Effect of seat surface inclination on postural control during reaching in preterm children with cerebral palsy.

Children with cerebral palsy (CP) often are hampered by dysfunctional postural control. (1,2) It is debatable whether postural control of sitting in children with CP can be enhanced by inclination of the seat surface. (3-5) Two studies (3,4) indicated that an anteriorly tilted sitting position induced a more upright sitting position, which was associated with reduced muscle activity in the lower extremities. McClenaghan et al, (5) however, found that a forward-inclined sitting position resulted in worse postural stability and that a posterior inclination of the seat surface resulted in better stability. Another series of studies (6-8) indicated that a horizontal orientation of the seat surface is advantageous because it is associated with reduced muscle activity of the lumbar extensor and hip adductor muscles and with better upper-extremity function. The conflicting findings of these studies reflect, among other things, the differences in methods (eg, different sitting positions), study groups (children with different forms of CP), ages (eg, 2.5-16 years and 4-15 years), and type of tasks performed (eg, reaching or drawing task). None of these studies consistently examined the effect of inclination of the seat surface on postural adjustments.

In the neural control of postural adjustments, 2 functional levels can be distinguished. (9) The first level of control involves direction specificity. Direction specificity means that perturbations inducing a forward sway of the body, such as reaching movements, are accompanied primarily by postural activity in the muscles on the dorsal side of the body, whereas perturbations inducing a backward body sway are accompanied by activity in the "ventral" muscles. The second level of control is involved in fine-tuning of the direction-specific adjustment on the basis of multi-sensorial afferent input from the somatosensory, visual, and vestibular systems. This modulation can be achieved in various ways, for instance, by changing the order in which the agonist muscles are recruited (eg, in a caudal-to-cranial sequence or in reverse order); by modifying the degree of the muscle contraction, which is reflected by the electromyography (EMG) amplitude; or by altering the degree of antagonist activation.

Only a few studies have addressed postural adjustments during voluntary reaching in children with CP. One study (1) addressed longitudinally the development of postural control during reaching in 7 infants between the ages of 4 and 18 months. Five of these children developed unilateral spastic CP, and 2 children developed spastic bilateral CP. (1)

A series of cross-sectional studies (2,10,11) examined postural control during reaching by means of surface EMG and kinematic recordings in 34 children, aged 2 to 11 years, with unilateral spastic CP and 24 children with bilateral spastic CP. From these studies, we learned that the basic level of postural control in children with CP who are able to reach usually is intact; only children with severe forms of CP had some problems with the generation of direction specific postural activity. (1,2) The children with CP mainly had difficulties in modulating postural adjustments to task-specific conditions. They exhibited a rather stereotyped top-down recruitment order of the dorsal postural muscles during reaching. Furthermore, they had difficulties adapting EMG amplitude to task-specific constraints, such as a heavy bracelet wrapped around the reaching arm. The latter deficit was more profound in children with bilateral spastic CP than in children with unilateral spastic CP.) (2)

The series of studies also indicated that the quality of reaching of the dominant hand of children with CP was significantly worse than that of age-matched children who were developing typically. The children with CP showed, in comparison with the age-matched controls, reaching movements that had a longer duration and that consisted more frequently of more than 1 movement unit (MU). Movement units are sub-movements of reaching, which are determined with the help of peaks in the velocity profile of the hand. Reaching movements consisting of 1 MU indicate the presence of an adequate feedforward planning of reaching. (12) These problems were more pronounced in children with bilateral spastic CP than in children with unilateral spastic CP. (10) It turned out that a better quality of reaching or postural control was associated with a more reclined pelvis position at reaching onset and with a more stable head and pelvis and a more mobile trunk during reaching. (11)

The aim of the present study was to evaluate the effect of seat surface tilting on postural adjustments during reaching and the quality of reaching in sitting children with CP. To this end, we studied a group of 58 preterm children with CP in 3 sitting positions: horizontal seat surface, seat surface tilted forward 15 degrees, and seat surface tilted backward 15 degrees. Surface EMG activity of arm, neck, trunk, and leg muscles and kinematics of posture and reaching were recorded while the children made reaching movements with the dominant arm. The following questions were addressed:

(1) Does the forward- or backward-tilted sitting condition affect the basic level of direction specificity during reaching?

