Development of a clinical measure of postural control for assessment of adaptive seating in children with neuromotor disabilities.

SF Fife, MSc, LIT, is Research Therapist, Therapy Department, Sunny Hill Hospital for Children, 3644 Slocan St, Vancouver, British Columbia, Canada V5M 3E8. Address all correspondence to Miss Fife.

IA Roxborough, BSR, OT/PT, is Director, Therapy Department, Sunny Hill Hospital for Children. RW Armstrong, MD, Phd, FRCPC, is Coordinator of Research and Medical Director of the Neuromotor Program, Sunny Hill Hospital for Children, and Assistant Professor, Department of Pediatrics, and Associate Member, School of Rehabilitation Medicine, University of British Columbia, T3252211, Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2B5.

SR Harris, Phd, VT, FAPTA, is Associate Professor, School of Rehabilitation Medicine, Universitv of British Columbia, and Faculty Clinical Associate, Therapy Department, Sunny Hill Hospital for Children.

JL Gregson, BSc, PT, is Physical Therapist, Positioning Assessment Unit, Sunny Hill Hospital for Children.

D Field, BSc, OT, is Occupational Therapist, Positioning Assessment Unit, Sunny Hill Hospital for Children.

This research project was approved by the Clinical Screening Committee for Research and Other Studies involving Human Subjects, University of British Columbia. It was supported by Grant '89-50, British Columbia Medical Services Foundation, A preliminary report of the project was presented at the Seventh International Seating Symposium, Memphis, Tenn, February 20-22, 1991.

Adaptive seating has been used increasingly over recent years as a therapeutic modality to improve postural control and functional performance(1-4) and to assist in the prevention of musculoskeletal contractures, deformities,(5) decreased respiratory function,(6) and pressure sores.(7) The number of individuals requiring specialized seating is not known. One of the most comprehensive surveys of seating needs was conducted in the Dundee district of Scotland.(8) Based on referrals elicited from hospital and community health professionals and agencies, self-referral by disabled persons, and direct assessment of 400 referred subjects, it was estimated that an average of 4.6 individuals per 1,000 persons in the total population surveyed (N=204,000) had seating problems. Older subjects with problems arising from stroke, arthritis, and general frailty constituted by far the largest group of inadequately seated subjects. Persons with cerebral palsy and associated disorders constituted the next largest diagnostic group. The latter group exhibited the most complex seating problems.

Attempts have been made to determine the nature and cost of specialized seating provided by the many seating clinics in North America. A survey of 320 members of the Wheeled Mobility and Seating Special Interest Group of the Association for the Advancement of Rehabilitation and Assistive Technologies generated a 13% response rate.(9) Respondents represented 43 seating centers from 22 states and 3 Canadian provinces. This relatively small percentage of respondents reported a total volume cost of $7,000,000 annually for new seating systems, Boenig et al state,

Adding volume from re-evaluations of seating and considering that our survey sample is a small segment of seating practitioners and suppliers in the United States and Canada, it is apparent that seating continues to grow as a significant area of rehabilitation treatment and equipment.(9(p10))

Based on our experience in the children's seating clinic of Sunny Hill Hospital for Children (Vancouver, British Columbia, Canada), the cost of individual seating systems ranges from approximately $400 to $5,000 and may require replacement at 2- to 4-year intervals. Mobilility bases, such as strollers or wheelchairs, could add a further $1,000 to $10,000 per individual. These costs do not include the professional time for assessment and follow-up.

As in many other areas of rehabilitation, the capacity to measure the success of seating system applications has not kept pace with the widespread and increasing use of this modality. The need for reliable and valid measurements to assess the effects of adaptive seating has become increasingly evident.

At a consensus conference on the efficacy of physical therapy in the management of cerebral palsy held in 1990, Campbell identified postural effects as potentially important outcomes of therapy, but noted that "clinically feasible tools for assessing postural alignment, control and stability are not currently available."(10(pg139)) She urged the development of such tools for use as outcome measures in future efficacy research.

