Toddlers body sway during
the performance of a dynamic task

*Posture and Gait Studies Lab, Department of Physical Education,
São Paulo State University at Rio Claro - São Paulo.
**Federal University of Campina Grande at Cajazeiras - Paraíba.
Lilian Teresa Bucken Gobbi*
Erlane Aguiar Feitosa**




    In order to verify the body oscillation of one-year old children while performing a dynamic and self-initiated task, three children were invited to stand on a force plate for 30 seconds in each trial and to perform the following task: Phase 1: children were told to keep the stand still position for the first 13 seconds; Phase 2: children reached and grasped a toy over the table at the left side and Phase 3: children brought the toy down to the chest. Mean sway amplitude in the medium-lateral direction was the dependent variable in each task phase. Toddlers were very unstable in phase 1 and always reached for the toy with both hands. Mean sway amplitude was larger in phase 3 compared with the other phases maybe because the reverse direction. The highest body oscillation of toddlers observed might represent a result of immaturity and lack of experience in balance tasks.
    Keywords: Postural control. Dynamic task. Development.

http://www.efdeportes.com/ Revista Digital - Buenos Aires - Año 11 - N° 100 - Septiembre de 2006

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    Independent walking is one of the most important landmark behaviours during the childhood. Around 12 months of age, the child learns how to stand with no support and is able to start independent locomotion. These behaviours are very important developmental landmarks for postural control (Barela, Polastri, & Godoi, 2000). The co-ordination between posture and movement challenges motor control of children and can be achieved in a feed-forward manner (Schmitz & Assaiante, 2002). Before the movement onset and while it is being carried out, the individual must anticipate the required postural adjustments in order to compensate for movement effects. It cannot be done in a retroactive manner because there is not enough time to compensate for the resulting disturbances in progress. These anticipatory postural adjustments take time to mature and are also dependent upon learning or experience (Schmitz & Assaiante, 2002).

    Different factors affect posture and movement in the first two years of life. Child's body changes forcing the differential activation of postural muscles, new integration of the sensory information from different sources and how to interact with the environment, and increased co-ordination of limbs.

    The development of sensory systems of children is closely related to motor development, with the former being the basis for optimal motor functioning. Maturation plays an important role for perception development but most of it is related to experience (Gallahue & Ozmun, 1998).

    Assaiante and Amblard (1992) purposed an ontogenetic model of the organization of balance control. According to their model, after the first two months, the infant organizes equilibrium with the descending commands and displays an articulated unit of head-trunk operation. During the development of the postural responses, the infants show a cephalocaudal gradient. The control appears first in the neck muscles, followed by those in the trunk and then in the leg muscles.

    However, after the upright stance, the equilibrium organization is under ascending control and the head-trunk operation is en-bloc, i.e., head and trunk work as a unit. It can be argued that the blocked neck joint enables visual and vestibular inputs to reach the effectors as directly and rapidly as possible.

    The temporal parameters, the greater double support and the shorter single-leg support phases, the spatial parameters, the larger base of support and the shorter step length, the muscular activity, co-contraction of postural muscles, and the kinematics of the centre of gravity, negative vertical acceleration at the end of the single stance phase, of the gait in the first six months of independent walking, characterize the first phase of the learning dynamic postural control. The infants are learning how to integrate posture and movement. In the second phase, the infants learn to accurately integrate sensory information available by tuning the fine adjustments of the gait parameters (Bril & Breniere, 1992; 1993).

    Leonard, Hirschfeld, and Forssberg (1991) studied the development of locomotor patterns of the infants with cerebral palsy to consider possible influences of supraspinal input on the development of locomotion. The data showed that locomotor patterns of infants with cerebral palsy are similar to normal infants during supported locomotion, but as they mature they retain some characteristics of their infant locomotor pattern. These findings suggest that the development of locomotion of both normal infants and those with cerebral palsy reflects maturational changes within the central nervous system. Changes involving increased specificity of cortical and subcortical connectivity precede and continue throughout the period of gait acquisition. Locomotor generator circuits, which probably exist in some form in the human, seem to be more dependent on descending influences than are those of lower animals. The maturation of the human locomotor pattern can be regarded as a process whereby spinal circuits become progressively more influenced by higher brain centres. In addition, it appears that supraspinal centres in the human have become increasingly more integrated and involved in the actual coordination of movement.

    Children's postural control has been assessed using a quasi-static task (e.g., Riach & Starkes, 1989; Assaiante, 1998; Barela, Jeka, & Clark, 1999; Barela et al., 2000). A dynamic task is more reliable than the quasi-static task since in real world children are more active and they frequently perform dynamic tasks. The use of a dynamic task in this study allowed for the observations of small adjustments in children's behaviour related to body sway, especially in preparation for action.

    Postural control and locomotion are crucial to human survival so it is necessary to understand how children mature in preparation for interacting with adult environment. The aim of this study was to document and characterize body oscillation while performing a dynamic and self-initiated task of children at age one year.


Participants: The participants were three children, whose characteristics are presented in Table 1.