(2) Which sitting position can be considered best for children with CP? Specifically, which condition is associated with less postural muscle activity, more efficient kinematics of postural control (better stability of the head, a more mobile trunk), (11) and a better quality of reaching (eg, more reaching movements consisting of 1 MU)?

(3) Is the effect of seat surface tilting on the various parameters of postural control and reaching influenced by (1) the type of CP (unilateral or bilateral spastic CP), (2) age, or (3) the severity of disability?

Method

Subjects

The original study group consisted of 58 children with CP, aged 2 to 11 years. Thirty-four children had unilateral spastic CP, and 24 children had bilateral spastic CP. Spasticity (excessive stretch reflex activity) was associated with dyskinesia in 7 children and with ataxia in 5 children. Children with a severe visual impairment and those who were not able to reach out for an object were excluded from the study. All except 1 of the children with unilateral spastic CP were able to walk without assistive devices. The children with unilateral spastic CP had level I disability (n=33) or level IV disability (n= 1) according to the Gross Motor Function Classification System (GMFCS). (13) In the group of children with bilateral spastic CP, 9 children could walk without assistive devices (GMFCS levels I-II), 10 children could walk with the help of assistive mobility devices (GMFCS level III), and 5 children had limited self-mobility (GMFCS levels IV-V). All children with CP were born preterm and were admitted to the neonatal intensive care unit of the University Medical Center Groningen. We were able to collect EMG data in 3 positions in 54 children with CP and kinematic data in 39 children with CP (Tab. 1).

The reference group consisted of 29 children with typical motor development. Nine children in this group were 2 to 4 years of age, 10 children were 5 to 7 years of age, and 10 children were 8 to 11 years of age (for details, see van der Heide et al (14)). Appropriate EMG data in 3 conditions were available in 29 children who were developing typically, and kinematic data were available in 18 children. All parents gave informed consent.

Procedure

The majority of the children sat without back and foot support, but some children needed extra support to be able to carry out the test. Back and foot support was provided to 5 of the 12 children with severe CP. Foot support only was given to 3 children with severe CP and to 1 child with moderately severe CP. The examiner (JCvdH) presented an attractive small object in the midline at arm's length distance of each subject. The instruction was to grasp the object with the dominant hand at a natural self-paced speed. The dominant hand was defined as the hand with which the child preferred to write or draw. The subjects were tested in 3 conditions. They started in the horizontal sitting position on a table.

Subsequently, a wedged platform was mounted on the table, which induced a 15-degree forward tilt of the pelvis or a 15-degree backward tilt of the pelvis of the seated child (Fig. 1). Forward- and backward tilted conditions were applied in random order. In the forward-tilted condition, the subjects were secured with a strap around the pelvis to prevent them from sliding from the platform. Ten to 20 trials were performed in each condition. Prior to testing, the children carried out some exercise trials.

[FIGURE 1 OMITTED]

Muscle activity was recorded with bipolar surface electrodes from the following muscles: deltoid, biceps brachii, sternocleidomastoid, neck extensor (at the C7 level), rectus abdominis, thoracic extensor (at the T10 level), lumbar extensor (at the L5 level), rectus femoris, and hamstrings on the side of the reaching arm. Electromyographic activity was recorded continuously with POLY,* a software program for long lasting polygraphic recordings (sampling rate=500 Hz). Children were included in the EMG analyses only if they had at least 3 trials per condition with an appropriate EMG recording (Tab. 2).