As the costs and sophistication of seating systems increase, so does the pressure from third-party payers to justify seating selections. Reliable and valid seating assessment measurements would increase accountability in developing individualized prescriptions for seating and would allow physical therapists to test clinical assumptions regarding anticipated seating outcomes. Furthermore, the development of standardized seating assessment tools would facilitate communication across different centers.

New concepts of motor control must be incorporated into the development of seating assessment instruments. Whereas earlier theories regarded posture as a static state representing summed responses of stretch reflexes, current thinking suggests a primary function of posture is the integration of movements into coordinated action sequences.(11) Thus, movement and posture are believed to be tightly integrated rather than separately controlled. This hypothesis suggests a comprehensive assessment of seating system outcomes should not be confined to assessment of postural alignment, but should include an assessment of the effects of changes of alignment on functional outcomes, such as control of the trunk and upper extremities, swallowing, and respiration, and on prevention of pressure sores. At Sunny Hill Hospital for Children, we are in the process of incorporating assessment of both alignment and function into a measure that could serve as an evaluation instrument for adaptive seating outcomes. The purposes of this article are to review the literature on seating assessments and to report on the development and pilot testing of the Seated Postural Control Measure (SPCM) and the Level of Sitting Scale (LSS).

Literature Review

The research literature on assessment of postural control and seating can be divided into two broad areas: (1) measures requiring complex instrumentation and (2) clinical evaluation scales.

Use of Measures Requiring Complex instrumentation

The most commonly measured variable in postural control studies is the amount of postural sway. Usually, sway characteristics are described on the basis of movements of the center of pressure recorded in force-plate studies. Riach and Hanes(12) used this technique to describe the maturation of postural sway in young children. Sway characteristics in sitting are now being studied in adults who have incurred a brain injury(13) and in children with cerebral palsy.(14) investigators at the Hugh MacMillan Rehabilitation Centre (Toronto, Ontario, Canada) have used a postural tracking system to monitor children's seated stability on horizontal and anteriorly tilted seats.(15)

If postural movement strategies are to be effective in returning the center of body mass to a position within the support base, the timing of muscle contractions is important. Delay in the onset of muscle responses can result in instability if the center of body mass moves outside the limits of stability before an effective corrective force can be generated by appropriate muscles.(16) The correct sequencing of muscle responses is also important to ensure appropriate alignment of the multiply linked body segments.(16)

These aspects of muscle coordination have been studied by examining electromyographic (EMG) and force-plate responses to perturbations of standing subjects. Such studies have been reported for newly standing children (ie, children during development of independent standing balance),(17) children with cerebral palsy,(18), and children with Down syndrome.(16) Woollacott and colleagues(19) described the development of motor coordination patterns in children without neuromotor disabilities, aged 3 months to 10 years, while standing and, in some subjects, while seated.

Other researchers have used videography in studying the development of postural control for sitting. In an unpublished study, Harbourne and colleagues(20) digitized movement trajectories from videotapes of infants who were at the "presitting" and "propped sitting" levels of development. The infants were manually supported in erect sitting and were videotaped while the support was withdrawn. The movement trajectories as well as EMG recordings demonstrated progressively organized movement control strategies in the children with more experience in sitting. in other unpublished studies,(21-22) no significant effects of supported versus unsupported seating on speed and smoothness of reaching were found in six children with spastic cerebral palsy and in five 4- to 5-month-old infants without neuromotor disabilities. The investigators in those studies caution that, because of small sample sizes and large individual variability, the effect of trunk support on function remains unclear. Reports in peer-reviewed publications of this and other research will add to our ability to use these and other findings. Further investigation of these findings is clearly important for providers of adaptive seating.

