Effects of infant care posture and weight on static postural balance

1.Introduction

Lifting, holding, and carrying an infant in-arms are typical care activities in parent’s daily life. Repetitive strain and inefficient postures during such infant care activities cause biomechanical loads on the musculoskeletal system [1, 2]. Especially, inappropriate postures that are biomechanically stressful [3, 4, 25] caused by an infant’s constant and unpredictable motions can induce various musculoskeletal diseases. Specifically, 49% of back injuries are related to infant care activities such as lifting an infant [5]. Some studies have reported that inefficient postures during infant care for a long period time can induce lower back pain [6]. Furthermore, repetitive infant carrying motion can lead to lumbar lordosis [7, 8]. Therefore, understanding biomechanical risk factors of infant care motions is important to prevent the onset of musculoskeletal diseases and suggest effective motion strategies or ergonomic interventions.

Therefore, some studies have performed quantitative motion analysis during sling-based infant carrying or carrying in-arms. Schmid et al. have demonstrated that carrying a dummy in front can induce lumbar lordosis and suggested that carrying an infant alternating on both sides using a sling could effectively prevent musculoskeletal diseases [9]. Additionally, some studies have compared ankle, knee, and hip joint moments between wearing a mannequin infant with a baby carrier and carrying a mannequin infant in arms. A baby carrier is useful for decreasing the loading knee abduction moment and the loading knee extension moment [10]. Kim et al. have investigated peak joint moments at the shoulder, lumbar, ankle, knee, and hip joints during lifting-up of infant dummies [11]. They found that the lumbar extension moment was the greatest during lifting-up of an infant dummy compared to other joints.

It has been reported that an altered posture during lifting motion can cause back pain [12] and increase the risk of falls [13]. Actually, some studies have investigated postural balance during lifting motion [12, 13]. However, no studies have investigated postural balance during various infant care motions yet. A previous study has suggested that falls during infant care activities might be associated with postural imbalance [20]. It is important to provide quantitative indicators of postural balance risk factors during infant care activities for suggesting an effective infant care strategy. Therefore, the aim of this study was to investigate COP (center of pressure) based measures during holding an infant dummy in-arms and carrying an infant dummy on back according to different infant dummy weights.

2.Method

2.2Experiments and analysis

In this study, self-developed infant dummies instead of real infants were used as shown in Fig. 1. Actually, some studies have also used infant dummies for analysis of infant care [9, 10]. Moreover, it has been suggested that carrying infant dummy is not significantly different from carrying a real infant [9]. In this study, two weights (4.6 kg and 9.8 kg) of infant dummies were used according to average weights of 1-month and 12-month infants in Korea [14]. Head circumference was 43.3 cm and height was 67.6 cm as mentioned in our previous study [11].

Figure 1.

Two postures in for measuring postural balance during infant care. Two postures include holding an infant dummy in subject’s arm (left) and carrying an infant on subject’s back.

A force platform (OR-7-2000, 464 × 508 × 84 mm; Advanced Mechanical Technology, Watertown, NY, USA) was used for assessing static postural balance during infant care posture. Two infant care postures (holding an infant in one’s arms and carrying an infant on one’s back) were selected as shown in Fig. 1. All subjects either held an infant dummy or carried the infant dummy on their backs without any assist devices such as slings and baby carriers. For holding posture, all subjects supported the dummy’s buttocks with their left hand and wrapped their right hand around the dummy’s back as shown in Fig. 1. For back carrying posture, all subjects slightly bent their backs and supported the dummy’s buttocks with both hands behind them. Static balance test was performed in a random order in four experimental conditions (2 infant care postures * 2 infant weights). All subjects were asked to naturally stand on the force platform in each infant care posture. The balance test was performed three times and mean values of three trials were used for subsequent analysis.

The COP was recorded for 60 s. The sampling frequency was 120-Hz. COP data were low-pass filtered with a zero-phase Butterworth filter of fourth-order and a 5 Hz cutoff frequency [16]. The mean value was subtracted from each time series for removal of the offset of anterio-posterior (AP) and medio-lateral (ML) COP data. COP time series were calculated as the COP displacement from averaged COP in the ML and AP. Many quantitative postural sway variables such as mean distance, COP area, mean velocity, and peak power were derived from the COP time series. The mean distance was calculated as average value of COP time series and the mean velocity was calculated as the mean absolute value of time derivative of COP time series. The peak power was defined as the peak value in the power spectrum of COP time series. Finally, COP area was defined as the area surrounding 95% of the points in the COP time series. These quantitative variables have generally been used to assess static postural balance ability in many previous studies [15, 16, 17, 18, 19, 22, 23, 24]. Two-way repeated measure analysis of variance (ANOVA) was performed to determine main effects of infant care postures, infant weight, and their interactions on COP variables. All statistical analyses were performed using SPSS ver. 16 for Windows (SPSS Inc., Chicago, IL, USA). The significant level was defined at p< 0.05.

Figure 2.

Representative center of pressure trajectories according to infant weight and infant care postures. AP, anterio posterior; ML, medio lateral.

3.Results

Figure 2 shows trajectories of representative COP according to dummy weight and infant care postures. Back carrying a 12-month infant dummy (9.8 kg) showed the greatest COP trajectory. In contrast, holding a 1-month infant dummy (4.6 kg) presented the smallest COP trajectory.

Figure 3.

Comparison of COP variables according to infant care posture and infant dummy weight. COP, center of pressure; AP, anterio posterior; ML, medio lateral; NS, non significance (*, P< 0.05; **, P< 0.01).