Simultaneously with the EMG recordings, movements were recorded kinematically with an ELITE optoelectronic movement recording system [dagger] in a 4-camera configuration at a sampling frequency of 50 Hz. Reflective markers were placed at the side of the body of the dominant hand on the following landmarks: (1) the condyle of the mandible, (2) 1 cm in front of the angle of the mandible, (3) the spinous process at C7, (4) the spinous process at T10, (5) the spinous process at L5, (6) the anterior superior iliac spine, (7) the proximal edge of the greater trochanter, and (8) the styloid process of the radius (Fig. 1). Sampling

of the kinematic data started some seconds before toy presentation and lasted for 8 to 12 seconds, depending on the reaching velocity of the child. We included children in the kinematic analyses only when at least 3 appropriate kinematic trials could be achieved in each sitting condition (Tab. 2). Gestational age and weight at birth did not differ for the children with unilateral spastic CP (mean gestational age [[+ or -] SD]= 29 [+ or -] 2 weeks; mean birth weight [[+ or -] SD] = 1,183 [+ or -] 356 g) and those with bilateral spastic CP (mean gestational age [ [+ or -] ]SD]=28.5 [+ or -] 1.7 weeks; mean birth weight [[+ or -] SD] = 1,248 [+ or -] 283 g) included into the present analyses. The whole session was recorded on videotape, time locked with the EMG recordings.

After the reaching sessions, the neurological condition of the children was examined. This examination consisted of a clinical evaluation of posture, motility, muscle tone (velocity-dependent resistance to stretch), and reflexes and included an assessment of the severity of the disorder. Three classes of severity were distinguished--mild, moderate, and severe--indicating that posture, motility, muscle tone, and reflexes were affected to a limited, moderate, or severe extent, respectively. In the analyses of the relationships between the severity of the disorder and postural dysfunction, the severity classification of the neurological examination was used, because this classification differentiated dysfunctional reaching and postural control in the various groups of children better than the GMFCS (for details, see van der Heide et al (2)).

Data Analysis

The videotape recordings were used to select reaching movements with the dominant arm, during which the children were in an appropriate state of attention. Such a selection procedure was needed in particular in the youngest children.

The onset of the reaching movements was determined on the basis of the kinematic data. For the EMG analysis, a computer algorithm was used for the detection of phasic muscle activity. The algorithm used a derivative of the root mean square of a full rectified signal (200-millisecond moving window) and marked significant deviations from a fixed detection level. The detection level was based on a long-term (3.7-second) mean baseline activity. Electromyographic activity bursts were detected when the activity exceeded the detection level for at least 50 milliseconds (see van der Fits et al (15)). The activity of the postural muscles was considered to be related to the arm movement when muscle activity started within a time window of 100 milliseconds before and 500 milliseconds after activation of the "prime mover" (ie, the arm muscle that was activated first [deltoid or biceps brachii]).

First, for the muscles involved in the postural adjustments, muscle activation rates were calculated (ie, the number of trials during which a specific muscle was activated divided by the total number of trials, times 100%). In addition, specific attention was paid to the rate of the en bloc pattern consisting of the in-concert activation of the neck, thoracic, and lumbar extensor muscles and the pattern variation index. The pattern variation index was obtained by dividing the number of different response patterns of the direction-specific postural muscles by the total number of trials, multiplied by 100%.

For the temporal organization of postural adjustments, we zoomed in on the onset latencies of muscle activation; the recruitment order of the neck, thoracic, and lumbar extensor muscles; and the rate of anticipatory postural muscle activation (for details, see van der Heide et al (2)).

For the analysis of EMG amplitude modulation, EMG amplitudes were determined by calculating the mean amplitude of the signal after subtraction of the baseline activity in 3 different time intervals. The first time interval consisted of the 100 milliseconds before the prime mover was activated, thereby covering anticipatory postural muscle activity. The second and third time intervals were 0 to 100 milliseconds (12) and 100 to 1,000 milliseconds (13), respectively, after prime mover activation.

Offline kinematic analysis was carried out with the help of the DataMonster 2.0 software package. (16) Kinematic analysis consisted of the calculation of spatial angles for the head (by a vector between markers 1 and 2) and the pelvis (by a vector between markers 6 and 7) in relation to the horizontal plane. In addition, the trunk angle was defined by calculating the angle of the 2 intersecting vectors between markers 3 and 4 and markers 4 and 5 (Fig. 1A). Arm movement onset was defined as the moment at which the velocity of the wrist increased more than 5% of peak velocity, and the moment at which wrist velocity declined to 5% of peak velocity at the farthest distance in space relative to the beginning of the movement was considered as the end of the movement. In the kinematic analysis, only trials with a clearly demarcated start and stop were included. The analysis focused on the kinematics of postural control and of reaching.

