges that need to be overcome for efficient execution of the desired activity. The postural
system acts to correct imbalances caused by external displacing forces or internally generated imbalances by the subconscious adjustment of tone in postural muscles.
- Biomechanics of Posture The human body can be considered to be made of multiple articulated skeletal segments. Postural control requires the maintenance of the orientation of these segments with each other and also the body position as a whole with respect to the environment. Postural balance involves control of the position and movement of the body’s centre of mass (COM). The COM represents the average location of the body’s mass and represents the point at which external translational forces and the moments of rotational forces act. In the standing and supine positions, the COM is located at the abdomen, approximately 2 cm anterior to the L2 vertebra. The body is also supported by its points of contact with the external environment which form a base of support. The centre of mass must lie within the base of support for postural equilibrium. Stability of the equilibrium is greater when the support base has higher surface area and decreases with increasing height of the COM from the base of support. The body is subject to external constraints imposed by physical forces like gravity and reaction forces from contact surfaces and internal constraints placed on the body due to the articulations between the segments of the body, inertia due to the mass of body segments and internal forces generated by muscular contraction. Postural balance and orientation is subject to a dynamic interplay of the constraints imposed on the body.
Figure 1 : The centre of mass (COM) is projected unto a support base formed due to reaction forces at points of contact. Human bipedal stance has a narrow support base
Gravity is the dominant external force acting on the body. Humans have developed a bipedal stance in which the entire body weight is borne on a narrow support base formed by the feet. The bipedal stance frees up the upper limbs for the performance of complex tasks requiring fine motor control and dexterity. In the upright position, the centre of mass has to be maintained at a height well above the support base formed by the feet.
C O M
Figure 3: Anti- gravity postural muscles of the back and lower limbs
- Postural Responses Postural reflexes can correct imbalances caused by external disturbances that can displace the COM away from the support base. Postural responses can be in the form of a restorative force that brings the COM back over the support base, e.g. Contraction of plantar flexors of angles to balance the body when the feet are displaced backwards in a fixed position while standing in a decelerating vehicle. Alternatively, the support base can be extended by spreading the legs or using the arms to draw support from an external surface, e.g. usage of support bars in a moving bus. The postural responses are developed in muscles whose contractions lead to the generation of a restoring force against the disturbance that elicited the response. The muscles involved in postural responses exhibit directional tuning- they exhibit differential activity based on the direction and amplitude of the postural disturbance. The muscles of the body possess different tuning curves. A Postural response recruits a synergy of differently tuned muscles as per the biomechanical requirements of the postural equilibrium or orientation. A postural synergy represents a specific pattern of activation in a group of postural muscles for the purpose of counteracting imbalances. Synergies represent motor modules that can be recruited in different combinations depending on the kinematic and kinetic requirements of the postural control. Postural responses can be integrated at different neural levels, as given in Table 1
Table 1: Common Postural reflexes
Reflex Sensor Stimulus Response Centre Stretch Reflex Muscle spindle Muscle stretch Contraction Spinal cord Supporting Reaction
Proprioceptors Contact with limb
Extension of limb to support body
Spinal cord
Labyrinthine righting reflex
Saccule(vestibular apparatus)
Movement of head
Head kept level Midbrain
Optical righting reflex
Photoreceptors External visual cues
Head kept level Cerebral Cortex
Stretch Reflex
The stretch reflex is a monosynaptic spinal reflex for the control of a muscle’s length. The muscle spindle is a sensory organ encased within the muscle. It contains specialised intrafusal fibres which are innervated by sensory neurons. These fibres are stimulated by stretch of the muscle. The stretch reflex arc has a single synapse between the sensory and motor neurons. Activation of the α motor neuron then leads to contraction of the extrafusal fibres. (Figure 4)
Figure 4: Stretch reflex
The ends of the intrafusal fibres are contractile and innervated by γ- motor neurons. γ- motor stimulation adjusts the length of the spindle, its sensitivity to the stretch. Muscle tone is determined by the activity in α and γ- motor neurons which are under the control of the descending motor tracts from supra spinal centres/
Sensory systems in postural control Information from multiple sensory systems is integrated to generate a sensory representation of the body’s position in space and also the relative orientation of the segments of the body. Three primary systems are involved in postural control
Somatosensory System
Vestibular System
Visual System
Somatosensory System : Proprioceptive afferents from sensory organs like muscle spindle, golgi tendon organs, cutaneous receptors and joint receptors contribute sensory information enabling the generation of an internal model of the orientation of body segments. Visceral proprioceptors are usually constantly stimulated due to gravity and help to sense any tilt in the upright posture or acceleration along the long axis of the body. Large diameter Ia afferents are involved in maintenance of the upright standing posture
Vestibular System : The vestibular system consists of the otolith organs (Utricle and Saccule) and the 3 semicircular canals in the inner ear. The Utricle and Saccule are sensitive to linear acceleration in the horizontal and vertical planes respectively. The semicircular canals are stimulated by rotational acceleration along the planes they are respectively aligned to. The vestibular system can sense the relative position of the body with respect to the gravitational vertical, thus allowing the construction of a frame of reference for body orientation in space.