Effects of seated positioning on resting EMG activity have been described for nondisabled subjects (23-24) and for subjects with cerebral palsy.(25) Although nondisabled adults showed decreased back muscle activity as the orientation of the backrest was inclined posteriorly,(23) children with mild to moderate cerebral palsy showed the least muscle activity with a vertical backrest.(25) in another study, children with cerebral palsy who could sit independently had increased lordosis and increased activity of the lumbar extensor muscles when they sat on seats with an anterior tilt as compared with horizontal seats. These findings contrast with those for nondisabled adults in which anterior seat tilt led to decreased EMG activity recorded at the vertebral levels of T-10 and L-3.(24) These contrasting findings, however, possibly indicate beneficial effects of anteriorly tilted seats.

The children with cerebral palsy, by reducing the excessive lumbar flexion associated with their posture on horizontal seats, sat more erect, and the adults with lessened back muscle activity, demonstrated increased sitting tolerance. In a pilot study of eight subjects with severe spastic quadriplegia,(27) there were no consistent EMG patterns across the subjects in response to seating system changes in orientation of the backrest or seat-to-back angle. Although responses to position change (ie, significantly increased or reduced EMG activity in one or more muscles) were evident, they appeared to be unique to individual subjects.

In examining the effects of different inclinations of the seat backrest while maintaining a 90-degree seat-to-back angle, Nwaobi(28) found that children and adolescents with cerebral palsy could use their upper extremities to activate a Switch significantly faster when positioned in a vertical orientation compared with 15 degrees of anterior incline and 15 and 30 degrees of posterior incline. When the effects of varying hip flexion angles (backrest maintained in the vertical plane while the seat angle was manipulated) or upper-extremity function were measured in similar groups of subjects, however, conflicting results were reported by Seeger et al(29) and Nwaobi et al.(30) Seeger et al found no effects of varying hip angles on response time, whereas Nwaobi et al found performance time optimal with hip flexion at 90 degrees.

Use of Clinical Evaluation Scales

Several clinical evaluation scales for assessing the effectiveness of seating systems have been described recently. According to Kirshner and Guyatt,(31) a clinical evaluation scale should include all items assumed likely to be affected by the intervention. Thus, items on speed and accuracy of independent wheelchair mobility should be included, even though not all children will attain independent mobility. Each item should have a sufficient number of defined levels in order to detect clinically meaningful change. The scale should also be reliable and feasible to administer in the clinical setting. Whereas the following scales are clinically feasible to administer, they have shortcomings that limit their value to clinicians.

Questionnaires to assess the caregivers' retrospective perceptions of subjects' behavioral changes after use of seating systems were described by Hulme and colleagues in 1983(32) and more recently by Murphy,(33) Although the behavioral information was valuable, these questionnaires did not directly assess changes in performance. An assessment instrument based on direct observations by therapists was also described by Hulme and associates.(34,35) This instrument was used to code various seated behaviors, including controlled sitting posture, head control, reaching, grasping, eve tracking, drinking, eating, sitting support, and alertness. The items were coded by two to six defined levels of task achievement or by multiple timed trials. The authors described interrater reliability of 80% agreement for this instrument, but they did not report test-retest reliability. Probabilistic statistical analysis of reliahility was not reported. We believe the time required for administration of this instrument (ie, 60-90 minutes) makes it impractical for routine clinical use.

A seven-level developmental scale of sitting ability, the Level of Sitting Ability Scale (LSAS), has been described for use in prescription of adaptive LSAS. When we examine whether SPCM scores respond over time in children who have undergone significant clinical change, we would be able to determine whether such responsiveness is similar for children with different degrees of need for adaptive seating. We therefore modified the LSAS to clarify definitions of scale levels and renamed it the Level of Sitting Scale. The LSS is described in Appendix 2.

Pilot Study

We conducted a pilot study to assess the interrater and test-retest reliability of the SPCM items and the LSS scores.

Subjects

The original sample of 45 children included all clients of the Positioning Assessment Unit at Sunny Hill Hospital for Children who were from the Greater Vancouver Regional District, who were less than 19 years of age, and for whom we had informed consent. The study was approved by the Clinical Screening Committee for Research and Other Studies Involving Human Subjects at the University of British Columbia (Vancouver, British Columbia, Canada). All subjects were nonambulatory and users of seating systems prescribed prior to initiation of the study. The sessions with the first 4 subjects were scheduled as practice trials, and their scores were not included in the data analysis. An additional subject did not participate because of her inability to attend during scheduled session times. The mean age of the remaining 40 subjects was 9.06 years, with a range of 1.67 to 18.5 years. Table I presents the characteristics of the subjects.

