Abstract
The development of reaching is crucially dependent on the progressive control of the trunk, yet their interrelation has not been addressed in detail. Previous studies on seated reaching evaluated infants during fully supported or unsupported conditions; however, trunk control is progressively developed, starting from the cervical/thoracic followed by the lumbar/pelvic regions for the acquisition of independent sitting. Providing external trunk support at different levels to test the effects of controlling the upper and lower regions of the trunk on reaching provides insight into the mechanisms by which trunk control impacts reaching in infants. Ten healthy infants were recruited at 2.5 months of age and tested longitudinally, until 8 months. During the reaching test, infants were placed in an upright seated position and an adjustable support device provided trunk fixation at pelvic and thoracic levels. Kinematic and electromyographic data were collected. Results showed that prior to independent sitting, postural instability was higher when infants were provided with pelvic compared to thoracic support. Associated reaches were more circuitous, less smooth and less efficient. In response to the instability, there was increased postural muscle activity and arm muscle co-activation. Differences between levels of support were not observed once infants acquired independent sitting. These results suggest that trunk control is acquired in a segmental sequence across the development of upright sitting, and it is tightly correlated with reaching performance.
Highlights
Sitting postural control and reaching are distinguishable yet interrelated motor milestones, which are progressively acquired during the first years of life
This study was motivated by the hypothesis that the development of sitting postural control and reaching behavior are highly interdependent functions
Full attempts were made to tease out the causal effects of postural control on reaching
Summary
Sitting postural control and reaching are distinguishable yet interrelated motor milestones, which are progressively acquired during the first years of life. The relation between posture and arm movements is evident in neonates. When newborns are fully supported, either in a reclined or upright sitting position, their usually chaotic arm movements are more coordinated and directed, indicating that pre-reaching movements are influenced by posture (Von Hofsten, 1982; AmielTison and Grenier, 1983). Newborns may not want to access the toy but rather pre-reaching movements may function to orient the infants’ attention to the goal (Von Hofsten, 1982; Campos et al, 2008). The hands attract considerable amount of attention and newborns will work to keep their hands in view (Van der Meer, 1997)
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