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. Author manuscript; available in PMC: 2025 Nov 1.
Published in final edited form as: Early Hum Dev. 2024 Sep 12;198:106122. doi: 10.1016/j.earlhumdev.2024.106122

Commercial Infant Products Influence Body Position and Muscle Use

Danielle N Siegel 1, Sarah Goldrod 2, Christopher Wilson 2, Andrew Bossert 2, Trevor J Lujan 1,2, Brandi N Whitaker 3, John L Carroll 3, Erin M Mannen 1,2,*
PMCID: PMC11560562  NIHMSID: NIHMS2024211  PMID: 39305831

Abstract

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; 7M/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 (≥ 4months) 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.

Keywords: Baby Gear, Incline, SIDS, Suffocation, Motor Development

1. INTRODUCTION

An infant’s musculoskeletal and motor development is significantly impacted by their interactions with others and their environment [1]. An infant’s environment ranges from being held in arms, lying on firm flat surfaces, to spending time in a multitude of different nursery products. In the U.S., infants younger than five months primarily spend their time in these nursery products [2] with an average of 5.7 hours (range 0–16 hours) per day in nursery products that are seated [3]. Common seated products like car seats, strollers, bouncers, and swings are designed for use over 30–60 minutes where the child is attended during activities like transportation, feeding, and playtime [4]. However, infants are not continuously attended and restrained in these products [3,5–7]. Additionally, excessive time lying supine in seated products, specifically car seats, has been linked to increased rates of deformational plagiocephaly and motor development delay, as well as decreased oxygen saturation levels [8–10]. Although these products are widely used, there has been insufficient research conducted on the biomechanical effects of placing infants in supine-lying commercial infant products. [11]. Therefore, more research is required to determine how various commercial infant products affect movement, motor development, and motor capabilities.

The American Academy of Pediatrics set recommendations for safe sleep in 1992, designed to decrease the occurrence of sudden infant death syndrome. These recommendations included a supine position, a firm flat separate sleep surface, and the avoidance of soft bedding [12]. Additionally, our previous biomechanics research on inclined sleep products revealed that increased suffocation-related hazards exist when infants lie supine or prone in inclined products [13,14] which contributed to President Biden signing the “Safe Sleep for Babies Act of 2021” (H.R.3182) to ensure that products marketed or intended for infant sleep have a 10° incline or lower. Despite these recommendations, many caregivers use seated products for prolonged periods as alternatives to a crib or bassinet to improve sleep [3,5–7]. In addition, our research focusing on awake infants in inclined positions found significant changes in muscle activation, specifically the erector spinae and abdominal muscles, and changes in rolling maneuvers compared to the flat surface [14,15]. These changes in muscle activation and rolling maneuvers indicate that infants may be able to achieve a roll prematurely in an inclined environment resulting in them lying in a prone position [15,16]. The unintended prone environment increases an infant’s risk of suffocation substantially if they are unable to keep their nose and mouth away from the surface of the inclined seated product.

In 2022, the National Electronic Injury Surveillance System recorded approximately 12,000 infant injuries treated in emergency departments related to the use of car seats used outside of a motor vehicle (infant carriers), bouncers, rockers, and swings. Many of these injuries were caused by infants rolling over within or out of a product entirely. While various injuries can occur in seated products, infants zero to six months are at a higher risk of experiencing breathing-related injuries caused by suffocation compared to older children [17], in part attributed to their less-developed respiratory system and arousal response [18,19]. Previous research shows that head-neck flexion is associated with increased total pulmonary resistance in both supine and semi-sitting positions [20]. Other studies indicate that head-neck flexion can significantly increase airflow interruption and even cause complete pharyngeal closure in infants [21–23]. Researchers have also shown that a slouched position can significantly decrease lung capacity, expiratory flow, rib cage structure and chest wall motion [24–27]. In addition, an infant’s head size in combination with their weak cervical spine musculature and laxity can subject the infant to uncontrolled and passive cervical spine movements where the infant’s positioning is dependent solely on their environment [28]. Because of the design features of some seated products, infants may be forced into higher head-neck flexion and a slouched position, increasing their risk for a breathing related incident.

