Exploring infant hip position and muscle activity in common baby gear and orthopedic devices

Abstract

Infant positioning in daily life may affect hip development. While neonatal animal studies indicate detrimental relationships between inactive lower extremities and hip development and dysplasia, no research has explored infant hip biomechanics experimentally. This study evaluated hip joint position and lower extremity muscle activity of healthy infants in common body positions, baby gear, and orthopedic devices used to treat hip dysplasia (the Pavlik harness and the Rhino cruiserabduction brace). Surface electromyography(EMG) and marker-based motion capture recorded lower extremity muscle activity and kinematics of 22 healthy full-term infants (4.2±1.6 months, 13M/9F) during five conditions: Pavlik harness, Rhino brace, inward-facing soft-structured baby carrier, held in arms facing inwards, and a standard car seat. Mean filtered EMG signal, time when muscles were active, and hip position (angles) were calculated. Compared to the Pavlik harness, infants exhibited similar adductor activity (but lower hamstring and gluteus maximus activity) in the Rhino abduction brace, similar adductor and gluteus maximus activity (but lower quadriceps and hamstring activity) in the baby carrier, similar but highly variable muscle activity in-arms, and significantly lower muscle activity in the car seat. Hip position was similar between the baby carrier and the Pavlik harness. This novel infant biomechanics study illustrates the potential benefits of using inward-facing soft-structured baby carriers for healthy hip development and highlights the potential negative impact of using supine-lying container-type devices such as car seats for prolonged periods of time. Further study is needed to understand the full picture of how body position impacts infant musculoskeletal development.

Introduction

Infant positioning in daily life, particularly in commercially available baby gear, is understudied. In early infancy, when babies experience rapid development, it is vital to understand the effect of daily body positioning environments on healthy musculoskeletal development. In this regard, a large body of literature has been dedicated to studying infant development from the context of diseases that may reduce the quality of life in adulthood.

The developing hip, particularly, has been studied in relation to developmental dysplasia of the hip (DDH), which is the most common congenital malformation present in 14% of infants, characterized by a hip joint that exhibits excessive laxity, resulting in instability or dislocation¹. Early diagnosis and prompt treatment of DDH are critical, since undiagnosed or improperly treated DDH can result in abnormal gait and function, low strength, and early degeneration of the hip and knee, including a doubled risk of early onset osteoarthritis and hip arthroplasty before age 40². In mild to moderate DDH, the Pavlik harness, a soft orthopedic harness with a 70-year-old design, non-surgically treats DDH in infants younger than 6 months old by holding the hips in flexion and abduction³. Additionally, a polypropylene hip abduction brace known as the Rhino cruiser is often used as a follow-up to unresolved instability in infants treated with the Pavlik harness. While the Pavlik harness reduces unstable hips to avoid surgical treatment with a 63%–93% success rate^4,5, the prescribed duration of treatment and weaning regimen varies widely amongst clinicians^6,7, and treatment success is evaluated solely upon ultrasound-guided morphological parameters, which may not effectively identify incomplete treatment or residual dysplasia that cause lifelong problems⁸. The discrepancy in treatment duration and weaning regimens is particularly concerning, since prolonged harness wear has been associated with reduced motor performance⁹, avascular necrosis¹⁰, and femoral nerve palsy¹¹. While the Pavlik harness treatment is the gold standard for DDH treatment, the mechanism through which the harness reduces unstable hips has only been explored through computational models^12,13, indicating a positive contribution of adductor muscles, but no experimental data exists to support these findings. Experimental data of infants in and out of the Pavlik harness and Rhino cruiser abduction brace has the potential to provide a more robust understanding of the biomechanics of the developing infant hip. Additionally, the experimental data may provide crucial insight into the short-term physiological effects of the restraints imposed by the Pavlik harness and the Rhino cruiser, particularly because full clinical trials of their usage have not been performed (to the best of our knowledge).

