Babywearing, the practice of transporting an infant or child in a carrier on the body, provides physical closeness of an infant to the mother or caregiver. The infant-mother proximity has been shown to have emotional, physical, and physiological benefits for the baby in both animal and human species. Stemming from survival instincts, separation from the mother is viewed as a life-threatening situation for offspring, causing them significant distress.1 Maternal carrying of the offspring has been shown to trigger a calming response demonstrated by central, motor, and cardiac signals in distressed infants.2,3 In addition, skin-to-skin contact of premature human babies with their mothers or other caregivers, called kangaroo care, has been shown to decrease infant mortality rates, increase breastfeeding success, reduce agitation and sleep apnea, and improve infant respiratory and temperature regulation.4–8 Furthermore, a separate study has linked babywearing to a decreased likelihood of hip dysplasia in infants and young children.9
Babywearing allows the caregiver to be physically close to the baby while remaining hands-free. Physical closeness benefits the mother as well as the baby. Mothers across species show increased oxytocin levels, responsiveness, and breast milk production when in close proximity to their infants.3,10 Babywearing offers mothers the benefits associated with infant proximity without the energetic burden of using arms to carry.11 New research highlights the strength of the mother-infant bond attributed to utilizing an infant carrier for as little as 1 hour each day.12 Two recent studies have identified biomechanical advantages for the caregiver of holding an infant with an aid (sling or carrier) compared with in-arms carrying during walking.13,14 However, the postural, fatigue, and biomechanical effects of infant-carrying methods on the woman's body during other tasks of daily living such as standing have not been studied.
Low back pain (LBP) is of particular concern for this population. Females are at an especially increased risk for LBP during pregnancy and postpartum, with 45% to 72% reporting LBP during their first pregnancy, increasing to 85% to 94% with subsequent pregnancies.15–17 Standing for prolonged periods has been shown to induce LBP in 40% to 70% of otherwise healthy people,18–21 and transient LBP development during standing is predictive for future clinical LBP.22 People who develop LBP during standing have consistently demonstrated biomechanical differences from those who do not, indicating suboptimal control strategies that may be considered risk factors for LBP.18,19,23 Standing is a critical component of many tasks of daily living, which caregivers of small children must accomplish while holding or carrying the infant. One previous survey study has examined musculoskeletal disorders in relation to traditional infant-carrying methods of Nigerian mothers, finding that back-carrying results in the most LBP during daily life compared with other infant-carrying methods.24 However, the impact of different methods of infant carrying on standing-induced LBP or risk factors for LBP has not been investigated.
The goal of this study was to quantify the postural differences of females holding infants in arms and in baby carriers compared with an unloaded condition during prolonged standing. We hypothesized that carrying an infant in a baby carrier and in arms would cause differences in postural sway and asymmetrical loading parameters when each were compared with an unloaded condition. A secondary goal was to subgroup females into those who develop LBP during standing (PDs) and those who do not (NPDs) to investigate potential differences in biomechanical parameters and responses to baby-carrying conditions.
Ten healthy females of childbearing age (27.4 ± 4.1 years), healthy body mass index (21.5 ± 2.5 kg/m2), and no previous babywearing experience volunteered from the university population to participate in this institutional review board-approved study (University of Denver, Denver, Colorado). Exclusion criteria included pregnancy within the past 9 months, orthopedic- or musculoskeletal-related disorders, chronic LBP, and neurologic disorders; all participants verbally confirmed that no exclusion criteria were met prior to enrollment in the study. Before participating, subjects were informed of the testing procedures and provided written informed consent.