The purpose of this study was to assess infant 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. We hypothesized that (1) infants would exhibit higher head-neck, torso-pelvis, and trunk flexion/extension in all seated products compared to the flat surface and (2) infants would have higher erector spinae and lower abdominal muscle activation at higher inclines in the supine position and the converse in the prone position.

2. METHODS

2.1. Participants

Following institutional review board approval, we recruited legal guardians of potential participants using community flyers, local, and social media platforms. Infants aged between two and seven months, born after at least 37 weeks of gestation, and falling within the 5th and 95th percentile for birth height and weight were included in this study. Infants with diagnosed orthopaedic or neurological conditions potentially affecting motor development were excluded. Prior to participating, guardians provided informed assent and completed an Ages and Stages Questionnaire, a developmental screening tool [29]. Infants were excluded if they scored below the age-appropriate cutoffs for gross motor and fine motor skills where an advanced professional assessment is recommended (gross motor cutoff score: 4month = 38.41, 6month = 22.25; fine motor cutoff score: 4month = 29.62, 6month = 25.14).

2.2. Experimental Procedure

2.2.1. Product Selection & Characterization

Four inclined seated infant products that featured unique designs were included in the study. Each of the products was evaluated using standard methodology for infant inclined products and a hinged infant weight gage recommended by ASTM international (ASTM F3118–17a). The firmness, surface curvature, material composition (soft goods) and products dimensions varied for each product. Seatback incline angles at the head ranged from 25.1° to 35.7° while the base angles at the thigh ranged from 22.9° to 44.0°, representing included angle and seat design differences. The width at the head ranged from 36.1cm to 54.3cm and width at the hinge ranged from 38.8cm to 58.3cm. The support surface/frame was also taken into consideration, with one product featuring a completely rigid support (carrier) and the other three products featuring a hammock design with no rigid support (bouncer, rocker, and swing). Material selections or the soft goods in the products included thick padding (carrier), a plush insert (bouncer), and minimal to no foam inserts or padding (rocker and swing). Details about the included products are presented in Table 1.

TABLE 1:

Product measurements and characteristics.

Inclined angle at head / thigh (deg) Max Product Height (cm) Width at head / hinge (cm) Thick plastic support? (Y/N) Insert/Pillows? (Y/N) Firmness (mm)a
Carrier 35.7 / 43.3 42 36.1 / 38.8 Y Y 12.73
Bouncer 28.0 / 22.9 67 54.3 / 58.3 N Y 12.72
Rocker 30.4 / 31.9 60.1 40.4 / 45.2 N N 15.63
Swing 25.1 / 44.0 74.6 38.7 / 42.4 N Y 16.76
Product Measurement Notes Product Photo
Carrier • Removable head and body insert graphic file with name nihms-2024211-t0005.jpg
• Thick, plush sides without insert
• Hard/supportive base and frame
• Shade cover and handle
• Five-point safety harness
• Rearfacing for infants 4–30lbs up to 32”
Bouncer • Detachable overhead mobile graphic file with name nihms-2024211-t0006.jpg
• Built-in music and vibration electronics
• Removable body insert and nonremoveable head insert
• No hard plastic support
• Vertical motion allowed
• Three-point safety harness
• Use from birth until infant starts trying to sit up or reaches 20lbs
Rocker • Detachable overhead mobile graphic file with name nihms-2024211-t0007.jpg
• Built-in vibration electronics
• Rocking motion allowed
• Optional stop for stationary mode
• Upright and inclined seatback positions
• No hard plastic support
• Three-point safety harness
• For infants and toddlers up to 40lbs
Swing • Detachable overhead mobile graphic file with name nihms-2024211-t0008.jpg
• Built-in sound and motion with 6 speeds
• Removable pillow insert
• Upright and inclined seatback positions
• Five-point safety harness
• For infants 6–25lbs
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a

Firmness values as determined by Mannen et al., 2023.