Animal model studies on hip development have indicated the effect of hip positioning and mobilization on the development and maturation of the hip. Studies in chick embryos showed that reduced movement and muscle activity detrimentally effects embryonic hip joint development, and may lead to early onset DDH¹⁴. Additionally, recent murine studies suggest that decreased gluteal and quadriceps muscle loading resulted in altered postnatal bone and joint shape and growth, meaning that inactive lower extremity muscles may contribute to the development or aggravation of DDH in neonates^15,16. Human infants, particularly in the first 6 months of life, are at a risk of developing or exacerbating DDH if their hips are immobilized for prolonged periods of time throughtight straight-legswaddling¹⁷, a finding which has been supported in murine models¹⁸. Yet,the role of inactive or active muscles to promote hip development in human infants has not been studied. Even in treating DDH with the Pavlik harness, the only contributive body position discussed in relation to closed reduction is passive supine lying^19,20, and the roles of active musculature and positioning²¹ are relatively unexplored.

Infants in the Western culture are increasingly spending a majority of their time in commercially available baby gear which have historically been designed solely based on rudimentary infant anthropometric data^22,23, without any regard to musculoskeletal development implications. Researchers have reported on the effects of infants spending excessive daily time in supine-lying car seats, finding adverse outcomes in the occurrence of deformational plagiocephaly²⁴, on oxygen saturation levels²⁵, and on leg movement²⁶. Conversely, babywearing (the act of carrying infants in a carrier or wrap upright on the caregiver’s body) has been shown to offer many emotional, physiological, and physical benefits to the infant. Babies carried by their caregivers in soft structured baby carriers exhibit decreased crying²⁷, an increased security of attachment²⁸, and responsive breastfeeding²⁹. Inward facing skin-to-skin contact of newborns (“kangaroo care”) through in-arms holding or aided by babywearing has been shown to decrease infant mortality rates for pre-term infants³⁰, increase breastfeeding success and duration³¹, reduce agitation³² and sleep apnea³³, and improve infant cardiorespiratory³⁴ and temperature regulation³⁵. An upright carrying position has also been suggested as a preventative measure against ear infections and an alleviation strategy for gastroesophageal reflux disease³⁶. Furthermore, it is theorized that inward-facing babywearing may lead to increased muscle activity of the lower limbs as the infant clings to the caregiver’s body³⁷. Our recent experimental infant biomechanics study has indicated that upright, inward-facing babywearing may positively influence neck muscle development³⁸. Outward or forward-facing baby carriers may not offer the same musculoskeletal benefits since they may place the infant spine in an extended position (instead of the desired C-shape) and extend and adduct the hip joint (instead of the desired M-position)³⁹.

Despite the growing body of evidence indicating that infant positioning in daily life and in commercially available baby gear (including upright caregiver-worn baby carriers and supine lying containment devices) may affect hip joint development and maturation, no research group has explored these effects from a biomechanics standpoint. The objective of this research was to evaluate hip joint position and lower extremity muscle activity in healthy infants in a common position (in caregiver’s arms) and in baby gear (inward-facing soft structured baby carrier and standard car seat) and compare those findings to those in orthopedic devices used to treat hip dysplasia (Pavlik harness and Rhino cruiser). We hypothesized that hip joint position and lower extremity muscle activity would be significantly different for all positions and baby gear, compared to the Pavlik harness.

Methods

Twenty-two infants between two to six months of age participated in this IRB-approved study. Infants included in this study were born full-term (>37 weeks of gestation), of normal birth weight (> 5 lbs. 8 oz.), between 5–95^th percentile in height and weight (CDC), and at least 1 week past vaccination and/or sickness. Participants were recruited from recruitment flyers and by word of mouth. This participant cohort is the same infant cohort from our previous publication exploring neck and back muscle activity³⁸. All tests from both studies were conducted on the same day, in the same session, and the task order for all tasks was randomized.

