Effect of Booster Seat Design on Children’s Choice of Seating Positions During Naturalistic Riding

The purpose of this naturalistic study was to investigate the effect of booster seat design on the choice of children’s seating positions during naturalistic riding. Data was collected through observations of children during in-vehicle riding by means of a film camera. The children were positioned in high back boosters in the rear seat while a parent drove the car. The study included two different booster designs: one with large head and torso side supports, and one with small head side supports and no torso side supports. Six children between three and six years of age participated in the study. Each child was observed in both boosters. The duration of the seating positions that each child assumed was quantified. The design with large side head supports resulted more often in seating positions without head and shoulder contact with the booster’s back. There was shoulder-to-booster back contact during an average of 45% of riding time in the seat with the large head side supports compared to 75% in the seat with the small head supports. The children in the study were seated with the head in front of the front edge of the head side supports more than half the time, in both boosters. Laterally, the children were almost constantly positioned between the side supports of the booster in both seats. The observed seating positions probably reduce the desired protective effect by the side supports in side impact, and may increase the probability of head impact with the vehicle interior in frontal impact.

Car Seat Position

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INTRODUCTION

Motor vehicle crashes are among the leading causes of morbidity and mortality in children 4 to 8 years of age [Bingham, Eby, Hockanson, et al., 2006; Subramanian, 2005]. A child’s safety is dependent on the adult’s choice of vehicle and child restraint. It is of utmost importance that children use correct restraints when seated in a vehicle, taking into consideration age, height, and weight. Belt-positioning boosters (BPB) are effective tools to help protect children from injuries, decreasing the probability of injury by as much as 45% compared to safety belt only [Arbogast, Jermakian, Kallan et al., 2009]. Durbin, Elliot and Winston (2003) have shown that seat belt syndrome related injuries to the abdomen and spine were nearly eliminated in crashes with children seated correctly on belt-positioning boosters compared to those restrained by safety belts alone.

Backless BPBs were introduced in Sweden in 1978. At that time, they were intended for use by 3 to 7-year-olds in combination with the vehicles’ safety belts. A few years later, the first high back BPB was introduced. Since then, the usage rate of backless and high back BPBs has increased. In the US, BPB use in the 4 to 8-year age group has increased from 15% to 63% between 1999 and 2007 [Partners for Child Safety Fact and Trend report, 2008].

The backs of the high back BPBs were initially intended to route the diagonal part of the safety belt in an optimal position over the child’s shoulder and chest. In recent years, the designs of the backs of the BPB have evolved towards large side supports both at the height of the torso and the head. The child restraint manufacturers emphasized two reasons for this; to provide improved side impact protection and to provide comfort for children by keeping them upright when relaxed or asleep to help provide protection at all times.

However, high back BPB with large side supports achieve the desired effect only if the child, particularly the child’s head, is contained within the BPB. Previous research on behavior in child restraint systems (CRS) has investigated the way in which children in the age group 0 to 8 years change their position, both within and outside the CRS [Charlton, 2010]. It is known from other research fields that sitting is not static; both children and adults vary their seating position and posture [Bentsen, 1971]. There are a number of reasons for this. For children, one reason is that their natural physical and mental development is achieved through movement, thought, contact with others, and sensing and experiencing their surroundings [Bentsen, 1971]. Children nearly always use several senses simultaneously to experience the environment; i.e. the visual, audile and haptic senses. Therefore, they are prone to change positions to be able to see, hear and touch at the same time. Another reason is that they may experience discomfort sooner than adults due to differences in the nervous system. Loading or pressure on body parts may be perceived earlier by younger children than older children and adults [MacGregor, 2008]. Furthermore, there is a gradual development of the muscles from birth to adulthood. The proportion of muscles in total body weight is smaller for a child compared to an adult. As a result, children feel fatigue earlier than adults and need to vary their seating position more often [Bentsen, 1971]. Also, to compensate for their relatively smaller amount of muscles children use their legs and feet for support while sitting rather than the seat back to stabilize the back [Bentsen, 1971].

