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Development of reaching during the first year: role of movement speed
E Thelen1, D Corbetta, J P Spencer
1Department of Psychology, Indiana University, Bloomington 47405, USA. thelene@ucs.indiana.edu
Insights
Infant reaching movements become smoother and straighter over the first year. Faster movements correlate with less straight paths, suggesting development involves stabilizing limb dynamics for better trajectory control.
Area of Science:
- Developmental motor control
- Infant biomechanics
- Motor learning
Background:
- Infant reaching develops from jerky to smooth trajectories within the first year.
- Limb dynamics, influenced by movement speed, are crucial for motor control development.
Purpose of the Study:
- To investigate the role of limb dynamics and movement speed in the development of infant reaching trajectories.
- To understand how infants learn to control their reaching movements over the first year of life.
Main Methods:
- Longitudinal observation of 4 infants from reach onset to 1 year.
- Weekly and biweekly assessments of reaching and non-reaching movements.
- Analysis of hand path straightness and smoothness in relation to movement speed.
Main Results:
- Improvements in reaching trajectory straightness and smoothness were non-linear, with periods of plateau and regression.
- Faster infant movements, both reaching and non-reaching, were associated with less straight hand paths.
- A correlation between movement speed and trajectory quality suggests an interplay between limb dynamics and control.
Conclusions:
- Infant motor development involves learning to stabilize reaching trajectories against self-generated perturbations related to limb dynamics.
- The findings support an equilibrium trajectory model of motor control, where development enhances the ability to manage movement speed and its impact on trajectory accuracy.
Abstract:
When infants first learn to reach at about 4 months, their hand paths are jerky and tortuous, but their reaches become smoother and straighter over the first year. Here the authors consider the role of the underlying limb dynamics, which scale with movement speed, on the development of trajectory control. The authors observed 4 infants weekly and then biweekly from reach onset to 1 year. Improvements in trajectories were not linear, but showed plateaus and regressions in straightness and smoothness. When infants' nonreaching movements were fast, their reaches were also fast, and faster reaches were also less straight. This is consistent with an equilibrium trajectory form of control, where development involves the increasing ability to stabilize the trajectory against self-generated movement perturbations.