Surface electromyography (EMG) electrodes (Delsys Inc., Natick, MA) recorded the bilateral muscle activity (1000 Hz) of adductors, quadriceps, hamstrings (biceps femoris), and gluteus maximus muscle groups. Marker-based motion capture recorded lower extremity movement through reflective markers placed bilaterally (Figure 1) on the anterior superior iliac spine (ASIS), posterior superior iliac spine (PSIS), greater trochanter, and medial and lateral epicondyles of the knee. Additionally,three-marker rigid bodies were placed on the anterior and posterior of the pelvis, and bilaterally on the lateral aspect of each thigh.

Figure 1:

(A) Infant fitted with bilateral EMG electrodes and motion capture markers in (B) Pavlik harness, (C) Rhino abduction brace, (D) Inward facing soft-structured baby carrier, (E) Caregiver’s arms, and (F) Car seat.

Infants participated in five 60-second positional tasks: lying supine on a mat while wearing a Pavlik harness, lying supine on a mat while wearing a Rhino hip abduction brace, held upright in a soft-structured inward-facing baby carrier (Boba, Inc.; Boulder, CO), held in-arms by caregiver (facing inwards), and buckled in a standard 5-point harness car seat (Evenflo Company Inc.; Miamisburg, OH) as shown in Figure 1. Infants’ chest circumference and hip girth were measured prior to fitting to ensure that the infants would be fitted with appropriately sized Pavlik harness and Rhino cruiser. For each infant, the Pavlik harness and the Rhino cruiser were fitted by authors SFS and EMM, who were trained by a pediatric orthopedic surgeon (author BR) and by instructional videos and pamphlets from the device manufacturers. The task order was randomized, and 30 seconds of usable data was extracted for each task. The data was considered valid and usable if the infants were awake, compliant, and not crying.

To avoid possible motion obstructions from the connecting wires between the EMG sensor enclosure and the EMG electrode head, the infants’ thighs were wrapped with soft cohesive self-adherent wrapping tape. Raw EMG waveforms were assessed for corrupted or missing data using power spectral analysis⁴⁰, and such data was excluded from analysis.

The raw EMG waveforms were band-pass filtered using a 4^th order Butterworth filter between 35 Hz and 500 Hz, to reduce contamination from movement artefacts, electrocardiogram signals⁴¹, and high frequency noise⁴². Additionally, to eliminate the effects of 60 Hz signal interference from nearby electronic sources, EMG waveforms were notch-filtered between 59.9 Hz and 60.1 Hz using a 4^th order Butterworth filter. EMG waveforms were then full-wave rectified, demeaned, and subjected to a low-pass 4^th order Butterworth filter with a cutoff frequency of 50 Hz to obtain the EMG envelope⁴³. The filtered and rectified EMG data was normalized to the Pavlik harness condition using custom MATLAB code (The MathWorks, Natick, NJ, USA). Mean EMG signal and the percent time that muscles were active above the Pavlik harness threshold (two standard deviations above the mean EMG signal of Pavlik harness) were calculated.

Due to the paired samples design in the intended analysis of the collected data, the differences between the EMG outcome variables at the Pavlik harness position and all other positions were evaluated for normality using Q-Q plots and Shapiro-Wilk tests. Since most of these differences did not demonstrate normality on the Q-Q plots and the Shapiro-Wilk test statistic (i.e. p < 0.05), the Friedman test was then conducted on the EMG data from all positions.

The dependent variables in the EMG data were the mean EMG signal and the percent time the muscles were active above the Pavlik harness threshold. The independent variable was infant position, which consisted of five groups: Pavlik harness, Rhino abduction brace, baby carrier,in-arms, and car seat. If the Friedman test was significant (i.e. an overall statistically significant difference was observed between the mean ranks of the EMG data from all positions), post-hoc pairwise Wilcoxon signed rank tests were used to compare the EMG data from all positions to the Pavlik harness position with a Bonferroni correction, resulting in a significance level of p < 0.0125 (for four pairs of comparisons). For each comparison, the effect size (d) was calculated as the Z-statistic divided by the square root of the sum of the number of observations in the two compared positions⁴⁴. Effect size was considered small if 0.1<d<0.3, medium if 0.3<d<0.5, and large if d>0.5⁴⁵. IBM SPSS Statistics for Windows, version 25 (IBM Corp., Armonk, NY, USA) was used for all statistical analyses.

