Course:KIN366/ConceptLibrary/PosturalControl

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Movement Experiences for Children
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KIN 366
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Instructor: Dr. Shannon S.D. Bredin
Email: shannon.bredin@ubc.ca
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Postural control refers to the multifaceted skill of intentionally retaining a specific body position for the purpose of performing specific movement behaviours (Horak, 2006). It is permitted by the communication between various sensorimotor systems, including the somatosensory, vestibular, and visuomotor systems, along with the musculoskeletal system (Gabbard, 2012; Horak, 2006). In other words, the neuromuscular system is responsible for controlling the positions of all associated limbs and body segments to provide a “reference frame” for subsequent motor action (Assaiante, Mallau, Viel, Jover, & Schmitz, 2005; Massion, 1994). In order to do so, the central nervous system is required to coordinate the effects of external forces on the body and intrinsic biomechanics on the movement capabilities of the muscular system (Massion, 1994).

From a developmental perspective, a child is required to learn to recognize and interpret a variety of different sensory inputs, from varying situations, in order to select and perform an appropriate motor response (Assaiante et al., 2005; Gabbard, 2012). In fact, a child will only be able to develop and demonstrate rudimentary movement behaviours (eg. rolling, standing, crawling, creeping, walking) after the establishment and mastery of the various critical postural strategies (Gabbard, 2012; Massion, 1998). In this sense, the ability to attain postural control mechanisms provides the link to the acquisition of all future complex motor skills (Gabbard, 2012; Kernell, 1998).

Related Concepts

Functions of Postural Control

According to Massion (1994), the postural control system is responsible for 2 important functions: (1) overcoming external disturbances, resulting from gravitational force, on the motor system and (2) providing a reference body orientation for the execution of different motor activities. The former, coined as an “antigravity function,” involves the combined tasks of joint stabilization and balance maintenance (Massion, 1998). Joint stabilization, facilitated by the musculature, is essential to overcoming gravitational pull while maintaining specific stationary positions and enduring the ground reaction forces resulting from locomotion (Massion, 1998). Balance maintenance refers to the system’s ability to establish and sustain a certain body position and involves the manipulation of the body’s centre of gravity such that it is oriented within the width of the body’s base of support (Gabbard, 2012; Massion, 1998). The latter function requires the neuromuscular system to use the specific orientation of body segments (eg. head, trunk, limbs, etc.) with respect to the environment as a means of making appropriate decisions and adjustments to the intended movement behaviour (Massion, 1998). In other words, the body needs to interpret the positioning of its own segments in order to calculate the relative positions of external objects from the surroundings and create a contextual motor plan to manipulate the environment through action (Massion, 1994).

Central Organization

As mentioned previously, the body’s ability to establish requisite postural control strategies is dependent on the interactions of various sensorimotor systems and the musculoskeletal system (Gabbard, 2012; Horak, 2006). These interacting systems, such as the visual, labyrinthine, and proprioceptive sensors, allow the body to use incoming sensory information to properly position a particular body segment in relation to the surrounding environment as well as other body segments (Clement, Gurfinkel, Lestienne, Lipshits, & Popov, 1984). This is accomplished by creating a hypothetical representation of body posture or what is known as a “postural scheme” (Massion, 1994). This “postural scheme,” derived from the multiple sensory inputs listed above, consists of numerous component reference values (eg. body geometry) that allow the neuromuscular system to establish an internal sensation of the ideal orientation of posture in a specific situation (Massion, 1994). By creating a “postural scheme,” the central nervous system can compare the actual posture mechanics (orientation of body segments and stability of posture) to the intrinsic model for posture, thereby providing feedback for the postural control system to make any necessary changes to body segment positions (Horak, 2006; Massion, 1994). In essence, the body can adapt to novel situations by manipulating its position toward the internal “postural scheme” so that succeeding movements can be completed with proper form for the purpose of creating a change in the external environment (Massion, 1994).

Important Physiological Systems: Defined

  • Somatosensory system

The somatosensory system is composed of a number of different sensory receptors, located throughout the body, that detect temperature, tactile information, pain, and proprioception (the internal awareness, as mediated by the central nervous system, of the relative positions of body segments to the external environment and to each other) (Tortora & Derrickson, 2012).