2.2.2. Testing Conditions

Infant participants wore only a disposable diaper that was provided by the researchers to maintain consistency between participants. Infants were placed both supine and prone on a firm foam playmat and then within the four infant products unrestrained (Table 1) for at least 90 seconds in each condition. Kinematic and electromyography (EMG) data was collected while infants were awake and not crying. The order in which the four infant products were tested was randomized where supine was tested first and then prone, to represent a scenario where an infant achieved a roll within the product. If an infant was upset or at risk of falling out of the product the trial was ended.

2.2.3. Kinematics

An 8-camera retro-reflective marker-based motion capture system (Qualisys, 100Hz) tracked infant kinematics using twenty-one markers positioned on specific body segments and anatomical landmarks. Custom 3-marker (6.5mm) rigid body clusters were placed on the forehead, and the anterior and posterior trunk and pelvis. Individual 9.5mm markers were placed on the shoulders as well as the anterior and posterior superior iliac spine (ASIS and PSIS, respectively) (Figure 1B). The marker data was truncated to include only 60 seconds of data from each condition and then the sagittal plane head-neck flexion, torso-pelvis flexion, and torso flexion were calculated in the supine position and the torso-pelvis and torso extension were calculated in the prone position (Figure 1C). Head-neck and torso-pelvis flexion/extension was calculated using the marker clusters to determine unit vector matrices and the local coordinate systems to define the angular orientation between adjacent body segments [13,30]. To account for each infant’s anatomical differences (i.e. head shape), all angles were normalized to the playmat condition. Torso flexion/extension was calculated with the shoulder and the ASIS (supine) or PSIS (prone) markers. The law of cosines was used to determine the flexion/extension angle where the hypotenuse of the triangle was the distance found between the markers when the infant was in the product and the two remaining sides were equivalent to each other at half the distance between the markers when the infant was in the playmat condition. A correction value was determined for each infant to account for the ASIS and PSIS marker anterior displacement from the frontal plane of the infant [31]. Additionally, for head-neck flexion, the average time an infant spent at >30° flexion was calculated for all products.

Figure 1:

Figure 1:

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(A) Experimental setup with motion capture markers, EMG, and SpO2 monitor. (B) Motion capture marker and EMG placement. For the motion capture markers, 6.4mm three-marker clusters were placed on the head as well as the anterior and posterior trunk and pelvis. 9.5mm individual markers were placed on the shoulders and the anterior and posterior superior iliac spine (ASIS and PSIS, respectively). The EMG placement is on the abdominal muscles (AB), quadriceps (QUAD), cervical paraspinals (CP), and erector spinae (ES). (C) Schematics defining Head-Neck, Trunk, and Torso-Pelvis flexion and extension where flexion is shown in blue and extension is shown in green.

2.2.4. Electromyography

Surface EMG electrodes (Delsys, Natick, MA; 2000 Hz) recorded bilateral muscle activity from the erector spinae, cervical paraspinals, abdominal muscles, and the quadriceps (Figure 1B). These muscle groups were chosen because the cervical paraspinals, abdominals and erector spinae muscle groups are known to influence spinal flexion and extension [13,14,32], and the quadricep muscles are active when infants use their feet to push against surfaces. Each sensor was placed by the same researchers and wrapped in flexible self-adhesive bandage to ensure placement throughout testing and consistency between participants.

The EMG data was extracted and truncated to include the same 60 seconds of data from each of the conditions as used in the kinematic analysis. For each infant, the playmat condition was used to normalize the data for the corresponding positioning (supine or prone). This condition was used for normalization because maximal voluntary isometric contractions, which are usually used to normalize EMG data in children or adults, are impossible to obtain in an infant population [9,13,14]. Using MATLAB, raw EMG waveforms were assessed with a power spectral analysis to remove corrupted or missing data. The data was then filtered and processed to obtain the EMG envelope for each trial [9,13,14,33]. The interquartile range method was implemented to remove sporadic errors [15,33] before the mean was taking for each testing condition. For normalization, the data was expressed as a percentage relative to the playmat condition, which was set at 100%. Values exceeding 100% indicate greater muscle activation compared to the playmat, while values below 100% indicate lesser muscle activation.