Single frame marker data (i.e. snapshots representative of infant position) were extracted from the Pavlik harness and the baby carrier conditions. Local coordinate systems of the hip and thigh were formulated using the markers on those segments. An XYZ Cardan rotation matrix representing the relative orientation between the hip (i, j, k) and thigh (I, J, K) local coordinate systems was used to calculate flexion (α), abduction (β), and external rotation (γ).

$$R=\left[\begin{array}{ccc}i.I& j.I& k.I\\ i.J& j.J& k.J\\ i.K& j.K& k.K\end{array}\right]=\left[\begin{array}{ccc}c\alpha c\beta & c\alpha s\beta s\gamma -s\alpha c\gamma & c\alpha s\beta c\gamma +s\alpha s\gamma \\ s\alpha c\beta & s\alpha s\beta s\gamma +c\alpha c\gamma & s\alpha s\beta c\gamma -c\alpha s\gamma \\ -s\beta & c\beta s\gamma & c\beta c\gamma \end{array}\right]$$

Results

Recorded and reported measurements of participants’ age, length, and weight are presented in Table 1.

Table 1:

Participant demographics and measurements: Mean (SD), [Range]

Sex
Male	13
Female	9
Measurements
Age at testing (months)	4.2 (1.6), [2.0 – 6.9]
Length at testing (cm)	60.7 (5.1), [50.8 – 71.1]
Weight at testing (kg)	6.6 (1.5), [3.6 – 10.0]
Length at birth (cm)	50.8 (3.0), [45.7 – 55.9]
Weight at birth (kg)	3.5 (0.5), [2.5 – 4.2]

Usable EMG data, after assessment for corrupted or missing data, was extracted for 22 participants in the Pavlik harness, 13 participants in the Rhino abduction brace, 20 participants in the baby carrier, 22 participants held by caregivers in-arms, and 21 participants in the car seat. For all muscles, infants exhibited significantly lower muscle activity in the car seat compared to the Pavlik harness (p<0.0125), both in mean muscle activity and in the percentage of active muscle time. Muscle activity of infants in the Rhino abduction brace was similar to the Pavlik harness, except in percentage of active muscle time of the gluteus maximus (p<0.05). However, it was observed that the hamstrings and gluteus maximus muscle groups generally exhibited lower muscle activity in the Rhino (medium effect size), compared to the Pavlik harness, both in mean muscle activity and percentage of active muscle time. Mean muscle activity and percentage of active muscle time for the adductors and gluteus maximus muscle groups were not significantly different for infants in the baby carrier, compared to the Pavlik harness. However, infants in baby carriers (compared to the Pavlik harness) exhibited significantly lower mean muscle activity for the quadriceps (p<0.0125) and the hamstrings (p<0.05), and had significantly lower percentage active muscle time for the quadriceps (p<0.05). Muscle activity of infants carried in-arms did not significantly differ from the Pavlik harness, but were highly variable. (Figure 2, Tables 2 & 3).

Figure 2:

(Top) Mean muscle activity (%) at the different positions, normalized to the Pavlik harness, and (Bottom) Percentage of time that the muscles are active above threshold (mean + 2 S.D. of Pavlik harness) for the different positions; *p < 0.0125 and ‡p < 0.05 compared to the Pavlik harness.

Table 2:

Mean muscle activity (mV) at the different positions and statistical analyses (Friedman test reported in line with the dependent variable; pairwise comparisons in table).