  • Vestibular system

The vestibular system consists of the organs within the inner ear which provide constant sensory information about balance (Angelaki & Cullen, 2008). The inner ear organs can detect rotational movements as well as forward or backward acceleration upon changes in head position (Angelaki & Cullen, 2008). Furthermore, the otolith organs (detecting acceleration changes) are constantly active due to the effects of gravitational forces (Angelaki & Cullen, 2008).

  • Visual system

The visual system provides information about the observable surrounding environment by perceiving and translating images from visible light to build an internal representation of the external world (Tortora & Derrickson, 2012). Since research has shown that eliminating the visual system results in enhanced postural sway (i.e. slight deviations from the intended postural position), it is suggested that visual information is essential for maintaining balance (Hansson, Beckman, & Hakansson, 2010).

  • Musculoskeletal system

The musculoskeletal system consists of the bones and joints of the human skeleton and the surrounding associated muscles that permit physical movement and provide a defined structure and shape to the human body (Tortora & Derrickson, 2012). The development of muscle strength will aid in supporting the mass of the body in order to maintain a variety of postural forms (Gabbard, 2012).

Developmental Perspective

Early Development Milestones

As discussed, one of the primary functions of establishing postural control is to ultimately allow the development and execution of other rudimentary motor skills (Gabbard, 2012; Massion, 1994). According to Gabbard (2012), research has shown a typical progression of postural control strategies developed throughout infancy and it occurs in a cephalocaudal (head to toe) manner. The achievement of the 4 postural movements (head control, rolling, sitting, and standing) during the 1st year of post-natal development is crucial for providing the child with opportunities to experience different sensory inputs (eg. visual information) from the external environment and also lays down the foundation for subsequent rudimentary movement processes including locomotor skills such as creeping, crawling, and walking (Gabbard, 2012; Massion, 1994; Massion, 1998).

Head Control

In the days immediately following birth, infants have little to no voluntary control over head movements at the neck (Gabbard, 2012). However, at approximately 1 month of age, a baby will be able to lift its head (i.e. flex the neck from a supine position) slightly for a few moments while supported at the neck (Gabbard, 2012). Furthermore, the infant will also be able to lift its head from a prone position for a brief duration (Gabbard, 2012). At this stage, the visual field experienced by the child is limited due to the small angle of neck flexion (or extension) as well as the child’s inability to maintain this position (Gabbard, 2012).

At 2 months, the infant will have improved ability to lift its head from a prone position resulting in a greater visual field for sensory experiences (Gabbard, 2012). At 3 months, the child has developed the necessary strength to lift not only its head but also its chest off the surface from a prone position assuming forearm support is present (Gabbard, 2012). Finally, at approximately 5 months, progression of head control is complete such that the infant can lift its head from a supine position without external support (Gabbard, 2012).

Rolling

In conjunction with the development of control over head and neck movements, the child also will begin to demonstrate body rolling capabilities (Gabbard, 2012). For instance, at approximately 2 months of age, an infant will be able to roll over onto its back starting from its side (Gabbard, 2012). Then at 4 months, a child will demonstrate the ability to roll onto its side from either a prone or supine starting position (Gabbard, 2012).

Beyond this stage however, infants require greater strength and control over hip and trunk movements in order to achieve more complex rolling patterns such as from supine to prone or from prone to supine (Gabbard, 2012). These more intricate rolling strategies are usually observed at approximately 6-8 months of age (Gabbard, 2012).

Sitting

In addition to head control and rolling, an infant will concurrently learn to sit in an erect position with external support at about 4 months of age (Gabbard, 2012). During this progression, attempts to sit unsupported are often characterized by a noticeable forward flexion of the lumbar region before finally being able to sit without support at approximately 6 months (Gabbard, 2012). However, an infant will not be able to place itself into a sitting position (from either supine or prone) until it masters the complex rolling patterns, at around 8 months, which indicate adequate upper body trunk stabilization (Gabbard, 2012). Moreover, the infant requires further time and development of strength and flexibility in order to sit down from a standing position (Gabbard, 2012).

Standing

Standing is the final postural movement achieved by an infant and is often attained (i.e. without additional support) at approximately 12 months (Gabbard, 2012). Preceding this monumental milestone, the child often is able to stand while being supported by environmental “hand-holds” at around 7 months and, following this, the child will be able to pull itself into a standing position with the proper affordances at about 9 months (Gabbard, 2012).

