P3.L229: Seated Infant Products Alter Body Position and Muscle Utilization

Background: Infant musculoskeletal and motor development is affected by their environments, varying from being held in arms, lying on a firm flat surface, to seated in commercial products. In the U.S., infants spend an average of 5.7 hours/day in seated products like infant carriers (defined as car seats used outside of the car), bouncers, rockers, and swings. Increased respiration-related hazards may exist while lying in inclined seated products due to the changes in body position. The purpose of this study was to assess how an infant’s muscle activation and body position are altered while placed supine in four commercial infant products (carrier, bouncer, rocker, and swing) compared to a firm flat playmat.

Methods: Thirteen healthy infants (4.2±1.4 mos; 7M/6F) participated. Surface electromyography electrodes recorded muscle activity from the cervical paraspinals, erector spinae, abdominals, and quadriceps (Fig. 1A). A retroreflective marker-based motion capture system tracked infant kinematics using custom rigid body clusters on the head, torso, and pelvis, and markers on the shoulders and anterior superior iliac spine (ASIS) (Fig. 1A). Infants laid supine for three minutes on a firm flat playmat (Fig. 1B) and unrestrained in a commercial infant carrier, bouncer, rocker and swing (Fig. 1C). EMG signal mean was taken over 60 seconds for each condition and muscle group, then normalized as a percent of the playmat mean. The head-neck and torso-pelvis flexion was calculated using the angular orientation between adjacent body segments from the local coordinate systems defined by rigid body clusters on each segment when the head, torso, and pelvis were visually aligned in the sagittal plane. Trunk flexion was determined using the shoulder and ASIS markers. All angles were normalized to the playmat condition to account for each infant’s anatomical differences. Paired t-tests (p < 0.05) compared the four products to the playmat.

Results: In all products tested, infants exhibited increased head-neck and torso flexion of up to 21° and 27° above the playmat(Fig. 2). Previous research has determined that head-neck flexion angles of just 15° to 30° can alter infant breathing patterns significantly. Trunk flexion was also significantly higher for all products (between 23° and 27° on average) compared to the playmat which relates to the conformity and/or concavity of the product longitudinally. For most products, older infants exhibited higher abdominal muscle activity, potentially indicating that older infants use their diaphragmatic muscles at a higher level in these seated products to overcome breathing challenges related to the flexed posture.

Conclusion: This research quantified body posture and muscle activity of infants lying in a variety of seated products, suggesting that to prevent potential respiratory-related hazards and musculoskeletal or motor developmental delays, prolonged use should be avoided.