Variable	Mean (95% Confidence Interval)	p	Z	Effect size ($Z/\sqrt{N}$)
Adductors, χ2(4) = 14.00, p = 0.007
Pavlik	4.98 (3.43 – 6.52)	-	-	-
Rhino	3.50 (2.05 – 4.95)	0.221	−1.223	0.24
Baby carrier	3.18 (2.16 – 4.20)	0.108	−1.605	0.25
In-arms	3.68 (2.10 – 5.26)	0.053	−1.932	0.29
Car seat	1.56 (1.07 – 2.05)	< 0.001	−3.569	0.56
Quadriceps,χ2(4) = 10.10, p = 0.039
Pavlik	1.83 (1.24 – 2.42)	-	-	-
Rhino	1.36 (0.85 – 1.88)	0.386	−0.866	0.19
Baby carrier	1.08 (0.85 – 1.30)	0.002	−3.07	0.51
In-arms	0.99 (0.74 – 1.23)	0.145	−1.459	0.24
Car seat	1.52 (1.07 – 1.97)	0.004	−2.896	0.51
Hamstrings,χ2(4) = 25.36, p < 0.001
Pavlik	2.59 (1.78 – 3.39)	-	-	-
Rhino	1.37 (0.94 – 1.81)	0.071	−1.804	0.37
Baby carrier	1.75 (1.28 – 2.23)	0.039	−2.068	0.37
In-arms	0.78 (0.62 – 0.93)	0.831	−0.213	0.04
Car seat	2.36 (1.90 – 2.82)	< 0.001	−3.636	0.61
Gluteus Maximus,χ2(4) = 18.80, p = 0.001
Pavlik	1.47 (1.03 – 1.91)	-	-	-
Rhino	0.96 (0.68 – 1.23)	0.050	−1.961	0.40
Baby carrier	1.36 (0.71 – 2.02)	0.309	−1.018	0.17
In-arms	0.80 (0.68 – 0.92)	0.936	−0.08	0.01
Car seat	1.45 (1.06 – 1.84)	0.006	−2.769	0.47

Table 3:

Active muscle time over threshold (%) at the different positions and statistical analyses (Friedman test reported in line with the dependent variable; pairwise comparisons in table).

Variable	Mean (95% Confidence Interval)	p	Z	Effect size ($Z/\sqrt{N}$)
Adductors, χ2(4) = 13.41, p = 0.009
Pavlik	4.57 (4.32 – 4.82)	-	-	-
Rhino	3.65 (0.14 – 7.17)	0.701	−0.384	0.08
Baby carrier	8.87 (0.07 – 17.66)	0.758	−0.308	0.05
In-arms	4.52 (1.37 – 7.66)	0.263	−1.12	0.17
Car seat	0.55 (−0.15 – 1.25)	< 0.001	−3.845	0.61
Quadriceps, χ2(4) = 13.90, p = 0.008
Pavlik	3.74 (2.97 – 4.52)	-	-	-
Rhino	2.09 (0.43 – 3.74)	0.646	−0.459	0.10
Baby carrier	1.93 (0.31 – 3.55)	0.039	−2.069	0.34
In-arms	3.92 (1.34 – 6.49)	0.616	−0.501	0.08
Car seat	1.09 (−0.02 – 2.20)	0.007	−2.689	0.48
Hamstrings, χ2(4) = 23.36, p < 0.001
Pavlik	3.97 (3.17 – 4.77)	-	-	-
Rhino	1.51 (0.16 – 2.85)	0.136	−1.49	0.30
Baby carrier	2.66 (0.47 – 4.85)	0.179	−1.344	0.24
In-arms	5.94 (1.34 – 10.54)	0.492	−0.686	0.12
Car seat	0.35 (−0.17 – 0.88)	< 0.001	−3.68	0.61
Gluteus Maximus, χ2(4) =13.33, p = 0.01
Pavlik	3.37 (2.74 – 4.01)	-	-	-
Rhino	1.05 (0.08 – 2.03)	0.060	−1.883	0.38
Baby carrier	2.63 (0.84 – 4.42)	0.523	−0.639	0.11
In-arms	6.59 (1.78 – 11.40)	0.136	−1.489	0.24
Car seat	2.00 (−0.53 – 4.52)	0.025	−2.249	0.39

Hip rotational angles calculated from motion capture data for the Pavlik harness indicate that the hip position in the baby carrier was not significantly different from the Pavlik harness (Figure 3, Table 4).