( For better clarity, please add a diagram here to show the orientation of SC Canals, and also mechanism of stimulation of haircells—attaching a few pix for your reference ) –
Separate module for vestibular contribution to posture, I have chosen to briefly summarise here, Please link to the other module
Visual System : The visual system supplies information about external space. Visual information guides the anticipatory postural responses during the performance of voluntary activities like walking and running. The visual flow – rate and direction of the translation or rotation of the visual scene across the retina contributes information of body position and movement. The visual system can also construct a frame of reference based on prior knowledge of positions of objects in the visual field. Visual system can provide feedforward control based on information about potentially destabilizing scenarios in the environment
Figure 6 : Sensory Integration of visual, vestibular and proprioceptive systems
There is sensory integration of the inputs derived from the three sensory systems. Input from a single modality can be ambiguous. The visual system cannot discriminate between head motion and the movement of the visual scene. The vestibular system cannot completely distinguish a head tilt from the body being externally accelerated. The somatosensory system cannot distinguish between similar body postures brought about by different biomechanical events. Sensory integration weights the sensory inputs according to their relevance and accuracy in a given situation and leads to the generation of internal models of body position and orientation. Excessive stimulation of any sensory modality can lead to a sensation of imbalance and inability to maintain posture – giddiness felt after rapid spinning due to excessive vestibular stimulation.
tune the postural responses that accompany voluntary movements and facilitates their adaptation to rapidly changing environments.
Cerebellum
The cerebellum’s function in posture can be studied through lesions in different functional regions of the cerebellum. Lesions of the lateral cerebellum affect arm and hand coordination during movements. A vestibulocerebellar lesion produces vertical instability. The anterior lobe of the cerebellum is involved in controlling the magnitude of postural responses. Lesions in the anterior cerebellum lead to hypermetria, postural responses with larger amplitudes and duration are produced. Antagonist muscles are then recruited to terminate the postural response. This is also poorly controlled leading to the production of a tremor and difficulty in execution of rapidly alternating movement – dysdiadochokinesia.
Cerebral Cortex
The cerebral cortex is responsible for the initiation of voluntary movements. The voluntary movements are accompanied by anticipatory postural responses that stabilise the body and provide a stable platform for the execution of purposeful muscle contraction. The supplementary motor cortex has been implicated in generation of anticipatory postural responses. The temporoparietal cortex is responsible for generation of internal models of postural orientation by sensory integration from somatosensory, vestibular and visual systems. Anticipatory postural movements are a part of the motor plan generated for movement. An efferent copy of the motor plan is also sent to the integrating sensors for comparison with sensory input. This helps to detect errors between the planned movement and the actual movement executed and leads to the generation of a appropriate corrective strategy. Reciprocal connections between motor cortices, cerebellum and basal ganglia are responsible for motor learning.
- Abnormalities of Posture and Equilibrium
Maintenance of a posture and balance is a prerequisite for performance of motor tasks. The postural control system is affected by disorders which affect the sensory systems or/and the neural centres involved in the control of posture. Vertigo is a sense of spinning of imbalance usually associated with vestibular disorders. Vertigo must be distinguished from other forms of dizziness like light- headedness and syncope.
Ataxia is a disorder in motor coordination affecting gait, balance and speech. Ataxia is classified based on the sources involved in the disorder
Sensory Ataxia
Sensory Ataxia is produced by disorders affecting the somatosensory system.
Peripheral neuropathy involving the large diameter sensory afferents for proprioception caused by diseases like vitamin-B12 deficiency and Diabetes mellitus.
Syphilitic Tabes dorsalis affects the spinal dorsal column pathway involved in proprioception.
Normally, sensory information derived from visual and vestibular system compensates for the lack of proprioceptive input and motor coordination and balance is maintained when subject can see the movements performed or external visual cues which help in maintenance of postural orientation. However, the subject faces difficulty in maintaining balance with the eyes closed – Romberg’s sign.
Vestibular Ataxia.
Unilateral vestibular dysfunction results in an imbalance of vestibular input leading to a sensation of vertigo, nausea and vomiting. Bilateral vestibular dysfunction doesn’t produce vertigo but is associated with disequilibrium.
Cerebellar Ataxia
Cerebellar ataxia produces a range of symptoms a wide based gait, lateral instability, inability to walk along a straight line while placing the foot right before the other foot, dysmetria and dysdiadochokinesia. Acute alcohol intoxication produces symptoms resembling cerebellar ataxia.
Parkinsonism
Parkinsonism is a group of motor disorders characterised by resting tremors, bradykinesia and rigidity and postural imbalances. A stooped posture, mask like face and shuffling gait is usually observed. There is a tendency to fall over due to inadequacy of the step size during gait.
Motion Sickness
Motion Sickness is produced when there is discordance between the vestibular and visual inputs to the medullary postural centres. In a closed cabin in a car or ship, there is vestibular stimulation due to the vehicular motion in the absence of a changing visual scene. This produces a sensation of nausea, vomiting and dizziness.
Abnormal Posturing
Involuntary postures are produced due to brain injury. The brain stem motor nuclei are under inhibitory control from descending influences from the cerebral cortex. Brain stem lesions abolishing this higher control release the descending tracts from inhibition leading to unopposed activity in the muscle groups they innervate leading to the production of abnormal postures. A lesion above the superior colliculus in the cerebral cortex produces decorticate rigidity and midcollicular lesions of the midbrain produce decerebrate rigidity.
