Research Article

The Effect of Visual Feedback on Static Standing Balance in Children with Spastic Diplegic Cerebral Palsy Compared to Normal Children under Altered Sensory Environment

Ali Tahmasebi1, Parvin Raji1*, Mostafa Kamali2

Editor | Mohammad Taghi Karimi, Department of Orthotics and Prosthetics, Faculty of Rehabilitation, Isfahan University of Medical Sciences, Isfahan, Iran.

Received | April 14, 2016; Accepted | May 14, 2016; Published | July 21, 2016

*Correspondence | Parvin Raji, Tehran University of Medical Sciences, Rehabilitation Faculty, Occupational Therapy Department, Tehran, Iran; Email: praji@tums.ac.ir

Citation | Tahmasebi, A., P. Raji and M. Kamali. (2016). The effect of visual feedback on static standing balance in children with spastic diplegic cerebral palsy compared to normal children under altered sensory environment. Health Rehabil. 1(2): 35-44.

DOI | http://dx.doi.org/10.17582/journal.hr/2016.1.2.35.44

Keywords | Visual feedback, Static standing balance, Spastic diplegic cerebral palsy

Introduction

Cerebral palsy (CP) is a non-progressive neuromotor disability with early onset occurring in developing brain of the fetus or infant. CP is one of the most prevalent reasons of childhood disability and the average life-long costs of this disability is estimated to be 11.5 billion dollars during 2000 in the United States only (Oskoui et al., 2013). The prevalence rate of CP is 2 cases per 1000 live births in the United States (Oskoui et al., 2013) and 2.06 cases per 1000 live births in Iran (Dalvand et al., 2012). Individuals with CP can experience different cognitive, sensory and motor disabilities concurrently (Liao and Hwang, 2003; Shumway-Cook and Woollacott, 1995; Campbell et al., 2006). With regard to the topography of the CP, the resulted motor impairments are divided into diplegia, hemiplegia, quadriplegia, monoplegia and triplegia (Laisram et al., 1992; Erkin et al., 2008). Investigation of the static standing stability is usually done in diplegic CP (Liao et al., 1997; Ledebt et al., 2005; Ferrari et al., 2010).

Static standing stability is the ability to maintain and/or control the center of mass of the body in the base of support provided by legs during standing (Umphred et al., 2013; Westcott et al., 1997). Poor standing stability can restrict motor abilities of an individual resulting in the reduced environmental exploration and interaction with friends and family members (Liao and Hwang, 2003; Shumway-Cook and Woollacott, 1995; Lepage et al., 1998) leading to a reduced quality of life (Ramachandran and Altschuler, 2009). Static standing stability is usually measured using postural stability and/or the amount of time during which the balance can be maintained (Rha et al., 2010). Investigating the excursion and velocity of Center of Pressure (COP) in mediolateral and anteroposterior planes, calculated from ground reaction force, is one of the most common methods to measure postural stability (Winter, 2009; Winter et al., 1990; Collins and De Luca, 1993; Doyle et al., 2007; De Kegel et al., 2011; Lin et al., 2008; Duarte and Freitas, 2010; Turbanski and Schmidtbleicher, 2010).

One of the techniques used to both measure and improve postural control is giving visual feedback information to the patient so that he/she can observe his/her COP excursions. In this method, the individual stands on a force plate and the position of his/her COP is shown on a monitor so that he can narrow his/her COP excursions following verbal directions. This method enables the individual to concurrently see and reduce his/her COP excursions in mediolateral and anteroposterior planes and it is widely used in the rehabilitation process of individuals with standing stability problems (Nichols, 1997; Sackley and Lincoln, 1997; Van Peppen et al., 2006; Wu, 1997). However, using this visual feedback technique is expensive and it is not applicable in clinical settings. On the other hand, using mirror visual feedback as an available inexpensive tool has proved to reduce COP excursions in elderly population (Vaillant et al., 2004; Hlavackova et al., 2009). To date, no study has investigated the effect of mirror visual feedback on static standing stability parameters in children with CP.

