Both dynamic and static balance is impaired after hemiplegia. Hemiplegic individuals exhibit a postural deviation that can be up to twice as high as their peers of the same age. Furthermore, weight transfer symmetry is also disrupted, with hemiplegic individuals transferring weight toward the healthy side at a rate of 61-80%. In addition, there is insufficient weight transfer in the frontal plane during the transition from the double support period to the single support period in hemiplegic patients. Maintaining postural control is essential for the performance of activities of daily living.
Postural stability refers to the ability to keep the center of gravity within the limits of the support surface to complete the desired movement and maintain a static posture. Static postural stability keeps the stationary center of gravity within the support surface. In contrast, dynamic postural stability is the ability to keep the moving center of gravity within the limits of the support surface. Postural and balance control are similar terms that refer to the ability to keep the body at equilibrium or bring it back to that point.
While standing or assuming a position, motor responses constantly change in response to information from the environment to control posture and ensure the body’s adaptation to the environment. Multiple sensory systems control body position and movement in space relative to the environment and gravity. The vestibular system provides kinesthetic inputs about head position and movements, while proprioception provides information about the position of limbs and body parts concerning each other. Visual information provides additional cues about body position concerning the environment.
Two types of postural control mechanisms can be identified. The first is compensatory postural corrections, in which the body’s center of gravity displacement is compensated for by moving one or more body segments to remain within the support surface. In the second mechanism, postural correction is made by a voluntary movement beforehand. The type and magnitude of voluntary postural correction depend on the direction and speed of the voluntary movement, with previous experiences playing a more significant role than sensory input.
In static balance, the center of gravity is kept within the support surface, and the activity of the muscles around the ankle is sufficient to maintain an upright posture. However, the support surface and the center of gravity are mobile in dynamic balance. Ankle muscles alone are inadequate to maintain balance during walking, requiring different control mechanisms to be activated.
The four fundamental skills required for balance during walking are:
- Establishing and maintaining continuous movement toward a goal
- Maintaining balance throughout the process
- Ability to adapt to changes in the environment
- Starting and ending the movement.
Postural control is essential for adapting the body to the environment while standing or moving. Several sensory systems control body position and movement in space concerning the environment and gravity. Kinesthetic inputs from the vestibular system provide information about head position and movements. Proprioception provides information about the position of limbs and body parts relative to others. Visual information provides information about body position concerning the environment.
Two types of postural control mechanisms can be identified. The first is compensatory postural corrections, which attempt to keep the body within the support surface by compensating for displacement in the body’s center of gravity by moving one or more body segments. In the second mechanism, postural correction is made by voluntary movement beforehand. The direction and speed of voluntary movement determine the type and magnitude of voluntary postural correction. In this case, previous experiences come to the fore rather than sensory input.
The center of gravity is kept within the support surface in static balance. The activity of the muscles around the ankle is sufficient to maintain balance in standing upright. The support surface and center of gravity are movable in dynamic balance. Ankle muscles alone are insufficient to maintain the balance of the whole body while walking. Therefore, different control mechanisms must be activated for balance while walking.
Four basic skills are required for balance during walking. The first is establishing the process of continuous movement toward a goal. The second is maintaining balance during this process. The third is the ability to adapt to any change in the environment. The fourth is starting and ending the movement.
Controlling balance while walking is a challenging skill. Two disadvantages contribute to this. The first is due to the use of a bilateral motor pattern, which includes two single limb support periods that are relatively long and together comprise 75-80% of the entire gait cycle. During these two periods, the vertical projection of the body’s center of gravity shifts forward and out of the medial border of the foot, creating potential mediolateral instability during a single period of support.
The second biomechanical disadvantage in human movements is due to body structure. Two-thirds of the body weight is in the upper body, and the upper body stores a large amount of potential energy. If the upper body cannot be controlled in an upright position, potential energy can easily turn into kinetic energy during a fall. Keeping the upper body posture upright in the heel strike and push phase during walking is challenging. Hip extensor rotational force is necessary to prevent falling forward during a heel strike, and hip flexor rotational force is required to avoid a backward fall during the pushing phase.
As a result of motor and sensory disorders after a stroke, it becomes difficult to maintain dynamic and static postural stability. As a result, independence in walking and activities of daily living is greatly affected. Loss of postural control is one of the most important causes of the falling problem in hemiplegic individuals, both during and after rehabilitation. Maintaining a static bipedal position is a simple skill for healthy individuals, but it may be difficult for stroke individuals.
The upright posture of stroke patients is characterized by weight-bearing asymmetries that develop with more weight being transferred to the unaffected side and spontaneous large postural sway in the frontal plane. Individuals prefer this asymmetrical posture. Motor weakness, asymmetrical muscle tone, somatosensory deficit, and changes in body image negatively affect postural instability.
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