2.2.5. Oxygen Saturation

To ensure safety, a pulse oximeter with a toe sensor was used to monitor the infant’s oxygen saturation (SpO2) levels (Nellcor PM10N, Figure 1A). Based on the clinical safety standard for SpO2, if the infants SpO2 reading was <90% for more than 10 seconds indicating moderate desaturation [34], the trial was ended.

2.2.6. Statistical Analyses

Statistical analyses were performed using the SPSS statical package (SPSS Inc., version 26; Chicago, IL). The muscle groups and sagittal plane angles were compared between the playmat condition (supine and prone), and the four different infant products were compared using paired t-test (p<0.05). After data collection was completed, we also decided to compare young (<4 months, N=7) and older (≥4 months, N=6) infants to determine if age influenced muscle activity or body position. This was done using the same paired t-tests comparing the playmat to each product for the respective age group, muscle group, and sagittal plane angles.

3. RESULTS

A two-sample a priori power analysis was performed based on normalized mean electromyography data from a previously published study on healthy infants in different positions [9] and determined that n=9 participants per test condition would be sufficient to produce significant results (1−β=0.8; α=0.05). Therefore, we enrolled infants until this minimum threshold of n=9 per testing condition was fulfilled resulting in thirteen healthy infants (4.2±1.4 months; 7M/6F). No infants were excluded due to their Ages and Stages Questionnaire results. All testing conditions had thirteen trials except for the prone position for the carrier (n=12) and the swing (n=9) where some trials were not completed due to an infant crying or at risk of falling out of the product. Table 2 shows the demographic and developmental screening results of our participants. The supine trunk flexion in the carrier could not be calculated as the ASIS was occluded due to the product configuration and thus was excluded from that analysis.

TABLE 2:

Demographics of the thirteen infants included for analysis

Mean STD Minimum Maximum
Age (Months) 4.2 1.4 2.1 7.3
Gestational Age at Birth (Weeks) 38.9 1.3 37.0 41.6
Height (cm) 60.9 3.7 54.0 66.0
Mass (kg) 6.8 0.8 5.9 8.7
Ages & Stages Gross Motor 47.7 11.5 25.0 60.0
Ages & Stages Fine Motor 47.7 9.7 30.0 60.0
Male Female
Sex 7 6
White Hispanic
Ethnicity 11 2
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3.1. Supine Positioning

Infants in all products exhibited higher head-neck, torso-pelvis, and trunk flexion on average compared to the playmat (Figure 2A). For head-neck flexion, the carrier resulted in the highest change compared to the playmat (21°; p=0.001), followed by the rocker (14°; p=0.001) and swing (5°). For trunk flexion, the bouncer (23°; p<0.005), rocker (23°; p<0.005), and swing (26°; p<0.005) resulted in significantly higher values compared to the playmat. The trunk flexion of infants in the carrier could not be calculated due to the ASIS marker’s being occluded by the product design. The torso-pelvis flexion was the highest on average for infants in the swing at 23° (p=0.004) more than the playmat, followed by the carrier (19°; p=0.005), rocker (13°; p=0.007), and then the bouncer (5°). Additionally, infants spent on average only 1% of the trial in head-neck flexion >30° on the playmat, and 33% of the trial in the carrier condition. For the other products infants spent between 7–10% of the time in head-neck flexion >30°, though these results varied between participants. For the carrier, six infants (3.3±1.0 months) never experience head-neck flexion above 30°, while three infants (4.6±1.0 months) spent the entire 60 seconds above the threshold.

Figure 2:

Figure 2:

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Figures A and B (top) represent our kinematic results where excursion plots are used to show the range of movements (minimum to maximum) that infants exhibited in each condition. Figures C and D (bottom) represent the EMG results shown as bar graphs depicting the mean muscle activation and the error bars represent the standard error of the data. An asterisk (*) represents significance when using a paired t-test to compare the means of each product to the mean of the playmat (p<0.05).