The results of literature review yield a controversy with regard to the effect of visual input on static standing balance in children with spastic diplegic CP. However, some researchers believe that visual input has no effect on static standing balance in children with spastic diplegic CP (Cherng et al., 1999; Donker et al., 2008; Liao et al., 1997), Nobre et al. (2009) and Rose et al. (2002) believe that visual input has a significant effect on static standing balance in children with spastic diplegic CP. Moreover, Saxena et al. (2014) states that visual input affect static standing balance only in children with spastic diplegic CP compared to children with hemiplegic CP. Importantly, the role and the degree of involvement of somatosensory inputs (including proprioception) in postural control is still a controversial matter (Simoneau et al., 1995).

Moreover, it has been suggested to use combining tasks requiring balance and usage of visual input such as mirror visual feedback to improve static standing balance (Saavedra et al., 2014). However, there is evidence showing that using mirror visual feedback can activate the mirror neurons system, which may adversely affect static standing balance (Nejati et al., 2013). With regard to the higher prevalence rate of gravitational insecurity among children with spastic diplegic CP (Case-Smith and O’Brien, 2014) and its effect on static stability, the aim of this study was to investigate the effect of visual feedback on static standing balance in children with spastic diplegic CP compared to normal children under altered sensory environment.

Methods and Materials

Thirty participants including 15 children with spastic diplegic CP and 15 typically developed children were selected conveniently for the study. The inclusion criteria were as follow: a) to be aged between 5 to 8 years old, b) not to use orthosis for walking, c) not to be afflicted with cardiovascular disease, d) not performing surgical operation 6 months prior to the initiation of the study, e) not having visual nor auditory impairments, f) not to be afflicted with epilepsy, g) not taking medicines affecting static standing stability, h) the ability to stand independently for 60 seconds, j) to be classified as level 2 or 3 on the Gross Motor Function Classification Scale and k) the ability to understand and follow verbal commands. Reluctance to continue participation in the study was considered as an exclusion criterion. It is important to note that children with spastic diplegic CP and typically developed children were matched based on age and body mass index.

The study was approved by the Ethics Committee for Human Experiments, Tehran University of Medical Sciences. An informed consent was obtained from the parents of children. Moreover, children stated their tendency to participate in the study and a small toy was given to them to thank their participation.

The children were instructed and demonstrated on how to perform the tests. Static stability was tested under 3 conditions: Eyes Open on Foam (EOF), Eyes Close on Foam (ECF) and Mirror Visual Feedback on Foam (MVFF). Children were instructed to stand for 60 seconds on a 5-cm-thick piece of foam which was placed on a force plate. In the MVFF condition, the mirror was placed in front of the children in a 1-meter distance (Hlavackova et al., 2009). Under the EOF condition, Children were instructed to look at a fixed point in a 1 meter distance in front of them. Children were demonstrated to stand on the force plate with bare feet, with their arms at their sides and their feet at shoulder width. Each condition was tested 3 times and the mean values were utilized for the final data analysis.

A Kistler force plate (Kistler Model 9285 Quartz Force, USA) was used to measure COP excursions. The following parameters were utilized to measure static stability during quite standing.

COP path length in mediolateral plane, COP path length in anteroposterior plane, COP excursion in mediolateral plane, COP excursion in anteroposterior plane, COP velocity in mediolateral plane and COP velocity in anteroposterior plane.

Normal distribution of the static standing stability parameters were calculated using Kolmogorov-Smirnov test. Descriptive statistics were utilized to describe demographic information of the participants. Independent t-test was used to compare mean values of static standing stability parameters between children with spastic diplegic CP and typically developed children. Paired t-test was used to compare each two of the three conditions (EOF and ECF, EOF and MVFF, ECF and MVFF) in children with spastic diplegic CP and typically developed children. Statistical

Table 1: Comparing mean values of static stability parameters (COP path length in ML, COP path length in AP, COP excursion in ML, COP excursion in AP, COP velocity in ML and COP velocity in AP ) in children with cerebral palsy and typically developed children during ECF, EOF and MVFF conditions

As shown in Table 1, there were no significant differences in mean values of static stability parameters in ECF condition between children with spastic diplegic CP and typically developed children (P˃0.05). However, significant differences were reported in mean values of COP excursion in mediolateral and anteroposterior planes in EOF and MVFF condition

