The present study addressed the question whether the deviant postural adjustments in children with spastic diplegia can be attributed to their crouched sitting position or primarily to their neural deficit. Postural adjustments during sitting in an erect and in a crouched position on a movable platform were assessed in 10 children, aged 3 to 7 years 6 months, with mild‐to‐severe forms of spastic diplegia and 10 age‐ and sex‐matched control children. Multiple surface EMGs of neck, trunk, and leg muscles and kinematics of head, body sway, and pelvis were recorded during forward and backward translations. The children with normal motor development showed a distinct adaptation of postural adjustments to sitting position. The children with cerebral palsy (CP) had a deficient adaptational capacity which was more pronounced in the erect than in the crouched position. Thus, the crouched sitting position did not induce postural deficiency but seemed to offer a solution to the sensory‐motor problem of the instability experienced. Children with severe diplegia exhibited a lack of direction specificity in the leg muscles during backward body sway, which points to a basic deficit in postural control. In addition, these children showed marked dysfunctions in the precise tuning of the postural adjustments to task‐specific conditions. In the children with mild‐to‐moderate forms of CP the basic level of control was intact.

Postural adjustments during sitting on a moveable platform were assessed by means of multiple surface EMGs of neck, trunk and leg muscles and kinematics in three groups of children, aged 1 1/2-4 1/2 years. The first group consisted of 13 preterm children (born at a gestational age of 25-34 weeks), whose neonatal ultrasounds had shown distinct lesions of the periventicular white matter (PWM), The second group was the preterm control group, consisting of 13 preterm children with normal neonatal brain scans, matched to the PWM group with respect to gestational age at birth, birth weight, sex and age of postural assessment. The third group was formed by 13 healthy children born at term and matched to the PWM group with respect to sex and age at examination. In addition to the postural assessment an age-specific neurological examination was carried out. Three of the children of the PWM group developed a cerebral palsy syndrome, nine showed minor neurological dysfunction and one child was neurologically normal. In the preterm control group one child showed minor neurological dysfunction, while the remaining 12 children of this group and all children of the full-term group were neurologically normal. The postural assessment revealed that preterm birth was associated with two types of postural dysfunction, One dysfunction was related to the presence of a PWM lesion and consisted of a limited repertoire of response variation. The other dysfunction was not related to the presence of a PWM lesion, but to preterm birth itself. It consisted of a change in the ability to modulate the postural responses. Preterm children showed a higher sensitivity to platform velocity than full-term children, and they lacked the capacity to modulate EMG amplitude with respect to initial sitting position.

The present study investigated developmental changes in postural adjustments during preschool age. Postural responses during sitting on a moveable platform were assessed in 21 healthy children aged 1% to 4% years. Multiple surface EMGstof neck, trunk, and leg muscles were recorded during forward and backward translations. Comparable data were available for 11 infants seen three times between the ages of 5 and 10 months. The data revealed the existence of a transient period between the ages of 9 to 10 months and 21/2 to 3 years, during which perturbations in a sitting position are accompanied by high activity in the direction‐specific agonist muscles and in the antagonist muscles. After this period, agonist activity became more variable, particularly so during backward translations, and antagonist activity disappeared. These changes could be attributed to biomechanical factors and to maturation of the nervous system.

Children with cerebral palsy (CP) display postural problems, largely interfering with daily life activities. Clarification of neural mechanisms controlling posture in these children could serve as a base for more successful intervention. Studies on postural adjustments following horizontal forward and backward displacements of a movable platform in ten school-age children with spastic diplegia and non-disabled controls revealed that sitting CP children, like standing CP children, show direction specific postural adjustments, indicating that the basic pattern of muscle coordination in these conditions is conserved. Dysfunctions are especially present in the modulation of the response pattern of ventral muscles during forward translations. They consist of: (1) a stereotyped and non-variable activation of all ventral muscles; (2) an abnormal top-down muscle recruitment; and (3) an excessive degree of antagonistic co-activation. The altered patterns of muscle coordination could be the result of two interacting mechanisms, the primary deficit due to the early brain damage and a compensation due to the postural instability. Especially the latter dysfunction furnishes opportunities for therapeutic help.