The two raters were an occupational therapist (DF) and a physical therapist (JLG). Each rater had at least 5 years' experience in pediatrics, including 2 or more years in the adaptive seating field. They were generally familiar with the measure, as they had participated in its development. Their training consisted of study of the Administration Guidelines, informal administration of some of the items in the raters' regular practice, and supervised (by SEF) conduct of the complete study procedure with the first four subjects. Therefore, persons without the same degree of familiarity with the measure may not be able to obtain the same degree of reliability.

Procedure

Data were collected over a 6-week period. Subjects attended two assessment sessions, which were approximately 3 weeks apart. We assumed that a 3-week interval would be adequate for the raters to forget scores from the first assessment and that the children would not undergo significant clinical change between sessions.

Both the LSS and the SPCM were administered to subjects at each of the two assessment sessions. Each rater independently administered the LSS while the other rater was outside of the testing room. The SPCM was then administered by each rater to the subjects in two different conditions: (1) sitting with their prescribed seating system (condition 1) and (2) sitting without their prescribed seating system (condition 2). The rationale for administering the SPCM under both seating conditions was that we proposed to use the SPCM to compare alignment and function outcomes under both conditions.

If children were not independent sitters (ie, if they scored lower than 5 on the LSS), they were provided with Tumbleform Feeder Seats' for condition 2. We used this procedure to avoid providing the subject with "hands-on" support, which is difficult to standardize. The Tumbleform seats with a lap belt were placed on a modified wheeled base[double dagger] that could be reproducibly oriented in space. The raters jointly selected an orientation of the base judged to be optimal for the safety, comfort, and function of the individual subjects. If subjects scored 5 or higher on the LSS, they sat on a bench with their feet supported while the SPCM was administered in condition 2.

The order of testing (alignment and function) for each subject was alternated between test conditions. The order of test conditions was alternated in successive subjects. For each subject, this sequence was maintained in the second session. Both raters assessed and independently scored the alignment items simultaneously. One rater administered the function items while the other rater observed, and each rater independently scored the items. The same rater administered the functional items to a given subject at both sessions, and the administering rater was alternated for successive subjects.

Subjects attended in pairs for sessions of approximately 2 hours' duration. This procedure allowed for rest periods and repositioning of subjects between administration of the LSS and the SPCM under the two test conditions.

Data Analysis

Interrater and test-retest reliability for SPCM and LSS items were assessed by examination of agreement tables. The percentage of agreement (number of agreements/total observations X 100) and the Kappa statistic were used as indexes of estimated reliability. The Kappa statistic reflects the percentage of agreement not attributable to chance alone.(43,44) Kappa values will be low, even if percentage of agreement is high, if the observed scores for an item fall mainly in one category. in this case, there is a greater probability that chance alone is responsible for agreement. Haley and colleagues,(45), in assessing reliability of the Movement Assessment of infants,(40) interpreted Kappa values as follows: poor, if less than .40; fair to good, if .40 to .74; and excellent, if .75 or higher. We chose a Kappa value of .40 as our minimum level of acceptable reliability. The percentage-of-agreement statistic is included in Tables 2 through 4 because of its frequent use in the literature.

Results

Seated Postural Control Measure

Interrater reliability. Across the two seated conditions and the two test sessions, the overall mean of the item Kappa coefficients was .45 for the alignment section and .85 for the function section (Tab. 2). Within the four data sets, the number of alignment items with Kappa values under the acceptable level of.40 ranged from 6 to 12 out of a possible 22 items. In the function section, only 1 of 12 items had a Kappa value of less than .40. There were 5 alignment and 11 function items with Kappa values of .40 or better across all four data sets.