All products exhibited unique combinations of muscle activation levels in the supine position (Figure 2C). The carrier resulted in lower CP (M=97.0%, SE=4.4%), ES (M=74.7%, SE=0.8%; p=0.009), and QUAD (M=90.3%, SE=1.2%) muscle activation and slightly higher AB (M=110.1%, SE=4.5%) muscle activation when compared to the playmat. The bouncer resulted in all muscle groups exhibiting higher activation levels compared to the playmat with the AB (M=160.4%, SE=12.7%; p=0.003) and QUAD (M=139.1%, SE=12.5%; p=0.045) muscle activation being significant. While in the rocker, infants exhibited higher CP (M=145.1%, SE=32.1%) and QUAD (M=110.1%, SE=3.3%) with lower ES (M=77.0%, SE=5.0%; p=0.017) and AB (M=94.8%, SE=5.9%) muscle activation compared to the playmat. In the swing, all muscle activations were higher than what was exhibited on the playmat ranging from 130% to 145% with the AB (M=140.5%, SE=12.2%; p=0.009) and QUAD (M=144.7%, SE=4.2%; p=0.002) being significant.

3.2. Prone Positioning

Opposite of the supine position, infants exhibited more extension on average in all products compared to the playmat (Figure 2B). For the trunk extension, all products (carrier: p<0.005, bouncer: p<0.005, rocker: p<0.005, swing: p=0.009) resulted in significantly more extension compared to the playmat with mean extension angles ranging from 14° to 23° (extension values are shown as negative numbers on the graph). Similarly, all infants experienced significantly more torso-pelvis extension in the carrier (26°; p=0.003), bouncer (16°; p=0.007), and rocker (17°; p=0.001). Infants also experienced slightly more torso-pelvis extension in the swing (9°) although the results were not statistically significant.

The prone position resulted in a large variance in muscle activation, particularly for the AB muscles, compared to the supine position (Figure 2D). For the CP muscle activation, the carrier (M=67.6%, SE=15.1%; p=0.014), bouncer (M=75.5%, SE=6.7%; p=0.006), and swing (M=67.1%, SE=2.5%; p<0.001) resulted in significantly lower muscle activation when compared to the playmat. Infants in the rocker exhibited the highest muscle activation at 112% (SE=16.8%). For the ES muscles, the carrier (M=88.3%, SE=7.2%), bouncer (M=73.3%, SE=2.0%; p<0.001), and rocker (M=68.3%, SE=8.8%; p=0.001) were lower and the swing (M=143.1%, SE=41.0%) was higher when compared to the playmat. The rocker exhibited the lowest ES muscle activation at only 68.3% of the playmat and the swing was the highest at 143.1%. When looking at the AB muscle activation, infants in the carrier (M=218.9%, SE=62.9%; p=0.036), bouncer (M=158.8%, SE=6.7%; p=0.001), and rocker (M=119.1%, SE=1.9%; p=0.024) had significantly higher values. Finally, all products had lower QUAD muscle activation compared to the playmat, but only the rocker was significantly different (M=74.6%, SE=4.5%; p=0.001).

3.3. Younger vs Older Infants

3.3.1. Age Comparison for Supine Positioning

Although our study was not powered to examine differences between younger and older infants, we noticed some interesting trends and therefore examined them separately. The kinematic results showed more statistically significant differences for older infants compared to younger infants (Figure 3A & B). For younger infants, trunk flexion was significantly higher compared to the playmat for the bouncer (p<0.001), rocker (p<0.001), and swing (p=0.002) and torso-pelvis flexion was significantly higher for the carrier (p=0.017). Older infants exhibited significantly higher head-neck flexion compared to the playmat in the carrier (p=0.011) and rocker (p=0.009), trunk flexion in the bouncer (p=0.013) and swing (p=0.013), and torso-pelvis flexion in all four products examined (carrier: p=0.004, bouncer: p=0.015, rocker: p=0.015, swing: p=0.014).