For the analysis of posture, we zoomed in on: (1) angular positions at movement onset and (2) angular displacements during the entire duration of the reaching movement. We did not include position and angular displacement of the pelvis in the analyses because alterations in these parameters mirrored the various conditions. For the parameters describing the reaching movements, we used: (1) reaching duration, (2) maximum reaching velocity, (3) index of curvature, (17) (4) the proportion of trials during which the reaching movement consisted of 1 MU, and (5) the length of the first MU (the transport MU) relative to total movement path (for details, see van der Heide et al (10)) An MU consisted of one acceleration and one deceleration in the velocity profile of the wrist marker. (12)

Statistical analyses were performed using the SPSS (version 10.1) ([double dagger]) computer package. For the evaluation of the effect of sitting condition and additional support on most of the kinematic and EMG parameters, we mainly used the nonparametric Wilcoxon test. For the analysis of sitting condition on EMG amplitude, paired t tests were used. Next, we explored by means of analyses of variance (ANOVAs) whether seat-surface tilting was affected by the severity of disability or by age. The post hoc Bonferroni test was used to identify the loci of significance for these analyses. We realize that the ANOVA is a parametric test and thus not optimal for the present set of data, but it was the only way to get some idea of the effect of multiple factors on outcome. In the primary analyses on the effect of sitting condition, we considered differences of P<.05 as statistically significant, but in the ANOVAs, where so many comparisons were made, we took P<.01 as the cutoff for statistical significance.

Results

Effect of Forward and Backward Tilting on Postural Muscle Activity

The children with and without CP occasionally showed a direction-inappropriate trial in the horizontal condition (ie, they occasionally activated a ventral postural muscle prior to a dorsal postural muscle) (see van der Heide et al (2)). Tilting of the seat surface did not affect the occurrence of such direction-inappropriate postural muscle activity; it remained a rare phenomenon.

With respect to the other EMG parameters, tilting of the seat surface had a limited effect only. In the children who were developing typically, only backward tilting induced alterations in muscle activity; it resulted in a higher activation rate of lumbar extensor muscle (P<.05) and a higher mean EMG amplitude of the thoracic extensor in the 12 and 13 intervals (P<.01). This means that backward tilting was associated with more muscular effort. Backward tilting had an analogous effect in the children with unilateral spastic CP. It induced a higher activation rate of the lumbar extensor (P<.01), a higher amplitude of the thoracic extensor in the 13 interval (P<.05), a higher pattern variation index (P<.05), and a higher rate of en bloc activation of the postural muscles (P<.01). In the children with unilateral spastic CP, forward tilting also affected postural muscle activity; it induced a lower activation rate of the thoracic extensor (P<.05) and a longer latency to onset of the neck extensor (P<.05). In the children with bilateral spastic CP, the effect of seat-surface tilting on postural muscle activity was minimal. Only forward tilting resulted in a higher pattern variation index (P<.05).

We explored by means of ANOVA whether these effects of seat-surface tilting were affected by the severity of the disability or by age. We found a significant interaction between severity of the disorder and seating condition on the amplitude of the thoracic extensor in the 13 interval (F=5.7; df=4,26; P<.01). Post hoc analysis showed that, in the backward-tilted condition, the children with severe CP had a lower mean amplitude of the thoracic extensor, whereas the children with mild or moderate CP had a higher amplitude of the thoracic extensor. Back or foot support did not affect the relationships between seat-surface tilting and postural muscle activity.

Effect of Forward and Backward Tilting on the Kinematics of Postural Control

Tilting of the seat surface had a different effect on the initial position and displacement of the head and trunk during reaching in the 3 groups of children (Fig. 2). In the children who were developing typically, forward tilting resulted in a more anteflexed position of the head at the onset of reaching (P<.01), whereas backward tilting induced a more straight trunk at reaching onset (P<.05). In the children who were developing typically, tilting did not affect the degree of displacement of the head and trunk during reaching.

