THERAPY-Magazin
Expert report on postural control
Explore how integrated postural control mechanisms maintain balance and stability. Learn how predictive and reactive strategies, motor training, and sensory adaptation can enhance rehabilitation outcomes.
Marc Michielsen
Advanced Bobath Instructor
We all live with gravity – usually without worrying that we might fall over. Falling is prevented by an integrated postural control mechanism. In the second part of our expert report, you will learn more about how this mechanism is activated.
Activation of the postural control mechanism
A basic prerequisite for postural control is the ability to stand up straight and actively counteract gravity. A further prerequisite is the ability to select and perceive sensory input in order to develop the body schema and to align the body in relation to the environment. This article focuses on the neurophysiological mechanism responsible for the activation of postural muscles.
The neuromotor system that enables movement and balance is the system of medial descending neural pathways. It runs through the centre of the spinal cord and ends on both sides in the motor pools of the axial muscles in particular. It activates the head and torso and transmits commands to the legs, ensuring strength and balance.
The supraspinal “processor” for balance control has many points of departure. The power centre for our sense of balance is located in the brain stem (reticulospinal nuclei). The reticulospinal pathways enable an upright posture. The “shadow system” for balance control is a complex interconnected structure that connects the vestibular nuclei with the cerebellar neurons. The vestibulospinal pathways enable quick and precise reactive postural control. “Reactive” here refers to movement correction via direct communication channels through peripheral error feedback.
The reactive postural control follows the movements of the body like a shadow to keep us upright. Balance errors are detected by the cerebellum, which continuously compares movement transfers with the intended posture. In order for the error feedback to be useful in predictive movement control, subsequent movements must be corrected with recourse to previous movements. Through repeated trial and error, the cerebellum adapts our movements to new circumstances. Predictive mechanisms must be learned and updated by comparing predicted and observed results [1].
The neuromotor system that enables movement and balance is the system of medial descending neural pathways. It runs through the centre of the spinal cord and ends on both sides in the motor pools of the axial muscles in particular. It activates the head and torso and transmits commands to the legs, ensuring strength and balance.
The supraspinal “processor” for balance control has many points of departure. The power centre for our sense of balance is located in the brain stem (reticulospinal nuclei). The reticulospinal pathways enable an upright posture. The “shadow system” for balance control is a complex interconnected structure that connects the vestibular nuclei with the cerebellar neurons. The vestibulospinal pathways enable quick and precise reactive postural control. “Reactive” here refers to movement correction via direct communication channels through peripheral error feedback.
The reactive postural control follows the movements of the body like a shadow to keep us upright. Balance errors are detected by the cerebellum, which continuously compares movement transfers with the intended posture. In order for the error feedback to be useful in predictive movement control, subsequent movements must be corrected with recourse to previous movements. Through repeated trial and error, the cerebellum adapts our movements to new circumstances. Predictive mechanisms must be learned and updated by comparing predicted and observed results [1].
A sense of balance can be trained
In a series of experiments, the balance of trained ballet dancers and a control group of non-dancers was mechanically disturbed in order to measure the time until the muscles began stretching and the consistency of the stretching. The results support the assumption that the neuromuscular responses of ballet dancers are characterised by significantly faster long-latency reflexes (LLR) with significantly more consistent muscle stretching. These findings suggest a superior postural control mechanism in trained dancers and may explain their ability to maintain static balance even with a minimal base of support [6].
In a series of experiments, the balance of trained ballet dancers and a control group of non-dancers was mechanically disturbed in order to measure the time until the muscles began stretching and the consistency of the stretching. The results support the assumption that the neuromuscular responses of ballet dancers are characterised by significantly faster long-latency reflexes (LLR) with significantly more consistent muscle stretching. These findings suggest a superior postural control mechanism in trained dancers and may explain their ability to maintain static balance even with a minimal base of support [6].
Prediction of balance disorders
The cortex, which controls consciousness, is a superordinate entity for the co-ordination of the interconnected reticular, vestibular and cerebellar systems. It predicts the consequences of a movement based on previous experience and aligns the body in advance to counteract any shift in the body’s centre of gravity. The risks of a movement are automatically detected: the cortex can predict a fall. For example, the brain “knows” that the body can be thrown off balance when we reach a hand out to greet someone. Even the mere intention of shaking hands with someone invokes the postural knowledge stored in the body schema. During the earliest phases of a movement, in which corrections via peripheral feedback are not yet possible, the cortex shapes this movement from the body schema. This predictive movement control can be observed by means of a more co-operative alignment of the body and an increased alertness of the sensory systems. Without these postural adjustments by the cortex (via the cortico-reticulospinal pathways), there would be a risk of falling into the other person’s arms when shaking hands. The term “predictive” refers to planned movements. This command to move is known as an anticipatory postural adjustment (APA). The postural muscles are structured in such a way that they generate powerful forces against the respective standing surface in order to shift or maintain the body’s centre of gravity and to be able to control or prevent excessive movements of the joints due to indirect counteracting torques. To enable efficient balance control, these forces must have the necessary strength, speed and precision.