Test-retest reliability. Across the two seated conditions and two raters, the overall mean of the item Kappa coefficients was .35 for the alignment section and .29 for the function section (Tab. 3). Within the four data sets, the number of alignment items with Kappa values under the acceptable level of .40 ranged from 11 to 15 out of a possible 22 items. The number of function items with Kappa values less than .40 ranged from 8 to 10 out of a possible 12 items. Only one alignment and one function item had a test-retest Kappa value of .40 or better across all four data sets.

Level of Sitting Scale

The mean Kappa value for interrater reliability over the two test sessions was .60, with mean agreement of 69%. The mean Kappa value for test-retest reliability across the two raters was .55, with mean agreement of 64%. The results are presented in Table 4.

Discussion

Seated Postural Control Measure

Neither interrater nor test-retest reliability appeared to be consistently better between seating conditions, raters, or test sessions. As shown in Tables 2 and 3, percentage-of-agreement values for items were generally higher than Kappa values. This finding reflects the fact that the Kappa statistic corrects for chance agreement.

The low Kappa values for interrater agreement were likely due to a combination of three factors: lack of standardized use of the measure, inaccurate observations by raters, and fluctuations in the subjects' behaviors. Clarity of item definitions and test guidelines or the amount of rater training, for example, may not have been adequate to ensure standardized administration of the measure. The ability of trained therapists to observe and accurately detect small angular deviations while children are positioned in their seating systems may also explain the limited reliability of the alignment items.

In a clinical study of cervical spine motion, Youdas et al(46) reported only poor to fair interrater reliability for both goniometric and visual estimation measurements in patients with orthopedic disorders of the cervical spine. Measurement of spinal alignment, as in our study, is further hampered when children are positioned in closely fitted seating systems, because simple measuring instruments such as goniometers, inclinometers,(47) scoliometers,(48) flexible rulers,(49) and the Schober method(50) cannot be utilized. Williams and Callaghan,(51) however, reported that physical therapists produced results of similar accuracy when measuring a shoulder joint angle with visual estimation and three types of standard goniometers. In another recent study in a clinical setting, visual estimates of knee range of motion yielded interrater intraclass correlation coefficients of >.80.52 THUS, there is some support in the literature for therapists consistently estimating positions for the large peripheral joints. Even though therapists commonly differentiate among mild, moderate, and severe deviations from normal spinal posture, however, it has not yet been demonstrated that this can be done reliably. Although the raters in our study assessed alignment of body segments almost simultaneously for the interrater comparisons, some children were unable to sit quietly and frequently moved during the assessment. Moreover, these were the children likely to have less constraint built into their seating systems.

Poor test-retest results, especially those for the function section, were not likely due to the clinical condition of the subjects undergoing change in the short interval between tests. Subject behaviors, however, might well have changed, in part because of the subjects' familiarity with the procedures on the second visit. By examining within-subject change scores for one data set, we investigated whether there was a consistent trend toward higher or lower scores on the second test. The maximum possible change in item scores was 3. The mean within-subject change score between tests ranged from -0.09 to 0.23 for alignment items and from 0.14 to 0.17 for function items. The direction of change scores between tests indicated worse alignment on the second test for 15 of 22 alignment items. The direction of change scores for function items was evenly divided between positive and negative change.

We intend to examine change scores more thoroughly before designing the next version of the SPCM. This examination will help us determine whether there were any consistent patterns of change scores that could be attributed to the seating conditions, raters, or test sessions. Several alternatives will also be considered.

Items in both sections of the SPCM are being reviewed by our seating clinicians with the purpose of improving clarity of items and deleting the most unreliable items, particularly when the item content is not essential. An educational module for more standardized training of raters will be developed. Reduction of item levels, from four levels to three levels, will also be considered if acceptable reliability is not achieved in the next clinical trial. We will also determine whether reliability is enhanced when testing total SPCM scores, alignment and function section scores, and scores for relevant clusters of items such as all alignment items of the pelvis or trunk, all grasp items, and so on. Once acceptable reliability of the SPCM is attained (our goal is a Kappa value of -.40 for each item), we will determine whether the instrument can be used to detect significant clinical change, as may occur, for example, in children following recovery from injury or following treatment interventions. This will be done initially by using serial administrations of the measure on children following acute brain injury, because these children are likely to show more rapid change in status than children with cerebral palsy. Changes in clinical status will be monitored to determine whether the changes are accompanied by changes on the SPCM.