Postural control is organized in basic, direction specific synergies which can be adapted to task-related conditions. Studies on the development of postural adjustments in young sitting children revealed that largely variable, direction specific muscle activation patterns are already present in 5-6 month old children not able to sit without support. With increasing age, the variation in muscle activation patterns decreases, resulting in a selection of the most complete patterns of synergist activation at 9-10 months of age. The synergy of the dorsal extensor muscles (during a forward sway of the body) develops faster than the synergy of the ventral flexors (during backward body-sway). A 'fixed' extensor synergy is prominently present between 9 months and 3 years, i.e. during the period when standing and walking abilities develop. With increasing age the 'fixed' extensor synergy gradually dissolves. The flexor synergy shows a larger flexibility than the extensor synergy, a difference which can be attributed to differences in stability limits and differences in the degree of supraspinal modulation.

Nowadays, the controversy on “nature” and “nurture” in motor development focuses on the development of automatic motor patterns. The present paper discusses this issue within the framework of a recent study on the effect of maturation and training on the development of postural adjustments in sitting infants. This study revealed that already before the age of sitting without help, highly variable but direction‐specific muscle activation patterns are present. This suggests that postural adjustments develop via an innate repertoire of primary direction‐specific response patterns. The following developmental step consists of a decrease in muscle activation patterns, which results in selection of the most complete patterns of synergist activation. In accordance with Edelman's neuronal group selection theory, pattern selection turned out to be experience dependent, as it appeared to be guided by cues on head stabilization and enhanced by daily balance training.

1. The present study addressed the question of whether daily balance training can affect the development of postural adjustments in sitting infants. 2. Postural responses during sitting on a moveable platform were assessed in twenty healthy infants at 5‐6, 7‐8 and 9‐10 months of age. Multiple surface EMGs and kinematics were recorded while the infants were exposed to slow and fast horizontal forward (Fw) and backward (Bw) displacements of the platform. After the first session the parents of nine infants trained their child's sitting balance daily. 3. At the youngest age, when none of the infants could sit independently, the muscle activation patterns were direction specific and showed a large variation. This variation decreased with increasing age, resulting in selection of the most complete responses. Training facilitated response selection both during Fw and Bw translations. This suggests a training effect on the first level of the central pattern generator (CPG) model of postural control. 4. Training also affected the development of response modulation during Fw translations. It accelerated the development of: (1) the ability to modulate EMG amplitude with respect to platform velocity and initial sitting position, (2) antagonist activity and (3) a distal onset of the response. These findings point to a training effect on the second level of the CPG model of postural adjustments.

