Balance systems of human body

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Balance is a feeling derived from being whole and complete; it's a sense of harmony. It is essential to maintaining quality in life and work. 

Independence in the functional ability is essential to health and well-being. A key feature of our independence is mobility. Mobility is defined as the ability to independently and safely move oneself from one place to another. Mobility includes many types of tasks like the ability to stand up from the bed or  chair, to walk or run, and to navigate through often quite complex environments.

Balance is defined as the ability to maintain the body's equilibrium by maintaining center of mass over its base of support. Balance can be classified into static balance and dynamic balance.

• In static balance, the body maintains posture in fixed position in which center of mass is over the base of support and body is at rest. For example: squatting, two leg stance, one leg stance, painting while standing, washing utensils in basin while standing, etc.
• In dynamic balance, the center of mass is maintained over the base of support while body parts are moving. This is more challenging because the person has to maintain equilibrium of body without falling while performing any gross motor moving activities. For example: jogging, forward or backward or sideways walking, running zigzag, playground sports activities, climbing stairs etc.

Development of postural control and balance in our body : 

• The early development of infant follows a predictable sequence of motor behaviour referred to as motor milestones. As the infant progresses and achieves next milestone (skill development), they may appear to regress to earlier form of behavior as new, more mature and adaptive versions of these skill emerge.
• According to Reflex and Hierarchial theories of motor control, maturity of higher cortical structure occurs with integration of reflexes controlled at lower levels within CNS. This integration will lead to more functional postural and voluntary motor responses.
• According to dynamic system and ecological theories, postural control emerges from complex interactions of musculoskeletal and neural systems. The organisation and responses of the postural control system is determined by both the task and the environment.

• Primitive reflexes or lower level reflexes (ATNR, STNR, TLR, etc) allows the baby to make body movements before the cortex is fully matured. These primitive reflexes are essential is developing tone in infant's body i.e. trunk, upper limbs and lower limbs. It causes activation of muscle groups in synergy patterns (tonic movements) i.e. a movement of head causes an automatic movements in the limbs, hands or feet.
• Later, these reflexes (as the infant grows and achieve milestones) contributes in development of vestibular, proprioceptive, visual, tactile, auditory and perceptual systems in body. These are essential in developing head and trunk control, dissociation of trunk from limb movements and also dissociation of waist up movements from waist down movements.
• Simultaneous integration of the tonic reflexes and movements (by 6 months)  occurs alongwith development of righting reactions. These righting reactions produces orientation of the head in space and orientation of body in relationship to the head and ground. These reactions are considered automatic reactions that enables a person to assume the normal standing position and maintain stability when changing positions. 3 righting reactions are optical righting reactions, labyrinthine righting reactions and body-on-head righting reactions. These reactions develops the vestibular and proprioceptive senses of body.
• Further cortical maturation occurs with development of balance (equilibrium) reactions and protective reactions. Balance emerges is association with a sequentially organized series of equilibrium reactions, including tilting reactions, used for controlling the center of gravity in response to tilting surface. Tilting responses are purported to emerge first in prone, then supine, then sitting, then emerge in all fours and finally in standing. The protective extension responses protects the body from injury during a fall and develop first in the forward direction, then sideways and then backwards.
• Equilibrium reactions, protective reactions and Labyrinthine righting reactions remains lifelong. Efficient working of these reactions, nervous system and musculoskeletal system is essential for developing static and dynamic balance alongwith achieving higher level skills in mobility tasks.  Mobility within the environment requires proper functioning  and coordination between static and dynamic balance systems.

Balance systems contributing in controlling and maintaining our balance are :

A) The three primary peripheral sensory inputs contributing to postural and balance control are the bilateral receptors of the somatosensory, visual and vestibular system. 

1) Somatosensory system: 

• The feet, ankles, knee, hip, back, neck and eye muscles all furnish useful information for balance maintenance. Somatosensation is the dominant sense for the upright posture control and is responsible for triggering an automatic postural response.
• The somatosensations includes various
• Touch sensations such as light touch, vibration, firm pressure, temperature and texture discrimination.
• Proprioceptive sensations such as sensing the location, position and movement of the body part and/or joints.
• Special sensors sensitive to stretch, force or pressure in muscles, tendons and joints send input signals to brain about joint positions with respect to ground and trunk. 
• Conscious joint position sensation is carried by dorsal column medial lemniscus pathway and transmitted at the end to primary somatosensory cortex.
• Unconscious joint position sensations are carried by spino-cerebellar pathways and transmitted at the end to cerebellar cortex. 
• Detection of body sway by somatosensory system (mainly by proprioceptive senses in joints) provides feedback for automatic postural modification and responses.
• Disorders with affected sensations are peripheral nerve disease, Charcot marie tooth disease, diabetic peripheral neuropathy, GBS, Leprosy, posterior cord syndrome, spinal cord lesion etc.