Figure 3:

Eulerian hip rotational angles calculated from motion capture data in the Pavlik harness and the baby carrier.

Table 4:

Eulerian hip rotational angles (degrees) in the Pavlik harness and baby carrier

Variable	Mean (95% Confidence Interval)	p
Flexion
Pavlik harness	86.47 (81.74 – 91.20)	-
Baby carrier	82.03 (72.15 – 91.91)	0.352
Abduction
Pavlik harness	81.57 (64.76 – 98.38)	-
Baby carrier	77.80 (66.76 – 88.85)	0.959
External Rotation
Pavlik harness	8.08 (4.08 – 12.09)	-
Baby carrier	4.29 (1.72 – 6.87)	0.163

Discussion

The purpose of this research was to evaluate hip joint position and lower extremity muscle activity in healthy infants in various positions andbaby gear, and compare the findings to those in orthopedic devices used to treat hip dysplasia. In agreement with previous computational models^12,13, the Pavlik harness and Rhino cruiser devices currently used to treat DDH in infants resulted in high levels of mean adductor muscle activity, supporting the use of the devices for reducing infant hips. Additionally, our hip angle values for the Pavlik harness show agreement with the optimal 90° flexion and 70° abduction position expected in the Pavlik harness⁴⁶. However, compared to the Pavlik harness, infants in the Rhino cruiser exhibited lower hamstring and gluteus maximus activity, which may be due to the lack of the foot straps present in the Pavlik harness and the less flexed and more restricted positioning of the Rhino due to its solid plastic frame. The inward facing soft-structured infant baby carrier resulted in similar hip position and lower-extremity muscle activity for the infants in this healthy cohort, indicating possible applications for infants with hip instability, and in promoting hip-healthy product design. Conversely, infants were significantly less active in the car seat, which, in conjunction with the findings of Jiang et. al indicating that car seats significantly reduce infant lower-extremity movement²⁶, may indicate that passive container-type of baby gear do not promote active lower-extremity muscles in healthy infants. Although carrying infants in arms (facing inwards) exhibited similar levels of muscle activity to the Pavlik harness, carrying position varied among parents observationally. While all parents were instructed to carry their infants facing inwards,some infants were carried with hips flexed and abducted and some were not.

There is a dearth of experimental human infant data that can contextualize our findings and explain the developmental role of hip position and hip muscle activity in early infancy. Some insights can be gained from recent animal model studies. A fetal chick study has established a relationship between sustained static muscle loading (i.e. immobilization) and abnormal spinal development⁴⁷, and an embryonic chick study has shown that reduced movement and muscle activity detrimentally effects embryonic hip joint development, and may lead to early onset DDH¹⁴. The latter was also established in recent murine models, where decreased gluteal and quadriceps muscle loading resulted in altered postnatal bone and joint shape and growth, meaning that inactive lower extremity muscles may contribute to the development or aggravation of DDH in neonates^15,16. Murine models also show that restrictive leg positioning, specifically straight-leg swaddling, increased the prevalence of DDH compared to no restriction on leg positioning¹⁸. This finding is also apparent in anthropologic studies of populations where tight, straight-leg swaddling has been widely practiced historically, with such swaddling being identified as a likely contributor to the high prevalence of DDH in these populations¹⁷. Conversely, there is some early clinical evidence linking hip mobilization and accelerated recovery of DDH in pre-term infants, indicating that active musculature and positioning may play a role in healthy hip development²¹.

As parents continue to use baby gear, further research is needed to understand the musculoskeletal and biomechanical impacts of infant positioning for both DDH and healthy infants. A majority of commercial and scientific research is focused on the immediate safety (or the lack thereof) of infants in commercially available baby gear, specifically in the areas of preventing bodily injury and suffocation^48–50. There is a dearth of experimental research investigating the potential long-term effects of baby gear on infant musculoskeletal development. Our recent experimental study found that infant spinal muscle activity was influenced by infant positioning in common positions and baby gear, and thatthe findings could be impactful for infants who spent an increasing amount of time in commercially available baby gear³⁸. While there is some preliminary work in developmental literature relating infant body movements to the use of baby gear²⁶, there has been no robust exploration of the impact that the use of commercial infant products has on infant biomechanics.