Table 2: Comparing mean values of static stability parameters (COP path length in ML, COP path length in AP, COP excursion in ML, COP excursion in AP, COP velocity in ML and COP velocity in AP) during ECF and MVFF condition in children with cerebral palsy and typically developed children

Table 3: Comparing mean values of static stability parameters (COP path length in ML, COP path length in AP, COP excursion in ML, COP excursion in AP, COP velocity in ML and COP velocity in AP) during ECF and EOF condition in children with cerebral palsy and typically developed children

Table 2 compares mean values of static stability parameters between ECF and MVFF condition in children with cerebral palsy and typically developed children. COP path length in mediolateral and anteroposterior planes showed a significant difference between ECF and MVFF condition in children with cerebral palsy (P≤0.05). COP velocity in anteroposterior plane showed a significant difference between ECF and MVFF condition in typically developed children (P≤0.05).

Table 3 depicts that there were no significant differences in mean values of static stability parameters during ECF and EOF condition in typically developed children (P˃0.05). However, a significant difference was reported in mean value of COP excursion in anteroposterior plane during ECF and EOF condition in children with spastic diplegic CP (P≤0.05).

Table 4 compares mean values of static stability parameters during EOF and MVFF condition in children with cerebral palsy and typically developed children. There were no significant differences in mean values of static stability parameters during EOF and MVFF condition in typically developed children (P˃0.05). However, significant differences were reported in mean values of COP path length in mediolateral and anteroposterior planes, COP excursion in anteroposterior plane and COP velocity in mediolateral plane during EOF and MVFF condition in children with spastic diplegic CP (P≤0.05).

Discussion

Static standing stability is the ability to maintain and/or control the center of mass of the body in the base of support provided by legs during standing (Umphred et al., 2013, Westcott et al., 1997). Poor standing stability can restrict motor abilities of an individual resulting in the reduced environmental exploration leading to a reduced quality of life (Ramachandran and Altschuler, 2009). The results of literature review yield a controversy with regard to the effect of visual nput on static standing balance in children with spastic diplegic CP. Importantly, the role and the degree of involvement of somatosensory inputs (including proprioception) in postural control is still a controversial matter (Simoneau et al., 1995). Moreover, it has been suggested to use combining tasks such as mirror visual feedback to improve static standing balance (Saavedra et al., 2014). However, there is evidence showing that using mirror visual feedback may adversely affect static standing balance (Nejati et al., 2013). Therefore, the aim of this study was to investigate the effect of visual feedback on static standing balance in children with spastic diplegic CP compared to normal children under altered sensory environment.

As shown in Table 1, all mean values of static stability

Table 4: Comparing mean values of static stability parameters (COP path length in ML, COP path length in AP, COP excursion in ML, COP excursion in AP, COP velocity in ML and COP velocity in AP) during EOF and MVFF condition in children with cerebral palsy and typically developed children

to typically developed children that is in consistent with the result obtained by similar studies (Liao et al., 1997; Cherng et al., 1999; Donker et al., 2008; Nobre et al., 2009). However, in ECF condition there was no significant difference between children with spastic diplegic CP and typically developed children with regard to static stability parameters that is in contrast with the results obtained by Saxena (Saxena et al., 2014). Perhaps, the differences in mean values of age and body mass index of the participants, thickness of the used foam, using orthosis for standing by children with spastic diplegic CP (Saxena et al., 2014) may be responsible for such differences in the results.

As shown in the Table 2, all mean values of static stability parameters during ECF condition are lower compared to MVFF condition in children with spastic diplegic CP. That is, children with spastic diplegic CP experience a better static standing stability during ECF condition compared to MVFF condition. However, all mean values of static stability parameters during ECF condition are higher compared to MVFF condition in typically developed children. That is, typically developed children experience a better static standing stability during MVFF condition compared to ECF condition.