Figure 3:

Figure 3:

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Supine positioning comparing young (<4 months) and old (≥4 months) infants. Figures A and B (top) represent our kinematic results where excursion plots are used to show the range of movements (minimum to maximum) that infants exhibited in each condition. Figures C and D (bottom) represent the EMG results shown as bar graphs depicting the mean muscle activation and the error bars represent the standard error of the data. An asterisk (*) represents significance when using a paired t-test to compare the means of each product to the mean of the playmat (p<0.05).

The EMG results showed fewer statistically significant differences for both the younger and older infants (Figure 3C & D). However, when looking at trends, younger infants exhibited higher CP muscle activation and lower AB and QUAD muscle activation when compared to the older infants. For the younger infants, the muscle activation compared to the playmat was significantly higher for the CP in the bouncer (p=0.049) and significantly lower for the ES in the carrier (p=0.047). For older infants, the muscle activation compared to the playmat was significantly lower for the CP in the carrier (p=0.013), and significantly higher for the AB and QUAD in the bouncer (p=0.014 and p=0.043, respectively) and swing (p=0.002 and p<0.001, respectively).

3.3.2. Age Comparison for Prone Positioning

Similar to our results in the supine position, we see more statistically significant differences for the older infants compared to the younger infants (Figure 4A & B). While trunk extension for the younger infants was significantly different for all products examined compared to the playmat (carrier: p=0.005, bouncer: p=0.004, rocker: p=0.001, swing: p=0.029), torso-pelvis flexion resulted in more significant differences. For the older infants, significantly different trunk and torso-pelvis extension were exhibited in the carrier (p=0.014 and p=0.009, respectively), bouncer (p=0.041 and p=0.037, respectively), and rocker (p=0.003 and p=0.006, respectively) compared to the playmat.

Figure 4:

Figure 4:

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Prone positioning comparing young (<4 months) and old (≥4 months) infants. Figures A and B (top) represent our kinematic results where excursion plots are used to show the range of movements (minimum to maximum) that infants exhibited in each condition. Figures C and D (bottom) represent the EMG results shown as bar graphs depicting the mean muscle activation and the error bars represent the standard error of the data. An asterisk (*) represents significance when using a paired t-test to compare the means of each product to the mean of the playmat (p<0.05).

In the prone position, fewer differences in trends can be seen when comparing the younger and older infants (Figure 4C & D). For the younger infants, muscle activation compared to the playmat was significantly lower for the CP in the carrier (p=0.034), bouncer (p=0.03), and swing (p=0.01), the ES in the bouncer (p=0.01) and rocker (p=0.017), and the QUAD in the rocker (p=0.001). The AB muscle activation was significantly higher compared to the playmat in the bouncer (p=0.009). For the older infants, muscle activation was significantly lower compared to the playmat for the CP in the swing (p=0.004) and the ES in the bouncer (p=0.015) and rocker (p=0.022). The AB muscle activation was significantly higher for the rocker (p=0.036) compared to the playmat.

4. DISCUSSION

The purpose of this study was to assess infant 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. We hypothesized that (1) infants would exhibit higher head-neck, torso-pelvis, and trunk flexion/extension in all seated products compared to the flat surface and (2) infants would have higher erector spinae and lower abdominal muscle activation at higher inclines in the supine position and the converse in the prone position.

4.1. Body Position

Head-neck flexion and extension magnitudes were higher in all seated products compared to the playmat in both the supine and prone positioning (Figure 2A & B). On the playmat, infants exhibited a large range of motion as they were not confined by the product. However, in the products, infants maintained a head-neck position of >15°. A previous study found that neck flexion angles of just 15° to 30° increases airway collapsibility which can increase exhalation speed and decrease lung capacity on inhale [35]. Other research found that head-neck flexion angles of 45° were required to significantly inhibit breathing capabilities where airflow interruption increased by 34.5% and severe airflow interruption increased by 17.6% compared to the neutral or flat surface position [23]. While the infants in our study did not obtain this 45° head-neck flexion, they were placed in an ideal position awake and were monitored for a short amount of time. Infants in products for longer periods of time, or while asleep, may reach higher head-neck flexion and increase their risk for breathing related injuries due to the slouched body position that can occur when unrestrained. This risk may also be exacerbated during sleep as rapid eye movement (REM) sleep in children has been shown to increase upper airway collapsibility, reduce tone of the pharyngeal muscles, and decrease arousal response to hypoxia compared to non-REM sleep [36,37]. In the prone position, younger infants have a decreased ability to avoid suffocation [38,39]. Only 40% of infants switch from nasal breathing, the preferred method in infants, to mouth breathing when nasal occlusion occurs [40]. During REM sleep, this switch is even less effective [41] meaning that occlusion of the nose when the mouth is fully or partially covered in seated products still presents a suffocation risk.

