Toddlers actively reorganize their whole body coordination to maintain walking stability while carrying an object

Highlights

• Functional differentiation of limbs occurs when children perform multiple tasks.

• Children keep the body stable when walking and carrying objects at the same time.

• Transitions between stance and swing affect inter-limb coordination of the shoulders and hips.

• Whole-body coordination may be partitioned as a function of task requirements.

Abstract

Balanced walking involves freely swinging the limbs like pendula. However, children immediately begin to carry objects as soon as they can walk. One possibility for this early skill development is that whole body coordination during walking may be re-organized into loosely coupled collections of body parts, allowing children to use their arms to perform one function, while the legs perform another. Therefore, this study examines: 1) how carrying an object affects the coordination of the arms and legs during walking, and 2) if carrying an object influences stride length and width. Ten healthy toddlers with 3–12 months of walking experience were recruited to walk barefoot while carrying or not carrying a small toy. Stride length, width, speed, and continuous relative phase (CRP) of the hips and of the shoulders were compared between carrying conditions. While both arms and legs demonstrated destabilization and stabilization throughout the gait cycle, the arms showed a reduction in intra-subject coordination variability in response to carrying an object. Carrying an object may modify the function of the arms from swinging for balance to maintaining hold of an object. The observed period-dependent changes of the inter-limb coordination of the hips and of the shoulders also support this interpretation. Overall, these findings support the view that whole-body coordination patterns may become partitioned in particular ways as a function of task requirements.

Introduction

Young children learn the many possible combinations of body articulators that may be used to explore their environments as well as perform more goal-directed tasks [1], [2]. One of the major challenges in early childhood is discovering how to perform one action with the legs (e.g., locomotion) while simultaneously performing another action with the hands (e.g., manipulating objects). Children learn very early in life to preferentially use the upper body for more differentiated and skilled actions. Particularly, a walking child has free use of their hands for exploring the environment. This ability may play a role in partitioning a global pattern of swinging arms and legs into a variety of more loosely coupled ways of using their hands while locomoting.

The differentiated function of the hands is already apparent during infancy. For example, we conducted a longitudinal study of the differentiation of arms and legs in infants between the ages of 3 and 6 months, when they were presented with an overhead mobile [3]. For arm movements, the joint rotation of shoulders and elbows became more weakly coordinated from 3 to 6 months. By contrast, no consistent coordination changes were found between the hips and knees. Furthermore, the arms and legs were moving in more functionally independent patterns by 6 months of age. These infants were more likely to keep their hands away from the body to reach midline objects while moving their feet close to the body to maximize kicks. These findings suggest that the relaxation of coordination between joints may be a precursor for the functional differentiation between arms and legs during infancy and early childhood.

Another opportunity to examine the process of functional differentiation of arms and legs is the transition from crawling to walking. At this transition, children no longer need to use their upper extremities as support but are able to swing their arms freely in space during walking. Arm swing is important for walking stability because it reduces vertical angular moment, vertical ground reaction moment, and energy consumption [4]. Furthermore, walking increases children’s ability to physically interact with the environment [5], allowing them, for example, to carry objects [6]. In order to walk and carry objects at the same time, children need to assemble a set of functional units for locomotion and another set for carrying. Although children younger than 2 years old tend to employ a less mature, high guard posture while carrying objects during walking, their balance appears to be unaffected [7]. In fact, children fall less when carrying objects than when not carrying objects [6]. Children seem to be able to use their arms and legs to stabilize themselves during walking while simultaneously carrying objects. One possibility is that a relaxation in the coordination of the larger functional units of the upper and lower body allows children to perform these separate functions at the same time. Surprisingly, there have been few studies investigating how the coordination of the arms is modified in response to carrying objects during walking, where the dominant coordination patterns involve the functional units of the legs. Therefore, questions remain about how carrying objects affects inter-limb coordination of the arms and legs.

To begin to address these questions, we measure the coordination pattern and stability between two segments by calculating their continuous relative phase (CRP) [8]. Furthermore, as opposed to quantifying the overall coordination of the gait cycle, it is possible to separate the gait cycle into phases to further analyze important transition periods. For example, the transitions from double limb support to single limb support during stance and from accelerating to decelerating the leg during swing may require different coordination to maintain stability during gait. Our goal in this study was to determine the coordination by which children use their arms and legs for walking while simultaneously performing carrying. To answer this question, we set out to determine: 1) how carrying an object affects the coordination of the arms and legs during walking, and 2) if carrying an object influences stride length, width, and speed. We hypothesized that carrying a small object would change coordination patterns as measured by CRP during different periods of the gait cycle when the arms may play a greater role in locomotion, allowing them to both stabilize gait and perform the carrying function. Furthermore, we hypothesized that carrying a small object would not affect stride length, width, or speed.