Two 40 × 40-cm force plates embedded into the floor (Bertec, Columbus, Ohio) were used to capture ground reaction forces under each foot during the testing at 1000 Hz, similar to previous prolonged standing biomechanical studies.23
All testing took place at the University of Denver, Denver, Colorado. Participants' age, height, and weight were recorded. The Chalder Fatigue Scale, a commonly used self-administered questionnaire with high reliability of 0.83 to 0.90,25,26 was administered to determine whether participants exhibited mental or physical fatigue.27
Participants performed 15-minute quiet standing trials where they were instructed to look forward and stand comfortably with 1 foot on each force plate in 3 conditions: holding nothing (unloaded), holding an infant mannequin (6-month-old; Dietz, Freiburg, Germany) in arms (arms), and holding an infant mannequin in the baby carrier (carrier) (Figure 1). Trials were of 15 minutes' duration, as this length of time was chosen because it has previously been shown to be a long enough time to predict back pain in prolonged standing studies.28 Participants were not told to maintain a specific foot position throughout the testing, but only to remain standing comfortably with 1 foot on each force plate to replicate a real-life scenario. For the arms condition, participants were instructed to hold the mannequin as if it was a real baby who has good head control. Carrying strategy and position were self-selected by participants. For the carrier condition, participants self-fit the baby carrier after watching an instructional video from a certified babywearing consultant explaining correct techniques for fitting the All Position 360 soft-structured baby carrier (Ergobaby, Los Angeles, California). They practiced fitting the baby carrier with an inward-facing infant mannequin on themselves prior to the standing trials.
Trials were conducted in a random order for each participant. Before and after each 15-minute trial, participants completed a 10-cm Visual Analog Scale (VAS) survey for pain and discomfort described in 3 regions: back, hips, and legs.29,30 If a participant reported pain of greater than 6 cm on the 10-cm VAS at any point during the testing, the participant would be asked to discontinue participation in the study to prevent further discomfort. Participants were given approximately 5 minutes to sit and relax between trials to ensure they were well rested.
Symmetrical stance was defined as the participant standing with 50% ± 15% of their weight on each force plate, as determined from weight-normalized vertical ground reaction force recordings. Weight shifts were defined as shifts of more than 65% of body weight or body weight plus infant mannequin onto 1 leg for more than 1 second,23 determined using custom Matlab scripts (version R2016b; The MathWorks, Inc, Natick, Massachusetts). Average weight shift frequency was calculated by determining the total number of weight shifts during the 15-minute trial and dividing by time. Percent time in asymmetrical stance was defined as the amount of time in asymmetrical stance divided by total time in trial, multiplied by 100. Postural sway parameters were calculated from the force plate data using custom Matlab scripts and included center-of-pressure (COP) path length, 95% confidence ellipse sway area, and medial-lateral (ML) and anterior-posterior (AP) excursions, and root-mean-squared (RMS) or sway variability.
Secondary Analysis: PD Versus NPD
Because there was a clear divide in VAS scores between participants, we grouped participants into those who developed pain (PDs) during any testing condition (a >1-cm increase in VAS score during any standing condition) and those who did not develop pain (NPDs).18,19
Test of normality determined that nonparametric tests were appropriate for the variables of interest. To determine whether participants were properly rested between trials, VAS scores at time 0 were compared between the 3 carrying conditions (unloaded, carrier, and arms) using nonparametric Friedman tests (α ≤ .05). Nonparametric Friedman tests were used to determine whether differences exist between carrying conditions for the postural sway variables or weight shift variables (α ≤ .05). When a significant main effect was identified with a Friedman test, post hoc Wilcoxon signed ranks tests were used for pairwise comparisons, with significance adjusted for multiple comparisons (α ≤ .05/3 ≤ .017). Effect sizes were calculated from post hoc Wilcoxon tests. All statistical analyses were performed using SPSS software (version 18; SPSS, Inc, Chicago, Illinois).
In this study, we aimed to quantify the postural differences of females holding infants in arms and in baby carriers compared with an unloaded condition during prolonged standing. Our secondary goal was to explore biomechanical differences in PDs compared with NPDs in response to baby-carrying conditions.
All participants scored 3 or less on the Chalder Fatigue Scale (1.0 ± 1.2), indicating no underlying mental or physical fatigue. No significant difference was found in the VAS scores prior to any testing condition, indicating individuals were properly rested between trials. No individuals reached a 6-cm score on any VAS and therefore all completed testing.
A statistically significant difference was found between conditions in weight shift frequency (χ22 = 7.467, P = .024), although pairwise comparisons did not reach significance (P > .017) (Figure 2A). A statistically significant difference between all conditions in percent time in asymmetrical stance was found (χ22 = 10.903, P = .004) (Figure 2B). Individuals spent significantly more time in asymmetrical stance during the arms condition than in the unloaded condition (P = .012), with a large effect size (r = −0.80).
