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Published on: May 2, 2021
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.
Insights
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.
Abstract:
Infants of about 5 months of age who have just mastered the ability to reach succeed more frequently in contacting an object when they are seated upright than when they are supine or reclined. That effect of posture disappears in the subsequent months. Whether that effect can be attributed either to insufficient muscular strength or to insufficient control over the mechanically unstable arm was the subject of the present investigation. Kinematics and electromyography (EMG) of reaching movements of 8 sitting and supine infants at 12, 16, and 20 weeks of age were recorded. Maximum levels of shoulder torque as well as kinematic stability measures were similar in both postures. Coactivation levels and the frequency of on-off switching of muscles turned out to be higher in the sitting than in the supine posture. The authors suggest that the difference in reaching behavior resulted from the degree of error in the feedforward control signal that was allowed by the different postures rather than either insufficient muscular strength or insufficient control over the mechanically unstable arm.