Table 2 and Fig. 3 present results of two-way repeated measure ANOVA to investigate infant care posture effect, infant weight effect, and their interactions. Back carrying a 12-month infant dummy had the greatest amplitude in all COP variables. Main effects of infant care posture were significant in all variables (P< 0.05). Specifically, back carrying posture showed significantly greater mean distance and peak power, faster mean velocity, and wider COP area than holding posture as shown in Fig. 3 (P< 0.05).

There were significant weight effects mainly in AP direction, with significantly greater mean distance, greater peak power, and faster mean velocity when holding or back carrying a 12-month infant dummy than when holding or back carrying a 1-month infant dummy (all P< 0.01). Additionally, COP area when holding or back carrying a 12-month infant dummy was wider than that when holding or back carrying a 1-month infant dummy (P< 0.05). In contrast, there were no significant interactions between infant care posture and infant weight (P> 0.05).

4.Discussion

Little is known about postural balance during infant care motions. This study investigated static standing posture balance during holding an infant dummy in arms and back carrying an infant according to two weights. Main findings of this study were as follows. First, standing posture during back carrying an infant dummy showed the greatest postural sway size and peak power, the fastest sway speed, and the widest sway area. Second, back carrying posture showed greater sway size and peak power, faster sway speed, and wider sway area in both AP and ML directions than holding posture. Third, effect of dummy weight was prominent in AP direction, with greater sway size, greater peak power, and faster sway size when holding or back carrying a 12-month infant than when holding or back carrying a 1-month infant.

In this study, a back carrying posture had significantly greater mean distance, greater peak power, faster mean velocity, and wider COP area than a holding posture. These results indicate that back carrying an infant can induce more unstable postural balance condition, with greater body sway and greater amplitude of main frequency component, faster sway speed, and wider sway area in both AP and ML directions. That is, back carrying posture may induce inefficient level of steadiness. Thus, greater postural balance activity might be required to achieve the level of steadiness. Moreover, back carrying posture’s balance strategy might have a greater intensity of the main COP oscillation component. Effect of infant care posture might be due to difference in trunk posture during static standing between holding and back carrying an infant dummy. Specifically, most participants tended to flex their trunk more during back carrying an infant dummy. This flexed posture might have made it more difficult to maintain postural stability.

Particularly, back carrying an increased weight showed increased values in all COP variables. That is, back carrying a 12-month infant showed the greatest sway size, the greatest amplitude of main frequency, the fastest sway speed, and the widest sway area. This means that holding an infant in one’s arms is helpful for maintaining a more stable postural balance than carrying an infant on the back. Postural imbalance is associated with the risk of fall. Many studies have shown that increased COP variables contribute to deterioration of postural balance. Actually, a previous study has reported that falls while carrying an infant might be due to postural imbalance [20]. Our study suggests that assisting aid such as commercialized back carrier should be used to maintain a stable postural balance when carrying an infant on one’s back.

In most COP variables, effect of dummy weight was prominent in AP direction. This might be due to change of body center of mass (COM) caused by an increase of dummy weight. For example, back carrying an infant dummy may lead to greater sway distance and greater main COP component mainly in AP direction to maintain postural correction against gravity by body COM placed to the posterior direction with an increase of infant dummy weight. Moreover, a faster sway speed by a heavier weight means that more postural control activity in AP direction might be required to achieve the level of steadiness. In a previous joint moment study, heavier infant weight during lifting-up of an infant caused increases of lumbar and hip extension moments [11]. Particularly, the lumbar extension showed the greatest peak joint moment [11]. Our study and the previous study indicate that weight effect exists in postural sway and joint load. These results suggest that an assisting aid or device should be developed in order to prevent unstable postural balance as well as back pain by increased infant weight.

Previous studies have focused on joint moment analysis during lifting-up or carrying of infant [9, 10, 11]. They reported that various infant care activities could induce mechanical load on musculoskeletal systems. Although various biomechanical analyses during infant care motions have been performed, studies investigating postural balance considering various infant care posture or infant weight are insufficient. This study demonstrates that both infant care posture and infant weight can affect postural sway characteristics. However, this study has some limitations. Some studies have suggested that using sling or carrier can improve joint loading and prevent musculoskeletal diseases [9, 10]. Some study investigated gait variables carrying an infant dummy in a baby carrier [21]. We also should investigate if the sling or baby carrier can improve postural balance in a further study. In this study, COP was measured for 60 seconds. However, most babysitters usually stand for long time for holding or back carrying an infant. Long period standing test should be investigated to reflect real infant care situations. Additionally, a small number of young subjects participated in this study, although there might be age and gender differences. Further study should investigate larger populations including elderly subjects. Furthermore, this study only focused on static postural balance. COP based measure during dynamic situation should be investigated to reflect more real infant care conditions.

5.Conclusion

COP variables during holding an infant dummy in-arms and carrying an infant dummy on back were investigated according to the weight of the infant dummy. Posture for carrying an infant dummy on one’s back showed the greatest postural sway size and peak power, the fastest sway speed, and the widest sway area. Additionally, the back carrying posture showed greater sway size, greater peak power, faster sway speed, and wider sway area in both AP and ML directions compared to the holding posture. Effect of infant dummy weight was prominent in AP direction for most COP variables. Our results could contribute to the prevention of musculoskeletal diseases or prevention of fall due to various infant care activities by developing an assisting device to improve postural balance.