Although mean value of COP excursion in anteroposterior plane is significantly lower in ECF condition compared to EOP condition in children with spastic diplegic CP, with reference to the literature on the importance and contribution of each parameter in predicting static standing stability no definitive comment upon the answer to the effect of vision on standing stability under altered sensory environment in children with spastic diplegic CP may be made. The mean value of difference between ECF condition and EOP condition was 515.36 mm/min and 761.22 mm/min with regard to COP velocity in mediolateral and anteroposterior planes, respectively, in children with spastic diplegic CP. According to the literature COP velocity in mediolateral and anteroposterior planes have the most importance and contribution in predicting static standing stability (Doyle et al., 2005). The biomechanical interpretation of the results in comparing ECF condition and EOP condition is that children with spastic diplegic CP in ECF condition experience COP excursion in a narrower amplitude compared to EOP condition; however, the COP velocity especially in anteroposterior plane is higher in ECF condition compared to EOP condition which is consistent with the results obtained by similar studies (Cherng et al., 1999; Donker et al., 2008; Liao et al., 1997).

As depicted in Table 4, most of the mean values of static stability parameters, especially COP velocity in mediolateral and anteroposterior planes, during EOF condition are lower compared to MVFF condition in children with spastic diplegic CP. That is, children with spastic diplegic CP experience a better static standing stability during EOF condition compared to MVFF condition. However, there were no significant differences in mean values of static stability parameters during EOF condition and MVFF condition in typically developed children.

Collectively, the results of Table 2, 3 and 4 show that children with spastic diplegic CP experience the worst static standing stability in MVFF condition and the best static standing stability in ECF condition. Moreover, most of the mean values of static stability parameters in EOF condition place in the middle of this range, from the worst standing stability experience to the best standing stability experience. However, in typically developed children the worst static standing stability experience was in ECF condition and the best static standing stability experience was in EOF and MVFF condition.

It has been said that children with CP have an increased dependence on somatosensory inputs. These inputs are from joint receptors, muscle spindles and cutaneous receptors (Shumway-Cook and Woollacott, 2007) which are possible to be affected by musculoskeletal impairments that are common in children with CP (Saxena et al., 2014). Therefore, these structural impairments result in an altered sensory processing and organizing pattern affecting the ability to integrate multiple inputs from different resources, as a cognitive task, to adapt a proper stability (Bar-Haim et al., 2013). From this point of view, it can be said that children with spastic diplegic CP had the best static standing stability in ECF condition because this condition had the lowest load of integrating multiple sensory inputs from different organs compared to EOF and MVFF condition. Similarly, as the cognitive requirements of the task intensify, the children will experience a worse (EOF condition) and the worst (MVFF condition) static standing stability. However, typically developed children with an intact integrating multiple inputs process had the worst static standing stability experience in ECF condition, because the useful visual input had been eliminated, and the best static standing stability experience in EOF and MVFF condition with no difference between EOF and MVFF condition.

According to the results, using mirror visual feedback had the worst effect on standing stability of children with spastic diplegic CP. Therefore, the suggestion of Saavendra et al (Saavedra et al., 2014) about using combining tasks requiring balance and usage of visual input such as mirror visual feedback cannot improve static standing stability of children with spastic diplegic CP. However, according to Nejati et al. (2013) activation of the mirror neurons system, especially when using mirror visual feedback, may adversely affect static standing balance. With regard to the hypothesis of Nijati et al. (2013) and the similarity of results between children with spastic diplegic CP and typically developed children during MVFF condition, it can be posited that the child experience a worse standing stability during MVFF condition because watching his/her instability in the mirror can adversely affect the static stability. Therefore, the child scrambles to correct his/her instability leading to a more instability. From another point of view, children with neurological conditions such as CP need to hire compensating strategies to compensate for their musculoskeletal impairments so that they can experience a better standing stability. This extra cognitive task may be the reason because of which the children with CP have difficulty in dealing with environmental distractions or external cognitive loads such as combining tasks which reduce stability in children with CP (Reilly et al., 2008).

There are some key limitations and useful suggestions which should be acknowledged regarding this study. First, there is at present no actual measure of “standing stability”, merely inferences based on quantitative parameters such as maximum COP excursion, COP velocity, etc. None of these parameters have been shown to be solely indicative of the quality of standing balance, though they are certainly related. At present, clinical quantitative measures such as the “equilibrium score” obtained through use of a Neurocom balance testing system use a composite of several parameters for assessment. Alternatively, performance-based metrics are used such as the Berg Balance Scale (Downs et al., 2013). Each such method has its limitations.