Table 1. Individual values for body weight (kg); body height, thigh, leg and foot length, ankle height (all in cm);
age and walking experience (both in months); SD = standard deviation.

Procedures: Each child was invited to stand on a force plate, facing the experimenter, with the distance between the ankles corresponding to their hip width and arms relaxed beside the trunk. In order to fix foot position in the same place in all the trials, the child stepped on footprints fixed on a carpet over the force plate. The force plate (KISTLER, model 9286A, 40 x 60 cm) provided information regarding the centre of pressure displacement.

    A table was positioned at the left side of the child within a distance relative child's extending arm. A toy was resting on the table approximately 5 cm from the edge. A different toy with the same features was used in each trial in order to maintain the child's motivation to perform the task. Each child performed three trials, each one for 30 s, with data collected at 100 Hz. The collection time of each trial was divided into three phases. In phase 1, the child stood on a force plate without loosing foot contact and looked forward to the experimenter for 13 s. In phase 2, at the experimenter's sign, the child reached and grasped the toy with their left hand. In phase 3, after grasping the toy, the child brought it to their trunk and maintained it in this position until 30 s were past.

Results and discussion

    Standing still on a force plate is a very difficult task for a one year-old child, since it is a complex task (Barela et al., 2000). In order to do three trials of 30 s each, the experimenter utilized many resources to keep the child's attention and cooperation. Even with this experimenter behaviour, only three toddlers completed the data collection. Toddlers' common behaviour was related to both collection time and lack of cooperation. Furthermore, all the participants did not follow the instructions. They performed the task of reaching and grasping the toy with both hands turning the trunk sideways. Children with age between 12 and 13 months showed trends to demonstrate a bimanual grasping behaviour of large toys (Fagard & Jacquet, 1996, cited by Gabbard, 2000). Only approximately at 18 months of age, do children acquire the reaching, grasping, and releasing of objects in a relatively coordinated way (Gabbard, 2000).

    Mean sway amplitude, only in the medium-lateral direction, was the dependent variable for each task phase and the data is described below for each child. In general, toddlers showed different medium-lateral mean sway amplitude in each task phase (Figure 1).

Fig. 1. Mean sway amplitude (MSA) by participant according to the task phase.

    Different behaviours were observed in all three toddlers.

    Values presented on Figure 1 showed that child 1 demonstrated larger mean sway amplitude in all task phases than the other children. Conversely, child 2 swayed less than the others independently of the task phase. Interesting to note is that in phase 3, which represents the reverse behaviour after grasping for the toy, all the toddlers showed higher values of mean sway amplitude than the other phases. Maybe this movement reversion generates an increased body sway at this age. Considering the difficulty in unimanual grasp that a one year-old child presents, the reversion of movement direction can be explained by the increased postural demands especially after grasping the toy.

    Barela et al. (1999), in a developmental study, observed that children around one year of age and with lateral support showed mean sway amplitude in the medium-lateral direction around 0.5 cm. Our one year-old children, without lateral support, oscillated near 5 cm in phase 1. This could be due to immaturity and lack of experience in dynamic balance tasks.

    Comparing the results in Figure 1 and the individual data in Table 1, anthropometric (body mass, stature and segment lengths), developmental (chronological age, walking age and walking experience) and dependent variables (mean sway amplitude for each task phase) can be interpreted. Child 2, with less walking experience (only one month), showed the smallest mean sway amplitude in all task phases. Maybe the lack of experience favoured the freezing of joint degrees of freedom and, as a consequence, smaller values of mean sway amplitude were observed. In contrast, both lower stature and lower segment lengths of child 1 could reflect a lower centre of mass, which also can result in less oscillation. However, those effects were not observed: child 1 showed higher values of mean sway amplitude reflecting difficulties in the control of body equilibrium. On the other hand, Child 3, who was older and had more walking experience, presented intermediate results for mean sway amplitude. The more time available to explore the environment and body sources, more refined the postural control.

    These data might be illustrating the continuum in the development of postural and balance control, where at the beginning of independent walking the joint freezing of degrees of freedom normally are accompanied by muscle co-contraction. Following this behaviour, a time period with gradual control of joint degrees of freedom decreasing muscle co-contraction and increasing mean sway amplitude. More walking experience could promote a higher dynamic postural control (Bril & Breniere, 1992; 1993).

    Anticipatory postural adjustments are crucial to perform a dynamic task and they must be learned, especially because their organization depends on previous experience (Schmitz & Assaiante, 2002). The small amount of walking experience observed in children 1 and 2 could explain their behaviour.

    When participants were asked to perform phase 1 and 2, anticipatory postural adjustments were required to correct movement effects. In this case, the amount of experience could drive them or to freeze joint degrees of freedom or use somatosensory information in a poor manner.

    The head-trunk operation 'en-block' purposed in the Assaiante and Amblard's (1992) ontogenetic model only to stabilize visual and vestibular signs and observed at the begging of independent walking confirm our purpose since it reinforce the notion of freezing joint degrees of freedom. As well as, the gradual utilization of somatosensory information and the refinement of sensory integration change the balance and postural control to an ascendant mode, initially increasing body oscillation.


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