Neuromuscular Mechanisms of Postural Control

The development of postural control strategies (from head control to standing) is dependent on a number of interacting systems and intrinsic components such as the visual, vestibular, and somatosensory systems, as well as muscle activation interactions and body composition (Woollacott & Shumway-Cook, 1990).

Body Composition/Morphology

In children, the strategies selected in order to maintain postural control are often reliant on variable heights, leg lengths, and centre of mass (Woollacott & Shumway-Cook, 1990). In order to account for these individual morphological differences, humans may choose to employ 1 of 3 different strategies to maintain balance while standing: (1) an ankle strategy which emphasizes adjustments at the ankle joint to remain balanced; (2) a hip strategy which emphasizes hip angular adjustments; and (3) a suspensory strategy which involves lowering one’s centre of gravity to maintain balance (Woollacott & Shumway-Cook, 1990). Each of these adjustments occurs very quickly (within hundreds of milliseconds (ms)) and the timing of these adjustments differs between adults and children due to a child’s shorter body segments (McCollum & Leen, 1989).

For instance, an average adult will make a hip adjustment in approximately 173 ms to safely maintain proper stance (McCollum & Leen, 1989). However, in infants, a hip strategy movement is utilized over just 114 ms which is, in fact, too quick for an appropriate correction of position to be made (McCollum & Leen, 1989). In other words, it is unlikely or even impossible to observe a young child making adjustments at the hip in order to maintain proper upright posture. Instead, development of a hip strategy will occur as the neuromuscular system matures in order to provide for increased stability in posture (McCollum & Leen, 1989; Woollacott & Shumway-Cook, 1990).

Muscle Activation

In a study conducted by Hartbourne, Giuliani, and NacNeela (1987), it was discovered that, as more advanced postural control strategies were learned, different muscle activation patterns could be observed. For example, in their longitudinal study, children 3-4 months old who sat with a noticeable forward flexion of the back demonstrated extreme variability in the types of muscles used to attain a given posture as well as in the order in which muscles were activated (Hartbourne et al., 1987). However, as the children developed the ability to sit upright without support, Hartbourne et al. (1987) observed a distinct pattern of muscle activation among most children, although the types of muscles used still varied. With these findings, Hartbourne et al. (1987) decided that the emergence of a new postural strategy as an infant develops is partly due to the establishment of a stable and consistent muscle activation pattern as each child adapts to a preferred set of muscles to use to maintain an upright mature posture.

Furthermore, experiments conducted by Woollacott, Debu, and Mowatt (1987) have shown similar findings when examining postural sway in children during forward or backward movement stimulation. In the study, it was found that, as children matured, a greater number of muscles (thereby demonstrating increased diversity of muscular control) and a specific pattern of activation was engaged depending on the type of movement stimulation induced (Woollacott et al., 1987).

Sensory Systems (Somatosensory, Vestibular, Visual)

In a typical or normal situation (i.e. flat support surface with no external disturbances aside from gravity), an average adult relies heavily on the somatosensory system to maintain proper balance and posture (Woollacott & Shumway-Cook, 1990). The visual system will then dominate if a novel situation is presented (such as when the support surface is no longer flat) (Woollacott & Shumway-Cook, 1990). On the other hand, the vestibular system acts as a secondary sensory system and is constantly creating an internal “postural scheme” and comparing it to the actual positioning of the body segments in order to identify incongruent positions and provide feedback information for corrections to the postural stance (Hirabayashi & Iwasaki, 1995). However, the sensorimotor make-up of children has been shown to be different as Lee and Aronson (1974) found that children place a heavier reliance on visual input compared to adults. Research suggests that this additional reliance on vision among infants results from the proximo-distal developmental pathway of the human body (Lee & Aronson, 1974). The proximo-distal developmental strategy proposes that body segments nearer to the trunk (eg. upper arm) matures prior to body segments further away from the trunk (eg. fingers and toes) (Gabbard, 2012). In other words, children depend on visual information to maintain correct postural positions rather than somatosensory cues because of a lack of somatosensory experience at distal body segments such as the ankles which are needed for unsupported standing (Woollacott & Shumway-Cook, 1990).