Level of Sitting Scale

Considering the interpretation of Kappa levels,(45) the LSS reliability estimates were fair to good. The majority of disagreements, between raters or between tests, were by only one level. There was disagreement by more than one level in a maximum of four subjects across the two data sets. We anticipate that better standardization of administration procedures will enhance the interrater reliability of the LSS. It will then be used in studies of concurrent validity of the refined SPCM.

The generalizability of the reliability results reported in this article is limited on three counts. First, the two raters were involved in development and critique of the measures and thus were not subject to the same motivations/biases a rater from another center may have had. Second, although each rater independently administered the SPCM to one half of the subject sample, the presence of the other rater may have influenced interactions with the subjects. Third, the raters had special knowledge about the SPCM and interaction with the test developer. In addition, we did not use a weighted Kappa. We used the Kappa statistic to analyze agreement. This statistic does not take into account the magnitude of disagreement and treats all disagreements equally.

Conclusions

There is a great need in physical therapy for standardized measurement instruments that can be used to evaluate therapeutic outcomes and to aid clinical decision making. Currently, a clinical observation tool that can reliably define posture and motor performance under either of the test conditions in this study (ie, seated with or without postural support) is not available. The pilot SPCM is clinically feasible to administer in 20 minutes or less and has face and content validity, according to external seating experts. We believe there is potential for acceptable reliability after further refinement of this measure.

Acknowledgments

We thank Lynne Balfour, OT, Adrienne Falk Bergen, VF, Doreen Dewes, PT, Eric Ferguson, PT, Jessica Presperin, OT, Elaine Trefler, OT, and Diane Ward, OT, for participating as external seating experts in developing the SPCM. We also thank the staff of the Positioning Assessment Unit, Sunny Hill Hospital for Children, for sharing their clinical expertise, Christopher Dumper for assistance with the computer analysis, and Ruth Milner for statistical consultation. [Tables Omitted]

References

1 Bergen AF, Presperin Tallman T. Positioning for Function: Wheelchair and Other Assistive Technologies. Valhalla, NY: Valhalla Rehabilitation Publications Ltd; 1990:3-6.

2 Ward D. Positioning the Handicapped Child for Function. 2nd ed. Chicago, Ill: Phoenix Press; 1984.

3 Hetiderson B, compiler. Scaling in Review: current Trends for the Disabled. Winnipeg, Manitoba, Canada: Otto Bock Orthopedic Industry of Canada Ltd; 1989.

4 Taylor S, Trefler E. Decision making guidelines for seating and positioning children with cerebral palsy. In: Trefler E, ed. Sealing for Children with Cerebral Palsy. Memphis, Tenn: University of Tennessee; 1984:55-76.

5 Letts M, Rang M, Tredwell S. Seating the disabled. In: Bunch WH, Keagy R, Kritter AE, et al, eds. Atlas of Orthotics. 2nd ed. St. Louis, Mo: CV Mosby Co; 1985:440-486.

6 Nwaobi OM, Smith PD. Effect of adaptive seating on pulmonary. functionof children with cerebral palsy. Dev Med Child Neurol. 1986;28:351-354.

7 Crenshaw RP, Vistnes LM. A decade of pressure sore research: 1977-1987. J Rehabil Res Dei,. 1989;26:63-74.

8 Bardsley GI. The Dundee Seating Programme. Physiotherapy 1984;70:59 63.

9 Boenig B. Reger SI Little J. Survey of seat-Delivery Systems in the United States and Canada. Report of SIG 09, RESNA, Washington, DC, 1989.