Second, the use of quiet standing with eyes open and eyes close as a test of standing stability is the best-case scenario for investigating visual input and standing stability. However, it imposes limitations to detect a significant and reliable difference in standing stability between children with spastic diplegic CP and typically developed children. If, due to increased use of compensatory strategies, children with spastic diplegic CP need to use more cognitive resources to achieve standing balance performance, then distractions or other external cognitive loads (other than MVFF condition) such as doing a mentally demanding quiz for example should degrade their stability performance more than typically developed children. Future study designs should take these concepts into consideration.

Conclusions

According to the results of this study, children with spastic diplegic CP experience the best static standing stability in ECF condition. The standing stability of these children reduces as the cognitive load of the conditions intensifies in EOF and MVFF, respectively. Using mirror visual feedback worsens the standing stability of both typical children and children with CP and it cannot be used to improve static standing stability of children with spastic diplegic CP.

Acknowledgments

This study was funded by the Tehran University of Medical Sciences Research Committee with a reference number of 94-04-32-30935. The authors would like to pay their special thanks to parents and children who participated in this study.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this article.

Authors’ Contribution

Study concept and design was prepared by all the authors. Analysis and interpretation of data was done by Ali Tahmasebi. Ali Tahmasebi and Parvin Raji drafting of the manuscript. Mostafa Kamali critically revised the manuscript for important intellectual content.