A statistically significant difference between all conditions for sway area was found (χ22 = 7.8, P = .02) (Figure 3A). When compared with the unloaded conditions, individuals exhibited significantly greater sway area in the arms condition (P = .017), with large effect size found (r = −0.76). A statistically significant difference between conditions for ML sway variability (RMS) was found (χ22 = 7.8, P = .02) (Figure 3B). When compared with the unloaded conditions, individuals exhibited significantly greater ML sway variability (RMS) in the arms condition (P = .013), with a large effect size (r = −0.79). Friedman tests did not identify statistically significant differences between conditions for COP path or AP sway variability (P > .05).
Secondary Analysis: PD Versus NPD
Immediately following the 15-minute trial, 3 of 10 individuals in the unloaded condition, 3 of 10 individuals in the carrier condition, and 5 of 10 individuals in the arms condition reported an increase in pain (a >1-cm increase in VAS score) in at least 1 region of the body. Therefore, 5 participants were classified as PDs and 5 were NPDs. A post hoc power analysis comparing PD versus NPD time shifted revealed that our n = 5 per group sample only reached a power of 1 − β = 0.46 and that group sizes of n = 10 were required to achieve sufficient power (1 − β > 0.80). Because this secondary analysis was underpowered, only descriptive statistics are provided.
In the unloaded condition, NPDs shifted their weight 20 times more frequently and spent 36 times longer in asymmetrical stance than PDs (unloaded condition; Figure 4). For the PD group, the arms condition caused an increase in both frequency and time shifted by a factor of 3 compared with the unloaded condition, whereas the carrier condition caused a 5-times increase in frequency of shifts and a 10-times increase in time spent in asymmetrical stance compared with the unloaded condition (Figure 4). For the NPD group, each carrying condition increased sway frequency compared with the unloaded condition by a factor of 3 for the carrier and a factor of 4 for the arms. Both the carrier and arms conditions nearly doubled the time spent in asymmetrical stance compared with the unloaded condition. Little difference was observed between carrying styles for NPDs.
Caregivers spend hours each day holding and transporting infants.31 As with any load-carrying scenario, understanding the biomechanical impact of different carriage methods is the first step in optimizing the task and reducing injury risk for the caregiver. In our study, we analyzed the effects on postural control of 2 common infant-carrying methods: in arms and in a soft-structured baby carrier. Overall, utilizing the soft-structured carrier to hold an infant mannequin provided a more biomechanically similar experience to an unloaded condition in contrast to carrying in arms, which was significantly different from the unloaded condition during prolonged standing.
When compared with the unloaded condition, individuals carrying in arms had greater asymmetrical stance time, sway area, and ML sway variability. Taken together, these suggest a more destabilizing pattern during stance for in-arms holding. In contrast, the carrier condition resulted in more similar postural control variables to the unloaded condition. These differences between the arms and carrier conditions compared with unloaded are interesting, given that the load magnitude of the infant mannequin did not change, only the holding method.
Interpretation of postural sway patterns is challenging. On the one hand, we know that too much motion during standing may indicate instability and balance problems; on the other hand, too little motion is linked to pain development.28 There is likely an optimal range of weight shifting and COP motion that avoids the drawbacks of both too much and too little postural sway; however, those threshold values have not been defined in the literature. The increased sway area and ML sway variability during the in-arms condition may indicate that stabilization during this condition is more challenging, as the woman sways more often to maintain her center of mass over her base of support. However, without a carrier holding the infant in place, the caregiver can shift the weight of the baby during standing, which may reduce fatiguing effects on the lower extremities. One clear benefit of babywearing is the ability to remain hands-free. Without the need to hold the baby with at least 1 arm, caregivers may be more able to complete activities of daily living or to catch themselves if they were to lose their balance.