Further studies conducted by Shumway-Cook and Woollacott (1985) show that young children demonstrate significantly more postural sway during quiet standing compared to mature adolescents or adults. Moreover, postural sway was amplified further when children were asked to stand with their eyes closed (Shumway-Cook & Woollacott, 1985). These results confirm the hypotheses of previous researchers regarding the underdevelopment of the somatosensory system at distal segments of infants (Shumway-Cook & Woollacott, 1985). On the contrary, in a study conducted by Hirabayashi and Iwasaki (1995), it was found that, despite a child’s reliance on visual cues to initially maintain proper postural balance, the visual system would not fully mature until approximately 10 years after the completed development of the somatosensory system. Furthermore, the vestibular system takes even longer to fully mature than the visual system (Hirabayashi & Iwasaki, 1995).

These findings ultimately bring rise to another important research hypothesis that muscular responses to postural perturbations develop at a lower level of the central nervous system (and therefore emerge earlier in the development process) while sensory processes occur at a higher level of the central nervous system (and therefore emerge later in the development process) (Hirabayashi & Iwasaki, 1995).

Postural Control vs. Movement

As previously emphasized, establishing postural control is critical for the development and execution of subsequent motor actions (Gabbard, 2012). This is mainly because of the potential for movements (i.e. constantly changing body segment positions) to interrupt the regular postural balance strategies (Massion, 1998). For instance, Massion (1998) explains that the forces of the muscles of the moving segment may impact the ability of supporting body segments to maintain its original position. Furthermore, the movement of one body segment may, in turn, affect the position of the centre of gravity in relation to the base of support (Massion, 1998). Therefore, the central nervous system is required to compensate for the changing internal environment of the postural system. Massion (1998) proposes a combination of 2 strategies employed by the central nervous system to offset the sudden change in body posture: (1) postural reactions resulting from sensory input and (2) anticipatory movements (occurring before movement onset) resulting from an internal adaptation to the change in the initial body segment position.

In discussing the appearance of sudden postural reactions, Massion (1994) indicates that the sensory signals, derived from the trepidation of the initial postural position, do not target individual muscles; instead, the sensory inputs innervate groups of muscles (synergies) in order to reduce the degrees of freedom of the reactionary sequence. Massion (1994) notes that innervating muscle synergies as opposed to individual muscles provides a distinct advantage to the neuromuscular system since synergies are activated depending on the associated nerve supply only rather than on the particular joint at which a muscle crosses. In simpler terms, muscle synergy activation is more flexible and context-dependent (MacPherson, 1991). The second anticipatory strategy serves to prepare the sensorimotor system for a change in equilibrium by adjusting the location of the centre of gravity as well as providing secondary support for the movement process (Massion, 1998). This second strategy is both complex and unique since it requires the motor system to predict both the outcome of the intended action and the consequences of the movement of the postural position (Bernstein, 1984). Additionally, certain anticipatory strategies are observed during early infancy (eg. secondary neck muscle activation during reaching movements) while others are not seen until later stages of development (eg. limb muscle activation when loading an external object) (Massion, 1998). However, a significant and unanswered question remains with regards to anticipatory strategies: by what mechanism does the neuromuscular system operate under in order to accommodate both the accomplishment of the intended movement as well as the maintenance of postural constancy (Massion, 1998)?

Clinical Cases and Practical Applications

Cerebral Palsy

According to Diener, Dichgans, Bacher and Gompf (1984), patients suffering from cerebral palsy demonstrate poor motor function resulting from reduced equilibrium regulation, the perseverance of the primitive motor reflexes, and a damaged vestibular system. In addition, these children often present abnormal postural strategies upon dynamic perturbation (Horak, 1997). In a study conducted by Ferdjallah, Harris, Smith, and Wertsch (2002), it was found that, compared to healthy control subjects, children with cerebral palsy demonstrated poorer ankle stability and control and, therefore, avoided using an ankle strategy when posture was disturbed. In other words, cerebral palsy patients relied more on a hip strategy, specifically lower leg protraction or retraction and body transverse rotation, to correct for perturbed postural balance (Ferdjallah et al., 2002). Unfortunately, due to weakness in the ankle, these patients were also more susceptible to falling upon a change in the environmental stimuli (Ferdjallah et al., 2002).

