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Research Reports |
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.
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.
Address all correspondence to Dr Hadders-Algra at: m.hadders-algra{at}med.umcg.nl
Submitted October 27, 2006;
Accepted March 6, 2007
 | Abstract |
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Subjects: The subjects were 58 children with CP aged 2 to 11 years (34 with unilateral spastic CP, 24 with bilateral spastic CP).
Methods: During the task of reaching movements, surface electromyographic and kinematic data were recorded for posture and reaching with the dominant arm in 3 sitting conditions: horizontal seat surface, seat surface tilted forward 15 degrees, and seat surface tilted backward 15 degrees.
Results: In the children with unilateral spastic CP, forward tilting improved postural efficiency and quality of reaching. In the children with bilateral spastic CP, both forward and backward tilting of the seat surface was associated with more postural instability and did not affect the quality of reaching.
Discussion and Conclusion: The results suggest that, in terms of postural control and quality of reaching, children with unilateral spastic CP benefit from a forward-tilted position and children with bilateral spastic CP benefit from a horizontal sitting position.
| Introduction |
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 TopAbstract Introduction MethodResultsDiscussion and ConclusionsReferences |
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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 multisensorial 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 study1 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 studies2,10,11 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 submovements 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:
| Method |
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TopAbstractIntroductionMethodResultsDiscussion and ConclusionsReferences |
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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.
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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 [±SD]=29±2 weeks; mean birth weight [±SD]=1,183±356 g) and those with bilateral spastic CP (mean gestational age [±SD]=28.5±1.7 weeks; mean birth weight [±SD]=1,248±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 al2).
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 al15). 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 al2).
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 (I2) and 100 to 1,000 milliseconds (I3), 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 al10) 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)
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 |
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TopAbstractIntroductionMethodResultsDiscussion and ConclusionsReferences |
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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 I2 and I3 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 I3 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 I3 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.
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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.
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| Discussion and Conclusions |
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TopAbstractIntroductionMethodResultsDiscussion and ConclusionsReferences |
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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 control18,19 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 settings3–8; (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° in the present study compared with 5° in the study by McClenaghan et al5), 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 al5), 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 al5). 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 GMFCS 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.
| Footnotes |
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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.
* Inspector Research Systems, Amsterdam, the Netherlands. 
SPSS Inc, 233 S Wacker Dr, Chicago, IL 60606.
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