[FIGURE 2 OMITTED]

In the children with unilateral spastic CP, forward tilting induced a lesser degree of anteflexion of the head at the onset of reaching (P<.05), and backward tilting induced a more straight initial trunk position (P<.01) and a larger displacement of the head (P<.05). In the children with bilateral spastic CP, forward tilting produced a more straight position of the trunk at reaching onset (P<.01), a larger displacement of the head (P<.01), and a smaller displacement of the trunk (P<.05) during reaching. Backward tilting affected only stability of the head: the head of the children with bilateral spastic CP showed a larger displacement in the tilted position than in the horizontal condition (P<.05).

Additional analyses indicated that seat-surface tilting was not affected by age or the presence of back or foot support. Seat-surface tilting was affected, however, by the severity of the disability. A significant interaction effect of severity x position was found for initial trunk position (F= 12.2; df=4,26; P<.01). Post hoc analysis revealed that the children with severe CP had a more flexed trunk position in the backward-tilted condition, whereas the children with a mild or moderate form of CP had a more extended trunk position in this condition.

Effect of Forward and Backward Tilting on the Kinematics of Reaching

Seat-surface tilting had a limited effect on the parameters of reaching (Fig. 3). In the children who were developing typically, backward tilting was associated with more reaches consisting of 1 MU and with reaches during which the transport MU covered a larger proportion of the reaching movement (P<.05). These findings indicated that backward tilting of the seat surface improved the kinematic quality of reaching in the children who were developing typically. In the children with unilateral spastic CP, backward tilting had no effect on the kinematics of reaching, but forward tilting did have some effect. Forward tilting yielded reaching movements during which the transport MU covered a relatively larger part of the movement (P<.05). In the children with bilateral spastic CP, tilting of the seat surface did not affect the kinematics of reaching. Additional analyses revealed that none of these effects of seat-surface tilting were affected by age, severity of the disability, or additional back or foot support.

[FIGURE 3 OMITTED]

Discussion and Conclusions

The present study demonstrated that a tilted position of the seat surface had a differential effect on postural control during reaching and on the quality of reaching in the 3 groups of children studied. In the children who were developing typically, none of the 3 sitting positions could be classified as best: forward inclination of the seat surface had little effect on posture and reaching, whereas backward inclination was associated with better reaching quality but at the cost of increased postural effort. The children with unilateral spastic CP benefited most from the forward-tilted position, whereas the children with spastic bilateral CP benefited most from the horizontal position.

Strengths and Limitations of the Study

Postural control and reaching had been studied in children with CP who were born preterm. This means that the findings of the present study cannot be generalized immediately to the general population of children with CP because it is known that preterm birth without an associated lesion of the brain may affect postural control (8,19) and kinematic quality of reaching. (20) With respect to the different sitting conditions applied in the present study, various points should be kept in mind: (1) the present study used degrees of tilt that are at the higher end of the spectrum used in clinical settings (3-8); (2) in the forward-tilting situation, we used a strap to prevent the children from slipping off the seat surface, implying that the effects of forward tilt and that of the strap cannot be disentangled; and (3) in general, the children did not receive foot support, which is a relatively artificial situation, because in daily life children usually will receive foot support. We opted to study behavior without foot support because the no-support condition would impose a larger postural challenge and thereby increase the likelihood of finding an effect of specific sitting conditions. The strength of the study is that a large group of children with CP were studied in a controlled and standardized way.

Clinical Relevance

Our study showed that postural control in children with CP who were able to reach the basic level was not influenced by a tilted position of the seat surface. Maybe the absence of effect in the children with a severe form of CP who participated in the present study was related to the fact that their basic level of control was only mildly affected. Therefore, an effect of seat surface tilting on the ability to generate direction-specific adjustments in children with a more severe dysfunction of the basic level of postural control cannot be excluded.