However, the motor commands of the cortex only provide a good estimate. They can only make the connection and initiate the movement from a more or less efficient postural set. The cerebellum is also required to make the parameters (weight, direction, speed, etc.) more precise. Symptoms of paralysis, for example after a severe stroke, impair this system. The lack of motor experience reduces the ability to balance. Patients with corresponding symptoms show reduced and/or delayed anticipatory postural adjustments compared to healthy subjects [3]. In this context, any deliberate selective movement carries the risk of losing balance. Although it seems possible to shake someone’s hand, reach for something or lift one’s foot, the brain is being deceitful; stroke patients lack precision because not enough anticipatory postural adjustments are transmitted and they are transmitted too weakly and too slowly.
However, the motor commands of the cortex only provide a good estimate. They can only make the connection and initiate the movement from a more or less efficient postural set. The cerebellum is also required to make the parameters (weight, direction, speed, etc.) more precise. Symptoms of paralysis, for example after a severe stroke, impair this system. The lack of motor experience reduces the ability to balance. Patients with corresponding symptoms show reduced and/or delayed anticipatory postural adjustments compared to healthy subjects [3]. In this context, any deliberate selective movement carries the risk of losing balance. Although it seems possible to shake someone’s hand, reach for something or lift one’s foot, the brain is being deceitful; stroke patients lack precision because not enough anticipatory postural adjustments are transmitted and they are transmitted too weakly and too slowly.
“Cerebellar” balance can be improved through small mistakes
It is assumed that the learning process in the cerebellum is largely based on error feedback. But only small mistakes lead to successful learning. If the deviations or errors are too unpredictable, i.e. too close to the stability limit, the cerebellum cannot determine the cause of the error. As a “teacher”, it is not able to provide the motor systems with the sensory information required to adjust movements. The cortex learns to compensate for this with predictive cognitive strategies: The patient increases their base of support, stiffens their legs, takes a lot of corrective steps, their arms become overly active, they press their head tightly against their chest. In order to improve the cerebellar controlled sense of balance, stability limits must therefore be maintained. Only small errors within these limits cause the cerebellum to intervene, so that the ability to reactively correct postural errors can be improved.
It is assumed that the learning process in the cerebellum is largely based on error feedback. But only small mistakes lead to successful learning. If the deviations or errors are too unpredictable, i.e. too close to the stability limit, the cerebellum cannot determine the cause of the error. As a “teacher”, it is not able to provide the motor systems with the sensory information required to adjust movements. The cortex learns to compensate for this with predictive cognitive strategies: The patient increases their base of support, stiffens their legs, takes a lot of corrective steps, their arms become overly active, they press their head tightly against their chest. In order to improve the cerebellar controlled sense of balance, stability limits must therefore be maintained. Only small errors within these limits cause the cerebellum to intervene, so that the ability to reactively correct postural errors can be improved.
The cerebellum learns as a result of a small mistake and the sense of balance improves. When the errors are somewhat larger, the cortex looks for compensation possibilities. In the case of major errors, a fear of falling can develop and the stability limits are lowered through fixation.
Postural mechanisms can be observed
The strategies we use to keep our balance can be observed – if you know where to look. For minor swaying with an adequate footing on the supporting surface, the ankle strategy is usually used. The shifting centre of gravity is restored by movements around the ankles and the transverse tarsal joint. Balance corrections in response to stronger, faster disturbances, such as when standing in a moving bus, are controlled by sweeping, rapid movements of the hip joints in combination with inverse rotations of the foot joints. If the balance disturbance is too great and affects the body too quickly, a new base of support must be found with the help of a corrective step to restore balance. This step must be fast, precise and powerful. It often occurs even when the body’s centre of gravity is within the base of support [2, 4]. Older people often take a corrective step before the stability limits have been reached. As a last resort, several steps can be taken in order to escape the disturbances. The number of steps a person needs to take to intercept a fall provides insight into the effectiveness of this balance strategy. These posture control strategies – ankle, hip and step strategies – are sensorimotor solutions that vary from person to person. They are comparable with the different types of human locomotion: walking, striding, jogging, running and sprinting.
A trained observer will notice balance problems when sensory conditions change. Postural fixation, for example, is observed more often in dark rooms or when subjects are not wearing their glasses. When reaching for a cup on the top shelf, a corrective step may be necessary to counteract the dizziness caused by the head movement when looking up. These examples show the importance of re-evaluating sensory impulses. When environmental conditions change, the brain bases its strategic decisions on the sensory source that provides the most accurate information. Every sensory strategy is therefore a process of re-evaluation. The re-evaluation of the sensory impulses causes a slight time delay in the postural “processing mechanism”. Through training, this time delay can be kept to a minimum.