The results of our study highlight the need to start reevaluating how infants are positioned in the myriad of devices that are used to contain them on a daily basis. Particularly from a hip health perspective, if the Pavlik harness is considered to be the gold-standard of healthy hip position and lower-extremity muscle activity, it is imperative to consider curtailing non-essential use of supine-lying container-type devices such as the car seat, and start exploring the public health potential of carrying babies upright in hip-healthy positions⁵¹, both in arms and in baby carriers. The latter is particularly impactful in the case of infants who may have undiagnosed mild hip dysplasia. Interestingly, although the baby carrier may appear to restrict the movement of the baby, our results suggest that babies are still using their lower-extremity muscles while awake in the baby carrier by actively clinging to the caregiver, in agreement with Büschelberger’s theory³⁷. Unlike the supine-lying car seat, a baby carrier promotes direct contact with the caregiver who ismoving, which likely encourages babies to adapt to the movements, resulting in active muscles in spite of restricted movement. It is likely that the numerous emotional and psychological benefits of caregiver-infant contact and face-to-face interaction enabled by an inward-facing baby carrier or in-arms holding may also stimulate infant clinging, but future work is required to understand this complex relationship.

The study was not without its limitations. The infants enrolled in this study were recruited from the same geographic location within a 20-mile radius of Little Rock, Arkansas. Minority groups, while not excluded from our study sample, were underrepresented in the demographic context of the Little Rock area. The motion capture markers used in this study were placed by best anatomic approximation near the desired landmarks (ASIS, PSIS, greater trochanter, and medial and lateral epicondyles of the knee), since many of the landmarks had not fully formed in our infant cohort. The EMG data for each task had a duration of 30 seconds (isolated from the middle of approximately 60 second collections), and may not be ideally representative of real-life scenarios, particularly in relation to muscle activity characteristics in prolonged exposure to the positions or products evaluated in this study. The short collection duration for each position is due to the nature of infant experimental research, where a compromise between inclusion of several conditions and maintaining infant compliance over the whole data collection session (typically around 1 hour) was necessary. Our study used surface EMG electrodes to record muscle activity instead of more invasive fine-wire indwelling electrodes (which are considered to be more accurate in isolating muscle activity) due to considerations of infant pain and discomfort, difficulty in isolating individual muscles in infants, and the inherent practicality and time efficiency. Surface EMG, however, has been demonstrated to adequately represent muscle group activity when compared to fine-wire indwelling electrodes, provided all muscles in proximity have a common function and are not antagonistic⁵². While the Delsys Trigno EMG sensors used in this study were designed for use on adult humans, previous usage has been documented in infants of a similar age range⁵³. Additionally, the EMG sensors used on infants in this study were the Delsys Trigno Mini electrodes, which were designed to collect data from small muscles. Infant compliance was a challenge in this study, and data was only included in the analysis if the infant was awake, compliant, and not crying.

Our novel infant biomechanics study explored hip position and lower-extremity muscle activity of infants in common positions, commercially available baby gear, and orthopedic devices used to treat hip dysplasia. Overall, our findings provide a unique insight into the hip position and muscle activity characteristics of orthopedic devices that non-surgically treat developmental dysplasia of the hip, indicate the potential benefits of using inward-facing soft-structured baby carriers for healthy hip development, and highlight the potential negative impact of placing infants in container-type devices such as the car seat for prolonged periods of time. While the muscle activity of the musculature around the hip is critical to understand in the context of femoroacetabular development, infant positioning likely impacts the entire musculoskeletal system. Future studies should seek to explore other muscles and joints, and utilize longer testing durations in order to more comprehensively understand the biomechanical impact of spending long periods of time in different body positions, orthopaedic devices, and baby gear.