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Updated: May 11, 2026
Assessing Corticospinal Excitability During Goal-Directed Reaching Behavior
Published on: December 2, 2022
Sit happens: Does sitting development perturb reaching development, or vice versa?
Regina T Harbourne1, Michele A Lobo, Gregory M Karst
1Munroe Meyer Institute, University of Nebraska Medical Center, 985450 Nebraska Medical Center, Omaha, NE 68198-5450, USA. rharbour@unmc.edu
Insights
Infants learn to reach and sit simultaneously, using arms for support initially. This dual task development shows a nonlinear progression in reaching skill as posture control evolves.
Area of Science:
- Developmental motor control
- Infant biomechanics
- Motor learning
Background:
- Reaching and sitting are key infant motor skills, often studied separately.
- Infants coordinate arms for sitting soon after learning to reach.
- The dual role of arms in reaching and sitting support is not well understood.
Purpose of the Study:
- To investigate how infants use arms for reaching while simultaneously learning to support themselves while sitting.
- To test the hypothesis that arm use for sitting support temporarily perturbs reaching development, leading to nonlinear skill progression.
Main Methods:
- Longitudinal study of 11 infants from prop sitting to independent sitting (5-8 months).
- Utilized behavioral coding, kinematics, and electromyography (EMG) to analyze reaching and posture.
- Categorized sitting into three stages: arm support, brief unsupported, and independent sitting.
Main Results:
- Infants linearly increased object contact with hands while decreasing postural arm support.
- Trunk and pelvis movement showed a U-shaped curve, indicating challenged posture control in the middle sitting stage.
- Muscle activation strategies evolved from tonic early on to integrated lower extremity and reach-onset control later.
Conclusions:
- Reaching development influences and is influenced by sitting development in infants.
- The dual sit-reach behavior is a complex assembly, crucial for flexible environmental interaction.
- Arm support for sitting is a temporary but significant factor in the nonlinear development of reaching skills.
Abstract:
The development of reaching and of sitting during the first year of life is typically studied as separate yet related behaviors. Interestingly, very soon after learning to reach, 4-7-month-old infants start coordinating their arms with their trunk and legs for sitting. In this longitudinal study, we focused, for the first time, on how infants learn to use their arms for the dual tasks of reaching for objects while providing arm support as they learn to sit. We hypothesized that the use of arms for support during sitting development would be a temporary perturbation to reaching and result in a nonlinear progression of reaching skill. Eleven infants were studied monthly from the time they began to prop sit to the time of sitting independence (5-8 months of age). Behavioral coding, kinematics, and electromyography (EMG) characterized reaching and posture while infants sat as independently as possible. Results revealed significant changes across time in trunk movement and hand use as infants transitioned through three stages of sitting: with arm support, sitting briefly without arm support, and sitting independently. Infants used their hands more for contacting objects and less for posture support linearly across time. In contrast, changes in posture control as indicated by pelvis and trunk movement demonstrated a U-shaped curve with more movement of these two body segments during the middle stage of sitting than in the first or last stage. During the middle stage of sitting infants reached persistently even though posture control, measured by pelvis and trunk movement, appeared to be significantly challenged. Muscle activation consisted of tonic and variable combinations of muscle pairings in early sitting. As infants progressed to sitting without hand support, variable but successful strategies utilizing lower extremity muscles in a tight linkage with reach onset emerged to provide prospective control for reaching. Our findings support the contention that reaching both drives the development of sitting in infancy as well as perturbs sitting posture, factoring into the assembly of the complex dual sit-reach behavior that supports and expands flexible interaction with the environment.