10 Campbell SK. Efficacy of physical therapy in improving postural control in cerebral palsy. Pediatric {hysical Therapy. 1990;2: 13% 140.

11 Reed ES. Changing theories of postural development. In: Woollacott MH, Shumway-Cook A, ed. Development of Posture and Gait Across the Life Span. Columbia, SC: University of South Carolina Press; 1989:3 24.

12 Riach CL, Haves KC. Maturation of postural stability in young children. Dev Med Child Neurol. 1987;29:650 658.

13 Ferguson-Pell M, Snow J A system for monitoring trunk control and head position during a reaching task for persons with head Conference of the Association for advancement of rehabilitation Technology; June 24--30, 1988; Montreal, Quebec, Canada; pp 116117.

14 McClenaghan BA. Sitting stability of selected subjects with cerebral palsy. Clinical Biomechanics. 1989;4:213-216.

15 Reid DT, Sochaniwskyj A. Milner M. An investigation of postural sway in sitting of normal children and children with neurological disorders. Physical & Occupational Therapy in Pediatrics. 1991;11:19-35.

16 Nashner LM Shumway-Cook A, Marin O. Stance posture control in select groups of children with cerebral palsy: deficits in sensory organization and mucular coordination. Exp Brain Res. 1983;49393-409

17 Shumway-Cook A, Woollacott MH. Dynamics of postural control in the child with Down syndrome. Phys Ther. 1985;65:1315--1322.

18 Forssberg H, Nashner LM. Ontogenetic development of postural control in man: adaptation to altered support and visual conditions during stance. J. Neurosci. 1982;2:545-552.

19 Woollacott MH, Debu 13, Mowatt M. Neuromuscular control of the posture in the infant and child: is vision dominant? Journal of Motor Behavior. 1987;19:167-186

20 Harbourne RT, Guiliani C, MacNeela JC. A kinematic and electromyographic analysis of the development of sitting posture in infants. Ther. 1989;69:370. Abstract

21 Gross Al., The effects of a supported scating position on upper extremity control in children with spastic cerebral palsy. Physical & Occupational Therapy in Pediatrics. 1989; 9:143. Abstract.

22 Clary-Trimm BL. The effect of proximal support on upper extremity reaching. Physical & Occupational Therapy in Pediatrics. 1991; 10:134. Abstract.

23 Andersson BJ, Oretengren R, Nachemson AL, et al. The sitting posture: an electromyographic and discometric study. Orthop Clin North Am 1975;6;105 120.

24 Soderberg GL, Blanco MK, Cosentino TL, Kurdelmeier KA. An EMG analysis of posterior trunk musculature during flat and anteriorly inclined sitting. Hum Factors. 1986;28:483-491.

25 Nwaobi OM. Effects of body orientation in space on tonic muscle activity of patients with cerebral palsy. Dev Med Child Neurol. 1986; 28:41-44.

26 Milner M, Lotto W, Koheil R, et al. Dynamic Positional and Electromyographic Monitoring of Sitting Posture. Toronto, Ontario, Canada: Rehabilitation Engineering Department, The Hugh MacMillan Medical Centre; 1987. Annual report.

27 Fife SE, Roxborough LA, Cooper D, et al. Tonic electromyographic activity in seated subjects with spastic quadriplegia: a pilot study. In: Proceedings; of the Fourth International Seating Symposium, Challenge,,; 88: Seating the Disabled", February 18-20, 1988; Vancouver, British Columbia, Canada pp 16-20

28 Nwaobi O. Seating orientations and upper extremity function in children with cerebral palsy. Phys Ther. 1987;67:1209 1212.

29 Seeger BR, Caudrey DJ, O'mara NA. Hand function in cerebral palsy: the effect of hip flexion angle. Dev, Med Child Neurol. 1984; 26:601-606.