References

• • Bar-Haim, S., Al-Jarrah, M. D., Nammourah, I. & Harries, N. 2013. Mechanical efficiency and balance in adolescents and young adults with cerebral palsy. Gait & posture, 38, 668-673. http://dx.doi.org/10.1016/j.gaitpost.2013.02.018
• • Campbell, S. K., Palisano, R. J. & Vander Linden, D. W. 2006. Physical therapy for children, Saunders.
• • Case-Smith, J. & O’brien, J. C. 2014. Occupational therapy for children and adolescents, Elsevier Health Sciences.
• • Cherng, R.J., Su, F.C., Chen, J.J. & Kuan, T.S. 1999. Performance of static standing balance in children with spastic diplegic cerebral palsy under altered sensory environments. American journal of physical medicine & rehabilitation, 78, 336-343. http://dx.doi.org/10.1097/00002060-199907000-00008
• • Collins, J. J. & De Luca, C. J. 1993. Open-loop and closed-loop control of posture: a random-walk analysis of center-of-pressure trajectories. Experimental brain research, 95, 308-318. http://dx.doi.org/10.1007/BF00229788
• • Dalvand, H., Dehghan, L., Hadian, M. R., Feizy, A. & Hosseini, S. A. 2012. Relationship between gross motor and intellectual function in children with cerebral palsy: a cross-sectional study. Archives of physical medicine and rehabilitation, 93, 480-484. http://dx.doi.org/10.1016/j.apmr.2011.10.019
• • De Kegel, A., Dhooge, I., Cambier, D., Baetens, T., Palmans, T. & Van Waelvelde, H. 2011. Test–retest reliability of the assessment of postural stability in typically developing children and in hearing impaired children. Gait & posture, 33, 679-685. http://dx.doi.org/10.1016/j.gaitpost.2011.02.024
• • Donker, S. F., Ledebt, A., Roerdink, M., Savelsbergh, G. J. & Beek, P. J. 2008. Children with cerebral palsy exhibit greater and more regular postural sway than typically developing children. Experimental Brain Research, 184, 363-370. http://dx.doi.org/10.1007/s00221-007-1105-y
• • Downs, S., Marquez, J. & Chiarelli, P. 2013. The Berg Balance Scale has high intra-and inter-rater reliability but absolute reliability varies across the scale: a systematic review. Journal of physiotherapy, 59, 93-99. http://dx.doi.org/10.1016/S1836-9553(13)70161-9
• • Doyle, R. J., Hsiao-Wecksler, E. T., Ragan, B. G. & Rosengren, K. S. 2007. Generalizability of center of pressure measures of quiet standing. Gait & posture, 25, 166-171. http://dx.doi.org/10.1016/j.gaitpost.2006.03.004
• • Doyle, T. L., Newton, R. U. & Burnett, A. F. 2005. Reliability of traditional and fractal dimension measures of quiet stance center of pressure in young, healthy people. Arch Phys Med Rehabil, 86, 2034-40. http://dx.doi.org/10.1016/j.apmr.2005.05.014
• • Duarte, M. & Freitas, S. M. 2010. Revision of posturography based on force plate for balance evaluation. Brazilian Journal of physical therapy, 14, 183-192. http://dx.doi.org/10.1590/S1413-35552010000300003
• • Erkin, G., Delialioglu, S. U., Ozel, S., Culha, C. & Sirzai, H. 2008. Risk factors and clinical profiles in Turkish children with cerebral palsy: analysis of 625 cases. International journal of rehabilitation Research, 31, 89-91. http://dx.doi.org/10.1097/MRR.0b013e3282f45225
• • Ferrari, A., Tersi, L., Ferrari, A., Sghedoni, A. & Chiari, L. 2010. Functional reaching discloses perceptive impairment in diplegic children with cerebral palsy. Gait & posture, 32, 253-258. http://dx.doi.org/10.1016/j.gaitpost.2010.05.010
• • Hlavackova, P., Fristios, J., Cuisinier, R., Pinsault, N., Janura, M. & Vuillerme, N. 2009. Effects of mirror feedback on upright stance control in elderly transfemoral amputees. Archives of physical medicine and rehabilitation, 90, 1960-1963. http://dx.doi.org/10.1016/j.apmr.2009.05.016
• • Laisram, N., Srivastava, V. & Srivastava, R. 1992. Cerebral palsy: an etiological study. The Indian Journal of Pediatrics, 59, 723-728. http://dx.doi.org/10.1007/BF02859408
• • Ledebt, A., Becher, J., Kapper, J., Rozendaal, R. M., Bakker, R., Leenders, I. C. & Savelsbergh, G. J. 2005. Balance training with visual feedback in children with hemiplegic cerebral palsy: effect on stance and gait. Motor control-campaign, 9, 459.
• • Lepage, C., Noreau, L., Bernard, P. & Fougeyrollas, P. 1998. Profile of handicap situations in children with cerebral palsy. Scandinavian Journal of Rehabilitation Medicine, 30, 263-272. http://dx.doi.org/10.1080/003655098444011
• • Liao, H.F. & Hwang, A.W. 2003. Relations of balance function and gross motor ability for children with cerebral palsy. Perceptual and motor skills, 96, 1173-1184. http://dx.doi.org/10.2466/pms.2003.96.3c.1173
• • Liao, H. F., Jeny, S. F., Lai, J. S., Cheng, C. K. & Hu, M. H. 1997. The relation between standing balance and walking function in children with spastic diplegic cerebral palsy. Developmental Medicine & Child Neurology, 39, 106-112. http://dx.doi.org/10.1111/j.1469-8749.1997.tb07392.x