The strategies we use to keep our balance can be observed – if you know where to look. For minor swaying with an adequate footing on the supporting surface, the ankle strategy is usually used. The shifting centre of gravity is restored by movements around the ankles and the transverse tarsal joint. Balance corrections in response to stronger, faster disturbances, such as when standing in a moving bus, are controlled by sweeping, rapid movements of the hip joints in combination with inverse rotations of the foot joints. If the balance disturbance is too great and affects the body too quickly, a new base of support must be found with the help of a corrective step to restore balance. This step must be fast, precise and powerful. It often occurs even when the body’s centre of gravity is within the base of support [2, 4]. Older people often take a corrective step before the stability limits have been reached. As a last resort, several steps can be taken in order to escape the disturbances. The number of steps a person needs to take to intercept a fall provides insight into the effectiveness of this balance strategy. These posture control strategies – ankle, hip and step strategies – are sensorimotor solutions that vary from person to person. They are comparable with the different types of human locomotion: walking, striding, jogging, running and sprinting.
A trained observer will notice balance problems when sensory conditions change. Postural fixation, for example, is observed more often in dark rooms or when subjects are not wearing their glasses. When reaching for a cup on the top shelf, a corrective step may be necessary to counteract the dizziness caused by the head movement when looking up. These examples show the importance of re-evaluating sensory impulses. When environmental conditions change, the brain bases its strategic decisions on the sensory source that provides the most accurate information. Every sensory strategy is therefore a process of re-evaluation. The re-evaluation of the sensory impulses causes a slight time delay in the postural “processing mechanism”. Through training, this time delay can be kept to a minimum.
Stability limits are real
The Limits of Stability (LOS) are defined as the maximum distance a person can lean in all directions from an upright, vertical position without falling, taking a corrective step or reaching for support. The ability to assume any body position within these limits is crucial for basic actions, such as reaching for objects, standing up from a sitting position (or sitting down from a standing position) and walking.
Within our respective stability limits, we feel safe and stable in our movements. In this state we are able to explore the environment by evaluating sensory impressions and through motor actions. For this reason, restoring balancing ability is a crucial component of motor behaviour and is necessary for performing everyday activities independently [4]. Our sense of balance has a strong influence on our daily life. After a fall, many people develop a fear of falling again, even if they have not injured themselves. This fear leads to avoidant behaviour, which in turn leads to reduced mobility and a lack of physical fitness. This in turn increases the actual risk of falling [7].
A trained observer will notice balance problems when sensory conditions change. Postural fixation, for example, is observed more often in dark rooms or when subjects are not wearing their glasses. When reaching for a cup on the top shelf, a corrective step may be necessary to counteract the dizziness caused by the head movement when looking up. These examples show the importance of re-evaluating sensory impulses. When environmental conditions change, the brain bases its strategic decisions on the sensory source that provides the most accurate information. Every sensory strategy is therefore a process of re-evaluation. The re-evaluation of the sensory impulses causes a slight time delay in the postural “processing mechanism”. Through training, this time delay can be kept to a minimum.
The Limits of Stability (LOS) are defined as the maximum distance a person can lean in all directions from an upright, vertical position without falling, taking a corrective step or reaching for support. The ability to assume any body position within these limits is crucial for basic actions, such as reaching for objects, standing up from a sitting position (or sitting down from a standing position) and walking.
Within our respective stability limits, we feel safe and stable in our movements. In this state we are able to explore the environment by evaluating sensory impressions and through motor actions. For this reason, restoring balancing ability is a crucial component of motor behaviour and is necessary for performing everyday activities independently [4]. Our sense of balance has a strong influence on our daily life. After a fall, many people develop a fear of falling again, even if they have not injured themselves. This fear leads to avoidant behaviour, which in turn leads to reduced mobility and a lack of physical fitness. This in turn increases the actual risk of falling [7].
A trained observer will notice balance problems when sensory conditions change. Postural fixation, for example, is observed more often in dark rooms or when subjects are not wearing their glasses. When reaching for a cup on the top shelf, a corrective step may be necessary to counteract the dizziness caused by the head movement when looking up. These examples show the importance of re-evaluating sensory impulses. When environmental conditions change, the brain bases its strategic decisions on the sensory source that provides the most accurate information. Every sensory strategy is therefore a process of re-evaluation. The re-evaluation of the sensory impulses causes a slight time delay in the postural “processing mechanism”. Through training, this time delay can be kept to a minimum.