30 Nwaobi O, Hobson D, Trefler E. Hip angle and upper extremity movement time in children with cerebral palsy. in: proceedings of the Eighth Annual RESNA Conference; June 24-28, 1985; Memphis, Tenn; pp 39-41

31 Kirshner B, Guyatt G. A methodological framework for assessing health indices. J Chronic Dis. 1985;38:27-36

32 Hulme JB, Poor R, Schulein M, Pezzino J. Perceived behavioral changes observed with adaptive seating devices and training programs for multihandicapped, developmentally disabled individuals, Phys Ther 1983;63:204-208

33 Murphy TE. The positioner chair: a classroom chair for children with a forward tilting seat. in: Proceedings of the Fifth International Seating Symposium, "Seating the Disabled"; February 16-18, 1989; Memphis, Tenn; pp 23-28.

34 Hulme JB, Gallacher K, Walsh J, et al. Behavioral and postural changes observed with use of adaptive seating by clients with multiple handicaps. Phys 7her. 1987;67:1060-1067.

35 Hulme JB, Shaver J, Acher S, et al. Effects of adaptive seating devices on the eating and drinking of children with multiple handicaps. Am J Occup Ther 1987;41:81-89

36 Green E. Assessment and measurement of sitting ability. in: Proceedings of the International seating conference; September 12-16, 1988; Dundee, Scotland. Abstract 33.

37 Mulcahy CM, Pountney TE, Nelham RL. Adaptive seating for the motor handicapped; problems, a solution, assessment and prescription. Physiotherapy. 1988;74:531-536

38 Russell DJ, Rosenbaum, PL, Cadman DT, et al. The Gross Motor Function Measure: a means to evaluate the effects of physical therapy. Dev Med Child Neurol. 1989;31:341-352.

39 Hardy MA, Kudar S, Macdonald JA. Assessment of Behavioral Components: Analysis of Severely Disordered Posture and Movement in Children with Cerebral Palsy. Springfield, Ill: Charles C. Thomas, Publisher; 1988.

40 Chandler LS, Andrews MS, Swanson MW. The Movement Assessment of Infants: A Manual. Rolling Bay, WA: Movement Assessment of infants; 1980.

41 Case-smith J. Posture and Fine Motor Assesment of Infants: Research Edition. Rockville Md: American Occupational Therapy Foundation; 1988.

42 Folio MR, Fewell RR. Peabody Developmental Motor Scales and Activity Cards; (PDMS). Allen, Tex: DLM Teaching Resources; 1983

43 Fleiss JL. Statistical Methods for Rates and Proportions. 2nd ed. New York, NY: John Wiley & Sons Inc; 1980.

44 Haley SM, Osberg JS. Kappa coefficient calculation using multiple ratings per subject: a special communication. Phy,; Ther. 1989;69: 970-974.

45 Plewis I, Bax M. Cited by: Haley SM, Harris SR, Tada WL, Swanson MW. Item reliability of the Movement Assessment of infants. Physical & Occupation Therapy in Pediatrics. 1986; 6:21-39.

46 Youdas JW, Carey JR, Garrett TR. Reliability of measurements of cervical spine range of motion: comparison of three methods. Phys Ther. 1991;71:98-106.

47 Bendix T, Biering-Sorensen F. Posture of the trunk when sitting on forward inclining seats. Scand J Rehabil Med. 1983; 15:197-203.

48 Amendt LE, Ause-Ellias KL, Eybers JL, et al. validity and reliability testing of the Scoliometer(R). Phys Ther, 1990;70:108-117.

49 Frey JK, Tecklin JS. Comparison of lumbar curves when sitting on the Westnofa Balans(R) Multi-chair, sitting on a conventional chair, and standing. Phys Ther. 1986;66:1365-1369.

50 Miedaner JA. The effects of sitting positions on trunk extension for children with motor impairment. Pediatric Physical Therapy. 1990; 2:11--14.

51 Williams JG, Callaghan M. Comparison of visual estimation and goniometry in determination of a shoulder joint angle, Physiotherapy. 1990;76:65'-657.

52 Watkins MA, Riddle DL, Lamb RL, Personius WJ. Reliability of goniometric measurements and visual estimates of knee range of motion obtained in a clinical setting. Phys Ther. 1991; 71:90-97.