In the children who were developing typically, none of the 3 sitting positions could be qualified as best. In the children with unilateral spastic CP, the forward-tilted sitting position was associated with better postural control and a better quality of reaching. In this position, the children with unilateral spastic CP showed less postural muscle activity of the trunk muscles and a slower recruitment of the neck extensor muscles. The latter finding can be considered favorable because it means a reduction of the stereotypic cranial-to-caudal recruitment order, which is one of the postural deficiencies of children with CP. (2) Other beneficial effects of forward tilting in the children with unilateral spastic CP were less anteflexion of the head at the onset of reaching and a better quality of reaching. The latter finding meant that the forward-tilted sitting position was associated with reaching movements during which a greater part was covered by the transport MU. Previously, we showed that more anteflexion of the head at the onset of reaching is associated with better reaching performance in children with CP. (11) Again we would like to emphasize that it is unclear whether the positive effect of the forward-tilted sitting position in the children with unilateral spastic CP was the result of the combination of forward tilting and the presence of the securing strap around the pelvis or a pure effect of seat surface inclination. Backward tilting of the seat surface had a negative effect in the children with unilateral spastic CP. It was associated with higher levels of trunk muscle activity and more head displacement during reaching.

In the children with bilateral spastic CP, the forward-tilted sitting condition induced a more straight sitting position, a result that is in accordance with the findings of Myhr and von Wendt, (3,4) who also studied mainly children with this type of CP. Forward tilting of the seat surface in the children with bilateral spastic CP also was associated with more instability of the head and less mobility of the trunk during reaching, both of which are known to be related to worse postural control and a worse quality of reaching. (11) This result is accordance with that of McClenaghan et al, (5) who reported that the forward-tilted condition resulted in a less stable sitting position in children with CP.

Unlike McClenaghan et al, (5) however, we found that the backward-tilted position induced less postural stability in the children with bilateral spastic CP. The discrepancy in findings between the 2 studies could be due to differences in the seat inclination angle used (15[degrees] in the present study compared with 5[degrees] in the study by McClenaghan et al (5)), the difference in tasks (reaching in the present study compared with a variety of upper-extremity tasks, such as drawing, in the study by McClenaghan et al (5)), and the severity of the disability of the children studied (mix of severities in the present study versus only children with a mild or moderate form of CP in the study of McClenaghan et al (5)). On the basis of the present data, we conclude that children with bilateral spastic CP might benefit especially from a horizontal sitting position.

The effects of seat surface inclination on postural adjustments and on the quality of reaching were not influenced by age and were influenced to a limited extent by the severity of the motor disorder. These findings mean that the effects of seat surface inclination on posture and reaching hold true for the whole age range studied (2-11 years). Children with a severe form of CP occasionally reacted differently, but not better or worse, on seat surface inclination than children with milder forms of CP.

The finding that the severity of the disability played a small role only in the effects of seat surface inclination most likely may be attributed to the task studied: postural control during reaching while sitting. We chose this task because of its high ecological validity. The drawback of the task, however, is that it challenges postural control to a limited extent only.

The gentle nature of the postural challenge involved in reaching while sitting is the major reason why the data of the present study showed a large variation. It is conceivable that more consistent results of severity of disability on the effect of surface inclination on postural control would be present in situations with a larger postural challenge, such as during external perturbations in stance. Here we also want to draw attention to the fact that severity of disability based on the findings of the neurological examination was more closely related to postural control parameters and the effect of seat surface inclination than the severity of disability expressed in terms of GM-FCS level. This finding might be surprising, but again this most likely has to do with the nature of the task we studied: postural control during reaching while sitting.

In conclusion, the results indicate that tilting of the seat surface differentially affects postural adjustments and quality of reaching in children with unilateral spastic CP and children with bilateral spastic CP. Children with unilateral spastic CP benefit most--in terms of less postural muscle activity, a better quality of reaching, and better postural stability during reaching--from a forward-tilted sitting position where forward slipping is prevented by a strap. Children with bilateral spastic CP benefit most from a horizontal sitting position.

