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The effect of posture on early reaching movements.
L Out1, A J Van Soest, G J Savelsbergh
1Institute for Fundamental and Clinical Human Movement Sciences Amsterdam-Nijmegen Faculty of Human Movement Sciences Vrije Universiteit, Amsterdam, the Netherlands.
Journal of Motor Behavior
|December 29, 2009
Summary
Infant reaching success improves when seated versus lying down at 5 months, likely due to posture affecting motor control, not strength. This difference diminishes as infants mature.
Area of Science:
- Developmental Psychology
- Motor Control
- Infant Biomechanics
Background:
- Infants aged ~5 months show improved object contact success when reaching from a seated posture compared to supine or reclined positions.
- This postural effect on reaching performance diminishes in later infancy.
Purpose of the Study:
- To investigate whether insufficient muscular strength or poor control of a mechanically unstable arm underlies the posture-dependent reaching differences in infants.
- To differentiate between strength limitations and motor control issues in early infant reaching behavior.
Main Methods:
- Recorded kinematics and electromyography (EMG) of reaching movements in 8 infants at 12, 16, and 20 weeks of age.
- Compared reaching in both sitting and supine postures.
- Analyzed shoulder torque, kinematic stability, muscle coactivation, and muscle switching frequency.
Main Results:
- Maximum shoulder torque and kinematic stability were comparable between sitting and supine postures.
- Muscle coactivation levels and the frequency of muscle on-off switching were significantly higher in the sitting posture.
- No evidence of insufficient muscular strength or unstable arm control was found to explain posture differences.
Conclusions:
- Postural differences in infant reaching success are likely due to variations in the feedforward motor control signal allowed by each posture.
- The findings suggest that the degree of error in motor commands, influenced by posture, is a key factor, rather than fundamental strength or arm stability deficits.