• • Lin, D., Seol, H., Nussbaum, M. A. & Madigan, M. L. 2008. Reliability of COP-based postural sway measures and age-related differences. Gait & posture, 28, 337-342. http://dx.doi.org/10.1016/j.gaitpost.2008.01.005
• • Nejati, V., Bazrafkan, F. & Asari Jami, S. 2013. Social cognition and motor control: Evidence from postural control of balance observer. Journal of Research in Rehabilitation Sciences, 9, 59-64.
• • Nichols, D. S. 1997. Balance retraining after stroke using force platform biofeedback. Physical therapy, 77, 553-558.
• • Nobre, A., Monteiro, F., Golin, M., Biasotto-Gonzalez, D., Corrêa, J. & Oliveira, C. 2009. Analysis of postural oscillation in children with cerebral palsy. Electromyography and clinical neurophysiology, 50, 239-244.
• • Oskoui, M., Coutinho, F., Dykeman, J., Jetté, N. & Pringsheim, T. 2013. An update on the prevalence of cerebral palsy: a systematic review and meta‐analysis. Developmental Medicine & Child Neurology, 55, 509-519. http://dx.doi.org/10.1111/dmcn.12080
• • Ramachandran, V. & Altschuler, E. L. 2009. The use of visual feedback, in particular mirror visual feedback, in restoring brain function. Brain, awp135. http://dx.doi.org/10.1093/brain/awp135
• • Reilly, D. S., Woollacott, M. H., Van Donkelaar, P. & Saavedra, S. 2008. The interaction between executive attention and postural control in dual-task conditions: children with cerebral palsy. Archives of physical medicine and rehabilitation, 89, 834-842. http://dx.doi.org/10.1016/j.apmr.2007.10.023
• • Rha, D.W., Kim, D. J. & Park, E. S. 2010. Effect of hinged ankle-foot orthoses on standing balance control in children with bilateral spastic cerebral palsy. Yonsei medical journal, 51, 746-752. http://dx.doi.org/10.3349/ymj.2010.51.5.746
• • Rose, J., Wolff, D. R., Jones, V. K., Bloch, D. A., Oehlert, J. W. & Gamble, J. G. 2002. Postural balance in children with cerebral palsy. Developmental Medicine & Child Neurology, 44, 58-63. http://dx.doi.org/10.1017/S0012162201001669
• • Saavedra, S., Bellows, D. & Da Costa, C. S. 2014. Commentary on “Analysis of Postural Stability in Children With Cerebral Palsy and Children With Typical Development: An Observational Study”. Pediatric Physical Therapy, 26, 331. http://dx.doi.org/10.1097/PEP.0000000000000059
• • Sackley, C. M. & Lincoln, N. B. 1997. Single blind randomized controlled trial of visual feedback after stroke: effects on stance symmetry and function. Disability and rehabilitation, 19, 536-546. http://dx.doi.org/10.3109/09638289709166047
• • Saxena, S., Rao, B. K. & Kumaran, S. 2014. Analysis of postural stability in children with cerebral palsy and children with typical development: An observational study. Pediatric Physical Therapy, 26, 325-330. http://dx.doi.org/10.1097/PEP.0000000000000060
• • Shumway-Cook, A. & Woollacott, M. H. 1995. Motor control: theory and practical applications, Williams & Wilkins Baltimore.
• • Shumway-Cook, A. & Woollacott, M. H. 2007. Motor control: translating research into clinical practice, Lippincott Williams & Wilkins.
• • Simoneau, G. G., Ulbrecht, J. S., Derr, J. A. & Cavanagh, P. R. 1995. Role of somatosensory input in the control of human posture. Gait & posture, 3, 115-122. http://dx.doi.org/10.1016/0966-6362(95)99061-O
• • Turbanski, S. & Schmidtbleicher, D. 2010. Postural control depends on testing situation. Sportverletzung Sportschaden: Organ der Gesellschaft fur Orthopadisch-Traumatologische Sportmedizin, 24, 123-128. http://dx.doi.org/10.1055/s-0030-1267402
• • Umphred, D. A., Lazaro, R. T., Roller, M. & Burton, G. 2013. Neurological rehabilitation, Elsevier Health Sciences.
• • Vaillant, J., Vuillerme, N., Janvy, A., Louis, F., Juvin, R. & Nougier, V. 2004. Mirror versus stationary cross feedback in controlling the center of foot pressure displacement in quiet standing in elderly subjects. Archives of physical medicine and rehabilitation, 85, 1962-1965. http://dx.doi.org/10.1016/j.apmr.2004.02.019
• • Van Peppen, R., Kortsmit, M., Lindeman, E. & Kwakkel, G. 2006. Effects of visual feedback therapy on postural control in bilateral standing after stroke: a systematic review.
• • Westcott, S. L., Lowes, L. P. & Richardson, P. K. 1997. Evaluation of postural stability in children: current theories and assessment tools. Physical therapy, 77, 629-645.
• • Winter, D. A. 2009. Biomechanics and motor control of human movement, John Wiley & Sons. http://dx.doi.org/10.1002/9780470549148
• • Winter, D. A., Patla, A. E. & Frank, J. S. 1990. Assessment of balance control in humans. Medical Progress through Technology, 16, 31-51.
• • Wu, G. 1997. Real-time feedback of body center of gravity for postural training of elderly patients with peripheral neuropathy. Rehabilitation Engineering, IEEE Transactions on, 5, 399-402.