SUMMARY To clarify the neural mechanisms controlling equilibrium during sitting, and the implications for the optimal sitting position for children with CP, automatic postural adjustments after perturbations of the support surface during sitting were investigated in seven children with spastic diplegia and in seven age‐matched controls. A sudden backward sway of the body evoked brisk responses in on ‘ventral’ muscles in both groups. However, the order of muscle recruitment, which in most non‐disabled children was caudal to cranial, was reversed in the children with diplegia, whose first response was in the neck flexors. The children with diplegia also co‐activated antagonistic neck and hip muscles. The authors concluded that although children with spastic diplegia may produce a d basic muscle activation pattern (first level of the central pattern generator, CPG), they cannot adjust the pattern in response to external changes (second level of the CPG). RÉSUMÉ Contrôle postural chez les enfants avec diplégie spastique: activité musculaire au cours des o perturbations lors de la position assise Pour éclairer les mécanismes nerveux contrôlant l'équilibre en position assise, et les implications pour une position assise optimale chez les enfants IMC, les ajustements posturaux automatiques après perturbations de la surface de support ont étéétudids chez sept enfants avec diplégie spastique et chez sept enfants appariés pour l'âge. Un balancement soudain du corps vers l'arrière provoquait des réponses brusques dans les muscles ‘ventraux’ des deux groupes. Cependant l'ordre du recrutement musculaire qui allait de la région caudale à la région cranierine chez la plupart des enfants valides, était inversé chez les enfants avec diplégie, dont la réponse première se situait aux muscles fléchisseurs du cou. Une co‐contraction antagoniste au niveau du cou et des hanches était aussi présente chez les enfants avec diplégie. Les auteurs concluent que, bien que les enfants avec diplégie spastique puissent présenter un pattern d'activation musculaire basale (premier niveau du generateur central de patterns, CPG), ils ne peuvent pas ajuster le pattern à des modifications externes (second niveau du CPG) ZUSAMMENFASSUNG Haltungskontrolle bei Kindern mil spastischer Diplegie: Muskelaklivität bei Veränderungen der Sitzposition Bei sieben Kindern mit spastischer Diplegie und sieben altersentsprechenden Kontrollen wurden die automatischen Haltungsanpassungen nach Lageveränderungen untersucht, um die neuralen Mechanismen, die das Gleichgewicht im Sitzen kontrollieren, und die sich daraus ergebenden Folgerungen für die optimale Sitzposition von Kindern mit Cerebralparese zu klären. Durch ein plötzliches nach hinten Kippen des Kórpers wurden bei beiden Gruppen lebhafte Reaktionen in den ‘ventralen’ Muskeln ausgelöst. Die Reihenfolge der Muskelaktivierung jedoch, die bei den meisten nicht behinderten Kindern von caudal nach cranial erfolgte, verlief bei den Kindern mit Diplegie umgekehrt, bei ihnen reagierten die Halsbeuger zuerst. Außerdem aktivierten die Kinder mit Diplegie auch antagonistische Hals‐ und Hüftmuskeln. Die Autoren kommen zu dem Schluß, daß die Kinder mit spastischer Diplegie zwar ein basales Muskelaktivitätsmuster entwickeln können erste Stufe des.zentralen Mustergenerators, CPG), aber sie können das Muster nicht an äußere Veränderungen. anpassen (zweite Svufe des CPG). RESUMEN Control postural en niños con diplegia espástica: actividad muscular durante perturbaciones en la sedestación Para esclarecer los mecanismos neurales que controlan el equilibrio durante la sedestación y las implicaciones de la posición sedente óptima para niños con PC, se investigaron los ajustes posturales automaticos después de perturbaciones en la superficie de soporte durante la sedestación, en siete niños con diplegia espástica y en sietc controles de la misma edad. Una brusca inclinatión hacia atrás del cuerpo dió lugar a respuestas bruscas en los músculos ‘ventrales’ en am bos grupos. Sin embargo el orden de reclutamiento muscular, que en la mayoria de los niños no incapacitados era caudo‐craneal, ocurría al revés en los niños con diplegia, cuya primera respuesta aparecfa en los flexores de la nuca. Los niños con diplegia tambien coactivavan los músculos antagonistas del cuello y de la cadera. Los autores concluyen que si bien los niños con diplegia espástica pueden producir un esquema básico de activación muscular (con un primer nivel en el generador central del patrón, GCP), no pueden ajustar el patrón como respuesta a cambios externos (segundo nivel de GCP).

1. The aim of the study was to find out whether the development of postural adjustments occurs via a coupling of simple muscle responses, such as stretch reflexes, or via selection from an innate repertoire of centrally generated response patterns. 2. Postural responses during sitting on a moveable platform were assessed in eleven healthy infants at 5‐6, 7‐8 and 9‐10 months of age. Multiple surface EMGs and kinematics were recorded while the infants were exposed to slow and fast horizontal forward (Fw) and backward (Bw) displacements of the platform. 3. From the youngest testing age onwards, largely variable but direction‐specific muscle activation patterns were present. Fw translations resulted predominantly in an activation of the neck flexor, the rectus abdominis and rectus femoris muscle, while the neck‐, thoracal‐ and lumbar extensor muscles (NE, TE, LE) and the hamstrings (Ham) showed varying amounts of inhibition. During Bw translations NE, TE, LE and Ham were preferably activated. The muscle activity could not be explained by simple stretch reflex mechanisms, but is likely to reflect centrally generated motor activity maturing in a predetermined way. However, indications for a contribution of stretch reflex mechanisms were also present. 4. With increasing age the variation in muscle activation patterns decreased, resulting in a selection of the most complete patterns. The ability to modulate the amplitude of the selected, most complete patterns during Fw translations, with respect to platform velocity and initial pelvis position, emerged at 9‐10 months.