Pain Developers Versus Nonpain Developers
While all participants in this study were healthy and did not have clinically diagnosed orthopedic conditions, some did develop pain while standing for a prolonged period. While unloaded, individuals who did not develop pain in this study shifted their weight more frequently and spent more time in asymmetrical stance. This is consistent with previous research demonstrating that NPDs have greater weight shift frequency than PDs.28 Female NPDs have also been shown to have greater asymmetry in weight shifting than female PDs during prolonged standing trials.23 Furthermore, decreased weight shift frequency in the first 15 minutes of standing has been shown to be predictive of LBP development with continued standing duration.28 Thus, our results are consistent with previous research suggesting that PDs remain more stationary during prolonged standing than NPDs and further support that PDs may benefit from increasing the time and frequency of weight shifting during prolonged standing to reduce the risk of pain development.28,32
When considering the infant-carrying method in each group, little difference was found in the NPDs, but carrying method did affect the observed biomechanics in the PD group. The carrier condition resulted in the highest frequency and time spent weight shifted for the PD cohort, with weight shifting measures approaching those of the NPDs in the unloaded condition. Thus, PDs may be more sensitive to an optimized and centered load that may allow for more freedom of movement. While we are unable to draw definitive conclusions based on these findings due to a small sample size, the carrying method appears to have a different impact on PDs compared with NPDs, and we suspect that the use of an infant carrier may be more beneficial for PDs. The VAS scores support the idea that increasing weight shifting may benefit PDs during load carriage, as only 3 of 10 participants reported pain or discomfort during the carrier trial compared with 5 of 10 during the arms trial.
Although not exclusionary for enrollment in this study, no participant had children. Junqueira et al33 did not find significant differences in postural changes between mothers and nonmothers during quiet stance with infants held in arms symmetrically at the front of the trunk. Participants in our study were not directed to hold the infant mannequin in any specific way, so it is possible that mothers with experience holding infants may utilize different postural strategies when compared with nonmothers. In addition, infant mannequins were used instead of living babies in the current experiment, and previous research has reported that mothers carrying their actual infants had slightly different spinal angles compared with carrying infant mannequins.33 While spinal angle differences were less than 2°, it is possible that utilizing living infants in our study would have produced varying results in all measures. However, we concluded that utilizing living infants would introduce a myriad of unnecessary variables for our study. Furthermore, our study limited the infant size to that of a 6-month-old baby. Caregivers likely utilize different carrying strategies based on infant size, age, and musculoskeletal development.
Although participants were able to watch the instructional video to appropriately fit the carrier as many times as they wanted, it was observed that many had questions as to the correctness of the fit of the carrier. The research team did not provide the participants with input regarding the fit of the carrier, but future studies should consider using a certified babywearing instructor to check for appropriate positioning of the carrier. Similar to the documented benefits of a lactation consultant for postpartum nursing mothers on breastfeeding duration and success,34 appropriate in-person babywearing instructions to new mothers or caregivers may further improve the biomechanical advantage provided by baby carriers.
In addition, a variety of carrier types and carrying styles exist in the marketplace: unstructured slings, structured carriers, forward-facing and inward-facing carriers, backpack carriers, and more. The carrier used in this study was chosen in part because it is a commonly used device and has been named a “hip-healthy” carrier by the International Hip Dysplasia Institute as the design may optimize the hip positioning of infants. Furthermore, front-carrying of infants has been shown to result in fewer musculoskeletal problems for the caregiver than back-carrying of infants, possibly due to the weight distribution of front-carrying being similar to that during pregnancy.24 Although no previous research has been done on the impact of carrier type on balance parameters of the caregiver during standing, the results of our study suggest that symmetric front-carrying with a soft-structured infant carrier may play an important role in reducing the biomechanical impact of holding an infant during prolonged standing, particularly for those females who develop LBP. Clinically, providers should consider how baby-carrying method may impact their individual patients, particularly those who may be susceptible to back pain.
Future studies should expand upon our prolonged standing pilot study to examine the impact of the many different infant-carrying methods on a variety of activities of daily living. As our sample size limited our interpretation of pain results, future studies could incorporate individuals with LBP. It is also critical to study postpartum mothers carrying infants, as this population is most likely to be impacted by infant-carrying methods. Finally, as more males and grandparents are becoming primary caregivers of infants, this study should be expanded to also include these populations.
Regardless of the biomechanical cost, mothers will continue holding and transporting infants until the end of time. The results of this study suggest that babywearing may offer the caregiver a biomechanically advantageous way to hold infants during prolonged standing, particularly for those who develop pain.
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