Dr Hadders-Algra provided concept/idea/ research design, fund procurement, institutional liaisons, clerical support, and consultation (including review of manuscript before submission). All authors provided writing. Dr van der Heide and Dr Fock provided data collection. Dr Hadders-Algra, Dr van der Heide, and Dr Otten provided data analysis. Dr Hadders-Algra and Dr van der Heide provided project management. Dr Hadders-Algra, Dr van der Heide, and Dr Stremmelaar provided subjects. Dr Hadders-Algra, Dr van Eykern, and Dr Otten provided facilities/equipment. The authors thank Dr Oebele Brouwer and Dr E. Brogren Carlberg for their critical and valuable remarks on a previous draft of the manuscript. Ms Lidy Kingma-Balkema and Dr Jolanda Schaap are kindly acknowledged for their skillful technical assistance.

The Medical Ethics Committee of the University Medical Center Groningen approved the study.

The study was supported by the Johanna KinderFonds (grant 19990021), the Dr W. M. Phelps-Stiching Voor Spastici (grant 99.058), the Gratama Stichting/Groninger Universiteits Fonds, and the Algemeen Welzijnsfonds Voor Geestelijk en Lichamelijk Gehandicapten.

This article was received October 27, 2006, and was accepted March 6, 2007.

DOI: 10.2522/ptj.20060330

References

(1) Hadders-Algra M, van der Fits IBM, Stremmelaar EF, et al. Development of postural adjustments during reaching in infants with CP. Dev Med Child Neurol. 1999;41: 766-776.

(2) van der Heide JC, Begeer C, Fock JM, et al. Postural control during reaching in preterm children with cerebral palsy. Dev Med Child Neurol. 2004;46:253-266.

(3) Myhr U, von Wendt L. Improvement of functional sitting position for children with cerebral palsy. Dev Med Child Neurol. 1991;33:246-256.

(4) Myhr U, von Wendt L. Influence of different sitting positions and abduction orthoses on leg muscle activity in children with cerebral palsy. Dev Med Child Neurol. 1993;35:870-880.

(5) McClenaghan BA, Thombs L, Milner M. Effect of seat-surface inclination on postural stability and function of the upper extremities of children with cerebral palsy. Dev Med Child Neurol. 1992;34: 40-48.

(6) Nwaobi OM, Brubaker CE, Cusick B, et al. Electromyographic investigation of extensor activity in cerebral-palsied children in different seating positions. Dev Med Child Neurol. 1983;25:175-183.

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

(8) Nwaobi OM. Seating orientations and upper extremity function in children with cerebral palsy. Phys Ther. 1987;67:1209-1212.

(9) Forssberg H, Hirschfeld H. Postural adjustments in sitting humans following external perturbations: muscle activity and kinematics. Exp Brain Res. 1994;97: 515-527.

(10) van der Heide JC, Fock JM, Otten E, et al. Kinematic characteristics of reaching movements in preterm children with cerebral palsy. Pediatr Res. 2005;57: 883-889.

(11) van der Heide JC, Fock JM, Otten E, et al. Kinematic characteristics of postural control during reaching in preterm children with cerebral palsy. Pediatr Res. 2005;58: 586-593.

(12) von Hofsten C. Structuring of early reaching movements: a longitudinal study. J Mot Behav. 1991;24:280-292.

(13) Palisano R, Rosenbaum P, Walter S, et al. Development and reliability of a system to classify gross motor function in children with cerebral palsy. Dev Med Child Neurol. 1997;39:214-223.

(14) van der Heide JC, Otten E, van Eykern LA, et al. Development of postural adjustments during reaching in sitting children. Exp Brain Res. 2003;151:32-45.

(15) van der Fits IBM, Klip AWJ, van Eykern LA, et al. Postural adjustments accompanying fast pointing movements in standing, sitting and lying adults. Exp Brain Res. 1998; 120:202-216.

(16) Otten E. DataMonster: a software package running on an Apple Macintosh computer for signal analysis and modelling for research in motor control. In: Winters JM, Crago PE, eds. Biomechanics and Neural Control of Movement and Posture. New York, NY: Springer-Verlag; 2000:665-667.

(17) Messier J, Adamovich S, Berkinblit M, et al. Influence of movement speed on accuracy and coordination of reaching movement to memorized targets in three-dimensional space in a deafferented subject. Exp Brain Res. 2003;150:399-416.