Trunk flexion is dependent on the shape of the product in the longitudinal direction. In our study, the seated products (bouncer, rocker, and swing) increased infants’ trunk flexion between 23° and 27° on average compared to the playmat in the supine position (Figure 2A). Researchers have shown that slouched position or higher trunk flexion can significantly decrease the forced vital capacity or the ability to achieve a full exhale compared to a normal sitting position [24–26] and that extreme spinal flexion, similar to a slumped posture, affects respiratory system compliance and chest wall motion during breathing compared to a normal sitting position [27]. Although some of these studies were conducted on adults, infants are more vulnerable and have a less robust respiratory system. Thus, when infant’s exhibit higher trunk flexion in certain products, their ability breathe may be negatively impacted, especially over prolonged periods of time.

All products exhibited higher torso-pelvis flexion in the supine position with the carrier, rocker, and swing exhibiting significant differences (p<0.05) when compared to the playmat (Figure 2A). Roll initiation for infants is largely dependent on torso-pelvis flexion [42,43]. Previous research shows that an infant’s initial body position in a product sometimes allows for easier achievement of supine-to-prone rolling [13–15]. More specifically, if a seated product allows for higher initial torso-pelvis flexion when lying supine, less movement is required to initiate the roll. Thus, products like the carrier, rocker, and swing, which subject infants to a higher torso-pelvis flexion compared to the flat surface, may facilitate rolling within or out of the product, one of the main causes of injury involving seated products. The higher head-neck, trunk, and torso-pelvis flexion/extension on average for all seated products compared to the playmat, confirms our first hypothesis.

4.2. Muscle Utilization

The muscle activation levels in all the seated products varied based on position and muscle group, with no clear trend (Figure 2C & D). Previous research from our lab found that during prone lying on an increasing incline the muscle activity of the erector spinae decreased while the abdominal muscles increased [14]. Additionally, during supine-to-prone rolling the converse occurred while in a device similar to commercial infant products [15]. In this study, only the abdominal muscles in the prone positioning followed this trend where all products exhibited abdominal muscle activation above that of the playmat, partially confirming our second hypothesis. The significantly higher muscle activation in the seated products (carrier, bouncer, and rocker) exhibited by infants could lead to muscle fatigue occurring more quickly, leading to an increased risk of suffocation if they cannot maneuver into a safe position. Abdominal muscle activity has also been found to be closely related to changes in intra-abdominal pressure and diaphragm function, [44,45] as well as aid against obstructed airways through forced exhalation [32]. With the significant increase in muscle activation levels, particularly the abdominal muscles, in both the supine and prone position, these products may affect an infant’s ability to breathe. Since fatigue of the abdominal muscles can lead to apnea episodes and decreased lung volume [45], products requiring high abdominal muscle activation could restrict rib cage expansion and reduce lung volumes, increasing the risk of positional asphyxiation events.

During prone lying, infants’ cervical paraspinal muscle activity was significantly lower for the carrier, bouncer and swing compared to the playmat (Figure 2D). Consistent with our contact data, some infants did not hold their heads up at all during prone lying and instead rested their face on the product during the entire collection. Younger infants (~2 months) have very little muscle strength and head-neck control [46], further increasing the likelihood of nose and/or mouth interaction with these products in the prone position. This can be exacerbated during sleep due to an infants’ preferred method of nasal breathing [40]. Some trends in the muscle utilization data indicate that the shape of the product is the main contributor to the body position of the infants. Younger infants who have minimal motor control or muscle strength are more at risk in products that induce higher head-neck and trunk flexion—body positions which can inhibit normal breathing, as they may not be able to maneuver into a more favorable position.