As a result of this research, Ferdjallah et al. (2002) recommend that developing appropriate postural control strategies is crucial for further advancement of the motor system; i.e. they suggest that mastering postural control will permit the development of other more complex motor skills in the future. Specifically, Ferdjallah et al. (2002) emphasizes the importance of establishing appropriate and safe hip and core strategies (lower limb protraction and retraction; body transverse rotation) that counteract the disturbances to static posture in order to minimize the risk of postural sway, instability, and, most importantly, falling.

Down Syndrome

Patients living with Down syndrome consistently report developmental delays in terms of acquisition of postural strategies, especially as it pertains to balance, and other voluntary motor skills (Frith & Frith, 1974). Previous research suggested that these deficiencies result from inadequate muscle tone among Down syndrome patients as well as impaired nerve excitability (Gilman, Bloedel, & Lechtenberg, 1981). However, according to Shumway-Cook and Woollacott (1985), insufficiencies in the postural control system may be a more plausible explanation for the aforementioned developmental delays. As evidence, Shumway-Cook and Woollacott (1985) point to a delayed postural strategy response to a perturbation leading to a loss of balance and an inability to restore a stable postural position.

Therefore, it is recommended that practitioners exploit therapeutic strategies that accentuate the development of muscle synergies necessary for the purpose of improving the strength and quickness of a postural reaction toward a disturbance event to an initial postural position (Shumway-Cook & Woollacott, 1985). In other words, Shumway-Cook & Woolacott (1985) recommend that therapy be directed at synchronizing specific groups of muscles (eg. synergies at the hip or ankle) necessary to respond to a stimulus such that postural imbalances can be restored efficiently. Finally, Shumway-Cook & Woollacott (1985) suggest that it may also be necessary to improve the central organizing processes underlying the adaptation of the neuromuscular system to novel situations.

Autism

Autism is first and foremost characterized by poor social skills as well as repetitive behaviours (Minshew, Sung, Jones, & Furman, 2004). However, deficiencies in motor control, coordination patterns, and postural stability should not be overlooked as symptoms of the disorder (Damasio & Mauer, 1978). In a study carried out by Minshew et al. (2004), autistic individuals were found to exhibit delayed development and reduced maturity postural control (compared to healthy controls). Based on the results, Minshew et al. (2004) suggested that the deficit in postural control strategies results from insufficient incorporation of the 3 primary sensorimotor systems (visual, somatosensory, vestibular) to provide the requisite information needed to construct the “postural scheme.” With these findings it can be deduced that autistic behaviour results not only from disorders in the neural systems involved with social interactions, but more generally, from poor neural organization in the central nervous system (Minshew et al., 2004).

General Development (Typically Developing Children)

As defined and emphasized throughout, the development of the postural control system is vital to the subsequent acquisition of more complex motor skills including the rudimentary locomotor tasks (Gabbard, 2012; Kernell, 1998). Therefore, it is critical for parents to offer their infants every opportunity to build up the necessary neuromuscular foundation in order to meet the typical postural control progression timeline.

Firstly, infants should be allowed to experience activities while in a prone position (under supervision) for the purpose of eventually developing the essential muscular strength (a rate-limiter) needed in the neck and torso to establish head control (Gabbard, 2012; Woollacott & Shumway-Cook, 1990). By developing head control, this will expose the infant to a greater visual field which will ultimately aid in the improvement of the visual sensory system (Woollacott & Shumway-Cook, 1990). Additionally, establishing head control will allow an infant to progress to more advanced forms of rolling further improving muscular core strength as well as introducing the somatosensory and vestibular systems to new situations (Gabbard, 2012).

Secondly, appropriate affordances (i.e. settings or objects that permit a child to further explore the immediate environment) should be present in the home setting so that the child will be afforded the opportunity to begin the process of standing with supportive “hand-holds” at approximately 6 months (Gabbard, 2012). Examples of such affordances include low tables or railings that an infant can grasp to support him or herself and, in the process, develop the lower body and core strength eventually needed for unassisted upright stance (Gabbard, 2012). Having the opportunity to stand permits the child to further explore the environment, thereby improving the visual system and, indirectly, the postural control system (Gabbard, 2012; Hirabayashi & Iwasaki, 1995).

References

Angelaki, D.E., & Cullen, K.E. (2008). Vestibular system: The many facets of a multimodal sense. Annual Review of Neuroscience, 31, 125-50. doi:10.1146/annurev.neuro.31.060407.125555.

