Introduction

Arm reaching skill develops at an early age1. Reaching is a foundational, fundamental skill as it allows infants to touch and/or gain possession of and explore a desired object. In order to provide early intervention for infants who do not optimally develop these important foundational skills, it is crucial to quantify and describe infants’ earliest practice of spontaneous and goal-directed arm movements and their developmental progression of reaching skills. To accurately quantify practice, it is necessary to record arm movement behavior across full days. Our goal here was to determine how patterns and characteristics of spontaneous and goal-directed arm movements produced across full days relate to the development of reaching skill and overall developmental rate in infants with typical development. This is necessary background information that will begin to inform what type of early intervention (type and amount of practice) is required to improve developmental outcomes for infants at-risk of developmental delays.

Reaching skill changes rapidly in the first year. Across just a few months, the baby progresses from not reaching for objects to reaching and grasping an object using the whole hand, to progressing further to pick up a tiny pellet using a skilled grasp1. Infants with typical development generally first learn to reach for objects at a very young age, usually between 3 and 5 months, with improvements made in straightness and smoothness during the first year24. For example, in younger infants, Bhat and Galloway5 reported on 13 infants, 8 weeks to onset of reaching, and described three phases of reaching. During the early phase, infants decreased their movement distance and velocity in the presence of a toy. During the mid-phase, infants increased the movement quantity, velocity, and smoothness; and decreased their hand–toy distance in the presence of a toy. During the late phase, infants continued to change their hand position to get closer and to contact the toy5. Gonçalveset al. also studied young infants (aged 4–8 months) longitudinally and found an increased number of touches and hits, and changes in time and distance kinematics during reaching trials6. Nelsonet al. studied 11–14-month-olds (53 infants) and found improvements in reach straightness and smoothness, kinematic changes and emergence of handedness as the infants matured7.

Although authors have described common general patterns occurring at each stage of reaching, infants proceed through the stages along their own unique timelines and developmental trajectories38. While it is likely that the amount and type of arm movement practice an infant participates in across days and months contributes to their rate of reaching skill development, whether infants’ practice of arm movements across the day in their natural environment is related to the progression of reaching skill has not been investigated.

One reason this fundamental question about the relationship between arm movement practice and the development of reaching skill has not been investigated has been the lack of feasibility of collecting detailed full-day information about arm movements. As described in the aforementioned previous studies, previous assessment of reaching skill has been limited to short measurements in laboratory settings using three-dimensional motion analysis and video equipment. To allow full-day assessment, we have developed the use of wearable sensors to allow the measurement of full-day infant arm movement activity in the natural environment9.

Advances in wearable sensor technology now allow us to quantify the number, type, and kinematic characteristics of bouts of infant arm movement made across a full day in the natural environment9. In the current study, we used wearable sensors to quantify full-day arm movement characteristics across days and months as infants learned to reach. We used video coding of a standardized reaching assessment to describe reaching skill progression and the Bayley Scales of Infant Development1 to measure developmental rate. Our aim here was to determine whether the amount and kinematic characteristics of arm movements made across the day in the natural environment was related to developmental status in infants with typical development as they learned to reach for objects using their arms.