Do not lose sight of stability limits
Patients monitor their own stability limits. One treatment goal is to activate the postural “processing mechanism” in order to increase its efficiency. Motor and sensory strategies can be improved through insulation and targeted exercise. Many patients suffer from an impaired joint alignment and a weakened joint environment. An extremity affected by paresis cannot control the body in an upright position and behaves more passively than limbs with movable joints. This results in hyperextension of the knee or continuous hyperextension of the leg. As a result, patients are forced to adopt an adjustment strategy by involving the limbs not affected by paresis [5]. The body compensates for this by increasing the activation of the non-paretic muscles and/or using a step strategy to maintain an upright posture. These patients do not like fast movements. Only slight internal or external swaying is tolerated. The stability limits of these patients should not be overestimated.
Patients monitor their own stability limits. One treatment goal is to activate the postural “processing mechanism” in order to increase its efficiency. Motor and sensory strategies can be improved through insulation and targeted exercise. Many patients suffer from an impaired joint alignment and a weakened joint environment. An extremity affected by paresis cannot control the body in an upright position and behaves more passively than limbs with movable joints. This results in hyperextension of the knee or continuous hyperextension of the leg. As a result, patients are forced to adopt an adjustment strategy by involving the limbs not affected by paresis [5]. The body compensates for this by increasing the activation of the non-paretic muscles and/or using a step strategy to maintain an upright posture. These patients do not like fast movements. Only slight internal or external swaying is tolerated. The stability limits of these patients should not be overestimated.
Is the ability to balance measurable?
It is common practice for many therapists to test balancing ability by inducing swaying in static posture and evaluating the corrective steps. This procedure is particularly popular with young therapists. Such “static” tests are indeed important for functional stability in everyday life. Maki and McIlroy give two reasons for this. Firstly, quasi-static movements and actions are actually responsible for a considerable proportion of falls (40 to 50%). Secondly, “static” tests provide revealing information with regard to the numerous falls that occur while walking. Corrective steps to compensate for shifts in the body’s centre of gravity share similarities with gait initiation and step adjustments when walking.
However, a single test or test method is not sufficient to assess the ability to balance. The result is also often debatable. The cause of a fall varies between individuals, as each person has different limiting factors and resources for postural control. The therapist should try to identify the relevant risk factors. It is important to evaluate the underlying physiological systems and the available compensation strategies in order to assess the risk of falling and to identify optimal intervention options for patients with balance disorders. Current clinical tools for balance assessment are not designed to help therapists identify the underlying postural control systems responsible for poor balance function [3].
It is common practice for many therapists to test balancing ability by inducing swaying in static posture and evaluating the corrective steps. This procedure is particularly popular with young therapists. Such “static” tests are indeed important for functional stability in everyday life. Maki and McIlroy give two reasons for this. Firstly, quasi-static movements and actions are actually responsible for a considerable proportion of falls (40 to 50%). Secondly, “static” tests provide revealing information with regard to the numerous falls that occur while walking. Corrective steps to compensate for shifts in the body’s centre of gravity share similarities with gait initiation and step adjustments when walking.
However, a single test or test method is not sufficient to assess the ability to balance. The result is also often debatable. The cause of a fall varies between individuals, as each person has different limiting factors and resources for postural control. The therapist should try to identify the relevant risk factors. It is important to evaluate the underlying physiological systems and the available compensation strategies in order to assess the risk of falling and to identify optimal intervention options for patients with balance disorders. Current clinical tools for balance assessment are not designed to help therapists identify the underlying postural control systems responsible for poor balance function [3].
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THERAPY 2020-II
THERAPY Magazine
Marc Michielsen
Advanced Bobath Instructor
Marc Michielsen studied Physiotherapy at the University of Leuven, Belgium, and is also an Advanced Bobath Instructor. His area of expertise is neurological rehabilitation, particularly following a stroke. After holding several positions as a senior physiotherapist at various hospitals, he has been the Head of Emergency Services at the Rehabilitation Center of Jessa Hospital since 2008. Michielsen has published several articles, abstracts, and other scientific papers in prominent journals.
References:
- Bastians (2006).
- Brown et al. (1999).
- Horak FB (2009). Postural Compensation for Vestibular Loss. Annals of the New York Academy of Sciences, 1164: 76-81. doi:10.1111/j.1749-6632.2008.03708.x.
- Lundy-Ekman L (2002). Neuroscience: Fundamentals for Rehabilitation, Elsevier LTD, Oxford, 2. Auflage.Maki BE und McIlroy WE (1999).
- Pérennou DA et al. (2008). The polymodal sensory cortex is crucial for controlling lateral postural stability: evidence from stroke patients. In: Brain Research Bulletin, 53(3), S. 359-365.
- Simons (2005).
- Tromp AM, Pluijm SM et al. (2001). Fall-risk screening test: a prospective study on predictors for falls in community-dwelling elderly. J Clin Epidemiol. 2001 Aug;54(8):837-44.
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