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The musculoskeletal and motor development of infants is affected by their environment, which varies from being held, lying on a firm flat surface, to seated in various nursery products. Nursery products can alter the body position of infants, particularly the position of the head/neck and trunk, which may inhibit an infant's ability to breathe. With U.S. infants spending an increasing amount of time in seated products, the purpose of this study was to assess muscle activation and body position in four commercial infant products (carrier, bouncer, rocker, and swing) during supine and prone positioning, compared to a firm flat surface. Thirteen healthy infants (age: 4.2 ± 1.4 months; 7 M/6F) were enrolled in this IRB-approved in-vivo biomechanics study. Surface electromyography sensors recorded muscle activity of the erector spinae, cervical paraspinals, quadriceps, and abdominal muscles and retro-reflective markers tracked movements to determine head-neck, trunk, and torso-pelvis flexion/extension in the sagittal plane. While supine, infants exhibited increased head-neck and trunk flexion of up to 21° and 27° above the playmat, respectively, in all seated products. While prone, high abdominal muscle activation compared to the playmat indicates that infants will fatigue faster in seated products. Additionally, the lower muscle activation levels exhibited in younger infants (< 4 months) compared to older infants (≥ 4 months) indicates that younger infants rely on the product design to maintain body position. However, offering infants a variety of environments to move within is important to avoid motor delay, therefore future work should explore how long-term use may impact an infant's development., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that have appeared to influence the work reported in this paper. EMM provides expert witness services related to some infant products., (Copyright © 2024 Elsevier B.V. All rights reserved.)