2) Visual system: 

• Peripheral visual system allows access and orientation to the wider environment, thus aids in making specific postural adjustments by feed-forward  or anticipatory postural response. 
• Vision helps you to see where your body and head is in relationship to the world around and to sense motion (of self and other moving object) between observing person and environment. 
• Incoming visual information from the environment are processed and stored in form of visual-perceptual memory related to any object or activities.
• Newly sitting infants rely heavily on visual inputs when controlling sway and that this dependence decreases with increasing experience in independent sitting, as infants rely more on somatosensory inputs. As the child grows their visual dominance, for controlling posture and balance, begin to decline after 3 years of age and their somatosensory system dominance (for postural balance control) increases. This means that chances of loss of balance or instability, when visual inputs are altered or disturbed, will decrease. This can also be called as sensory organization.

3) Vestibular system: The vestibular system parts which helps in balancing and maintaining equilibrium are semicircular canals and otoliths organs. Vestibular system is essential to balance, stabilize our head and body during movement and maintain posture. 

• The semicircular canals helps to detect head movements (like flexion-extension, side to side rotation, neck lateral flexion) during rotational and acceleration motion of body.
• Otoliths organs helps to detect linear acceleration, gravitational force and tilting movements. Utricle is specialized to detect movements in horizontal plane while saccules detects movements in vertical plane. 

The 3 Vestibular reflexes whose activation helps in effective functioning of any task are:

• Vestibulospinal reflex : sense falling/ tipping.
• Vestibulocollic reflex: acts on the neck musculature to stabilize the head if the body moves.
• Vestibulo-occular reflex: stabilize visual images during head movements.

Disorders in which this system is affected are vertigo, BPPV, acoustic schwannoma etc.

B) Motor output, recruitment and control: 

• Volitional postural movements are under conscious control. This voluntary movement is controlled by primary motor cortex and premotor cortex. This cortex prepares, generates and send neural impulses/commands to execute movements specific to task. The motor neural output is transmitted to motor neurons of spinal cord via corticospinal tracts.
• Volitional movements or voluntary control is also determined by body’s muscle strength, its motor neurons recruitment capacity, muscle mass. Generation of force (muscle force and motor neurones recruitment) produces movements effective in controlling the body’s position in space.
• Disturbance in this system causes paralysis or weakness. Examples in stroke, multiple sclerosis, motor neuron disease, myasthenia gravis, muscle dystrophy, spinal cord injury, cerebral palsy etc.

C) Coordination and checking of movements: 

• The cerebellum is important for making postural adjustments in order to maintain balance. It works through its connection with vestibular system, ascending proprioceptive tracts (spino-cerebellar tracts) and motor cortex. It modulates and send signals to motor cortex to compensate for any shifts in body postion or changes in load acting on muscles during activity performance. It is essential for bilateral coordination of body parts and efficient performing of midline oriented activities.
• Basal ganglia function in balance by checking unwanted movements, muscle tone control, motor programming, somatosensory integration, storing and automatic execution of motor plans. 
• Cerebellum and basal ganglia modify movements on a minute-to-minute basis. Balance between two systems allows for smooth coordinated movements.
• Disturbance in this system causes movement disorders. Example: Parkinson's disease, Chorea, spino-cerebellar ataxia, cerebellar stroke etc.

Fig: Balance system of body

Consideration of these balance systems for rehabilitating patients with balance disorders : 

• During rehabilitation, the primary goal of treatment is to help patient regain their functional independence by regaining mobility. To achieve mobility in patients, with balance problems or disorder, balance systems and posture are two important components to be worked on initially.
• Teaching, strengthening and learning balance strategies: Maintaining balance means keeping our center of gravity over our base of support, and that is where the balance strategies come into play. Remarkable mechanisms in how we maintain our balance as a bi-pedal species is by implementing various balance strategies, i.e. ankle strategy, hip strategy and stepping strategy. These strategies are used for stabilization in multiple directions, including anteroposterior, medio-lateral and also multi directional plane of motion. Using all three balance strategies effectively and in the correct sequence is an integral part of a fall prevention program.
• Therapists considers the person, their environment and their occupation when planning interventions for them. If any one domain is affected then it becomes a risk factor of fall or injury while performing task. For example, intrinsic (person), extrinsic (environment) and behavioral (occupation) problems. The fall prevention and management intervention maximize the potential to impact positively on an individual’s ability to carry daily activities (occupational performance). 

• Rehabilitation of balance disorders begins with remediation or restoration of underlying deficits along with teaching and learning compensatory strategies or adaptive strategies for the irreversible deficits. Intervention planning and therapeutic exercises will depend on which system is affected.
• Interventions aims at improving the quality of life of clients by enhancing functional performance capabilities and promoting independence in activities of daily living skill.

Thankyou for reading!!!!

(Dr. Ashwini Sangar, Dr. Sheetal Tatar-Dhande, Dr. Pallavi Khadse-Kolhe)