(18) Hadders-Algra M, Brogren E, Katz-Salamon M, et al. Periventricular leukomalacia and preterm birth have a different detrimental effect on postural adjustments. Brain. 1999;122:727-740.

(19) van der Fits IBM, Flikweert ER, Stremmelaar EF, et al. Development of postural adjustments during reaching in preterm infants. Pediatr Res. 1999;46:1-7.

(20) Fallang B, Saugstad OD, Grogaard J, et al. Kinematic quality of reaching movements in preterm infants. Pediatr Res. 2003;53: 836-842.

* Inspector Research Systems, Amsterdam, the Netherlands.

([dagger]) BTS, Milan, Italy.

([double dagger]) SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.

M Hadders-Algra, MD, PhD, is Professor of Developmental Neurology, Department of Developmental Neurology, University Medical Centre Groningen, University of Groningen, Hanzeplein 1, 9713 GZ Groningen, the Netherlands. Address all correspondence to Dr Hadders-Algra at: [e-mail].

JC van der Heide, PT, PhD, is Researcher, Department of Developmental Neurology, University Medical Centre Groningen, University of Groningen.

JM Fock, MD, is Pediatric Neurologist, Department of Developmental Neurology, University Medical Centre Groningen, University of Groningen.

E Stremmelaar, MD, is Pediatrician, Department of Paediatrics, University Medical Centre Groningen, University of Groningen.

LA van Eykern is Electronic Engineer, Department of Developmental Neurology, University Medical Centre Groningen, University of Groningen.

B Otten, PhD, is Professor of Neuromechanics, Center for Human Movement Sciences, University of Groningen.

[Hadders-Algra M, van der Heide JC, Fock JM, et al. Effect of seat surface inclination on postural control during reaching in preterm children with cerebral palsy. Phys Ther. 2007;87:861-871.]

Table 1.
Clinical Characteristics of the
Children With Cerebral Palsy (a)

                              EMG             Kinematic
Variable                      Analysis        Analysis

Uni-CP/bi-CP (n)              33/21           24/15
Severity disability (b)
  Mild                           32              24
  Moderate                       13               9
  Severe                          9               6
Age (y), mean (SD)                6.5 (2.3)       6.9 (2-3)
M/F (n)                       28/26           19/20
GA (wk), mean (SD)               28.6 (1.8)      28.8 (1.9)
Birth weight (g), mean (SD)   1,187 (325)     1,208 (328)

(a) EMG=electromyographic, uni-CP=subjects with unilateral spastic
cerebral palsy, bi-CP=subjects with bilateral spastic cerebral palsy,
GA=gestational age at birth, M=male, F=female.

(b) Severity of disability according to the results of the neurological
examination.

Table 2.
Number of Electromyographic (EMG) and Kinematic
Trials per Individual Included in the Analyses (a)

             Group    n    HZ

                           Median   Range

EMG trials   TD       29   11       9-18
             Uni-CP   33   10       7-25
             Bi-CP    21   11      10-14

Kinematic    TD       18    9       4-13
  trials     Uni-CP   24    6       3-10
             Bi-CP    15    6        4-9

             Group    n    FW

                           Median   Range

EMG trials   TD       29   11       9-14
             Uni-CP   33   10       7-13
             Bi-CP    21   10       6-13

Kinematic    TD       18    7       3-12
  trials     Uni-CP   24    5.5     3-11
             Bi-CP    15    7        3-9

             Group    n    BW

                           Median   Range

EMG trials   TD       29   10       8-14
             Uni-CP   33   11       7-14
             Bi-CP    21   10       9-13

Kinematic    TD       18    7.5     3-11
  trials     Uni-CP   24    6       3-12
             Bi-CP    15    7       3-10

(a) HZ=horizontal seat surface, FW=forward tilted seat surface,
BW=backward tilted seat surface, TD=subjects who were developing
typically, uni-CP=subjects with unilateral spastic cerebral palsy,
bi-CP=subjects with bilateral spastic cerebral palsy.