4.3. Young vs Older Infants

While the body positions analyzed were similar between older and younger infants, older infants in the supine position exhibited significantly higher abdominal muscle activations in the bouncer and swing, approximately twice that exhibited on the flat surface (Figure 3C & D). The same was not true for younger infants where abdominal muscle activity was not significantly different or nearly as high. This indicates that younger infants are relying on the products to maintain their body posture rather than utilizing their own muscles, meaning the shape of the product is the main contributor to the body position of these infants. With less control, younger infants may not be able to move from a position that inhibits breathing the same way older infants can. Additionally, our study showed higher cervical paraspinal muscle activation for younger infants compared to older infants. This increase in neck muscle activation may increase a younger infant’s risk of early fatigue, resulting in an inability to account for breathing related challenges due to position. Younger infants may not have the motor control, muscle strength, or experience to move into a more favorable position or overcome the increased work of breathing due to the compromised body position forced by the seated products. However, the older infants activating their muscles indicates that above a certain age threshold when infants have sufficient strength and motor control, seated products may be beneficial to encourage muscle use and provide infants a variety of body positions and opportunities to move throughout the day, which helps avoid gross motor milestone delays, head molding, shoulder retraction and torticollis [47,48].

4.4. Considerations

Bony landmarks for marker placements were sometimes estimated since bones are not fully developed in infants. A method of using local coordinate systems on each body segment was applied to minimize the errors caused by lack of clear anatomical landmarks [30]. Furthermore, surface EMG sensors have inherent limitations, however, our experimental design enabled infants to act as their own controls, mitigating the variability typically associated with EMG placement [13,14]. Sixty-second trials where infants were placed in the ideal positioning while fully awake, may not perfectly represent daily life or unsafe situations. In our study, infants were also placed in an ideal position unrestrained for a short period of time, however, over time, infants would slouch more in the product while unrestrained exacerbating our results. Future studies should consider how other body positions and individual product characteristics, such as firmness, curvature, and conformity impact an infant’s biomechanics and ability to breathe. Lastly, this study did not take into consideration CO2 rebreathing and sleep surface permeability which are important to understand and inform safety considerations for infant environments and products.

4.5. Conclusion

This study is the first to investigate different seated infant products in relation to body position and muscle utilization. During supine positioning, infants exhibited increased head-neck and trunk flexion compared to the playmat, a firm flat surface. Both head-neck flexion and trunk flexion can inhibit normal breathing, which leads to increased work especially in younger infants. In prone, the high abdominal muscle activation compared to the playmat indicates that infants will fatigue faster when prone in seated products. In combination with potential nose/mouth interactions, infants may be unable to move into a position which enables free airflow, increasing suffocation risk. Additionally, younger infants are subject to the design of the seated product to maintain their posture rather than their own muscles, meaning they may not have the strength to overcome a compromised body position that inhibits their breathing capabilities. However, offering infants a variety of body position and movement opportunities throughout the day is beneficial to avoid gross motor milestone delays, head molding, shoulder retraction, and torticollis [47,48]. Seated products offer some variety of body position for infants compared to a flat surface and if used, should only be used by older infants for short periods of time with.

Highlights.

Seated products impact body position, muscle use, and breathing hazards for infants

Product designs increase head-neck and trunk flexion of infants while supine

Abdominal muscles fatigue faster after rolling (while prone) in seated products

Younger infants are subject to the design of the products to maintain their posture

Acknowledgements

This research was part of a larger study funded by the U.S. Consumer Product Safety Commission (CPSC, Commission) under contract number 61320620D00002/61320621F1014. It has not been reviewed or approved by, and may not reflect the views of, the Commission.

We acknowledge support from the Institutional Development Awards (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under Grant #P20GM148321. We also acknowledge support from the Boise State University FaCT Core Facility.

Footnotes

Declaration of competing interests

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.

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