Assaiante, C., Mallau, S., Viel, S., Jover, M., & Schmitz, C. (2005). Development of postural control in healthy children: A functional approach. Neural Plasticity, 12(2), 109-18.

Bernstein, N. (1984). Human motor actions. Whiting, Amsterdam: Bernstein Reassessed. H.T.A.

Clement, G., Gurfinkel, V.S., Lestienne, F., Lipshits, M.I., & Popov, K.E. (1984). Adaptation of postural control to weightlessness. Experimental Brain Research, 57, 61-72.

Damasio, A.R., & Mauer, R.G. (1978). A neurological model for childhood autism. Archives of Neurology, 35, 777-86.

Diener, W.W., Dichgans, J., Bacher, M., & Gompf, B. (1984). Quantification of postural sway in normal and patients with cerebellar diseases. Electroencephalography and Clinical Neurophysiology, 57, 134-42.

Ferdjallah, M., Harris, G.F., Smith, P., & Wertsch, J.J. (2002). Analysis of postural control synergies during quiet standing in healthy children and children with cerebral palsy. Clinical Biomechanics, 17, 203-10.

Frith, V., & Frith, C.D. (1974). Specific motor disabilities in Down’s syndrome. Journal of Child Psychology and Psychiatry, 15, 293-301.

Gilman, S., Bloedel, J.R., & Lechtenberg, R. (Ed.). (1981). Disorders of the cerebellum. Philadelphia, PA: F.A. Davis Co.

Hansson, E.E., Beckman, A., & Hakansson, A. (2010). Effect of vision, proprioception, and the position of the vestibular organ on postural sway. Acta Oto-Laryngologica, 130, 1358-63. doi: 10.3109/00016489.2010.498024.

Hartbourne, R., Giuliani, C.A., & NacNeela, J. (1987). Kinematic and electromyographic analysis of the development of sitting posture in infants. Developmental Medicine & Child Neurology, 29, 31-32.

Hirabayashi, S., & Iwasaki, Y. (1995). Developmental perspective of sensory organization on postural control. Brain & Development, 17, 111-13.

Horak, F.B. (1997). Clinical assessment of balance disorders. Gait Posture, 6, 76-84.

Horak, F.B. (2006). Postural orientation and equilibrium: What do we need to know about neural control of balance to prevent falls? Age and Ageing, 35(2), 7-11. doi:10.1093/ageing/afl077.

Kernell, D. (1998). The final common pathway in postural control – Developmental perspective. Neuroscience and Behavioral Review, 22(4), 479-84.

Lee, D.N., & Aronson, E. (1974). Visual proprioceptive control of standing in human infants. Perception & Psychophysics, 15, 529-32.

Macpherson, J.M. (1991). How flexible are muscle synergies? In D.R. Humphrey & H-J. Freund (Ed.), Motor control: Concepts and issues (pp. 33-47). Chichester: John Wiley & Sons.

Massion, J. (1994). Postural control system. Current Opinion in Neurobiology, 4, 877-87.

Massion, J. (1998). Postural control systems in developmental perspective. Neuroscience and Behavioral Reviews, 22(4), 465-72.

McCollum, G., & Leen, T.K. (1989). Form and exploration of mechanical stability limits in erect stance. Journal of Motor Behavior, 21, 225-44.

Minshew, N.J., Sung, K., Jones, B.L., & Furman, J.M. (2004). Underdevelopment of the postural control system in autism. Neurology, 63, 2056-61.

Shumway-Cook, A., & Woollacott, M.H. (1985). Dynamics of postural control in the child with Down syndrome. Physical Therapy, 65, 1315-22.

Shumway-Cook, A., & Woollacott, M.H. (1985). The growth of stability: Postural control from a development perspective. Journal of Motor Behavior, 17, 131-47.

Tortora, G.J., & Derrickson, B.H. (2012). Principles of anatomy and physiology, 13th edition. Hoboken, N.J.: John Wiley & Sons.

Woollacott, M.H., Debu, B., & Mowatt, M. (1987). Neuromuscular control of posture in the infant and child: Is vision dominant? Journal of Motor Behavior, 19, 167-86.

Woollacott, M.H., & Shumway-Cook, A. (1990). Changes in postural control across the life span – A systems approach. Physical Therapy, 70, 799-807.