The sitting position depends on which positions are possible in a specific seat. The children’s ability to move depends on the design of the restraint system [Meissner, Stephens and Alfredson, 1994; van Rooij, Harkema, de Lange et al., 2005]. The design may promote a range of sitting positions, including less optimal positions. Research by Charlton et al. [Charlton, 2010] has provided a first understanding of how children sit in vehicles. However, there is limited knowledge of how the size of the side supports influences seating position. The purpose of this study was to investigate the effect of high back belt-positioning booster design on the choice of children’s seating position during naturalistic riding. Specifically, the head and torso positions in sagittal (fore-aft) and lateral (left-right) directions were studied together with the orientation of the face and gaze.
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METHODS

In order to increase the understanding of the natural sitting behavior of children during a car ride, a small naturalistic study was conducted to identify common seating positions. Six children between 3 and 6 years of age were observed. The children were recruited from a daycare center. The children were positioned in high back BPBs in the rear right seat of a car. Two markedly different BPBs were chosen. Both seats had adjustable headrests with multiple height positions. Also, the angle between the back and the cushion was adjustable, allowing contact with both the cushion and the seat back of the car. Seat X had small side supports for the head and no torso side supports, while seat Y had large head and torso side supports (Figure 1). The depths of the head side supports were 10.5 cm for seat X and 20 cm for seat Y. Seat X was developed in the late 1980s and the side supports were intended to support the head when resting and sleeping. Seat Y represents a typical modern high back BPB available in stores today. Seat Y is promoted as a high back BPB with side impact protection.
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Figure 1

Seat X to the left and seat Y to the right.

The cars used in the field tests belonged to the children’s families and were driven by a parent. Each child was taken for a ride with a duration of 40 to 50 minutes in each of the two BPBs. The route entailed both city and highway driving. The two rides occurred on different days, but at the same time of day and on the same route each time. The test order of the seats was alternated equally. Each car was equipped with a digital video camera. The camera was attached to the inner roof on the left side of the car, providing a side view of the child (see snapshots from the films in Figure 3).
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Figure 3

a) and b) seating position AA in seats X and Y – entire back and head in contact; c) and d) seating position AB in seats X and Y – entire back in contact and head in upright position; e) and f) seating position BB in seats X and Y – shoulder not in contact and head in upright position.

Data was collected in two ways; continuous film observations of children during the ride. From the films, the children’s seating positions were categorized and the duration of each sitting position was quantified. Also, the children’s different behavior and actions during the ride were documented. However, the comfort parameter seat stiffness was not measured.
Classification of seating positions

The children’s seating positions were systematized by differentiating the sagittal (fore-aft) from the lateral seating positions (left to right). To design a usable classification system, a number of head and torso positions in the x and y directions were defined. Arm and leg positions were not observed.

The sagittal torso positions were defined as: (A) the entire back including shoulders against the BPB back, (B) the entire back but not the shoulders against the BPB back, (C) child remains upright but no part of the back against the BPB back, and (D) the torso is leaning forward without contact with the BPB back. The sagittal head positions were: (A) head against the BPB back, (B) head upright relative to the torso, and (C) head leaning forward relative to the torso. These head and torso positions were combined to make up the sagittal seating positions as shown in Table 1. For example, in the AB position the child sits with the entire back against the BPB back, while the head is upright. The classification was derived from seating posture categories defined by Utriainen, Dahlman and Osvalder (2003).

The lateral seating positions were combined in the same way as the sagittal positions. The lateral torso positions were defined as: (a) the whole torso is within the BPB back, (b) one shoulder is outside the BPB back, and (c) one shoulder and part of or the whole thorax is outside the BPB back. The lateral head positions were: (a) between the head side supports, (b) resting against one of the head side supports, (c) partly outside the head side supports, and (d) completely outside the head side supports. The combinations of the lateral torso and head positions are shown in Table 2. In the results, the direction of the lateral movement is indicated by plus (+) meaning inboard, and by minus (−) meaning towards the side window. The face and gaze orientations were defined by five categories; forward, right, left, down or other.
Table 2

Definitions of the lateral seating positions. No arms or legs were drawn; the horizontal line shows shoulder width.
Head a Head b Head c Head d
Torso a An external file that holds a picture, illustration, etc. Object name is file57-finali11.jpgaa An external file that holds a picture, illustration, etc. Object name is file57-finali12.jpgab An external file that holds a picture, illustration, etc. Object name is file57-finali13.jpgac N/A
Torso b An external file that holds a picture, illustration, etc. Object name is file57-finali14.jpgba An external file that holds a picture, illustration, etc. Object name is file57-finali15.jpgbb An external file that holds a picture, illustration, etc. Object name is file57-finali16.jpgbc An external file that holds a picture, illustration, etc. Object name is file57-finali17.jpgbd
Torso c N/A N/A An external file that holds a picture, illustration, etc. Object name is file57-finali18.jpgcc An external file that holds a picture, illustration, etc. Object name is file57-finali19.jpgcd
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Two persons conducted the film analysis by analyzing three children each. Parts of the films were analyzed jointly and the analyses were partly repeated for quality confirmation. Any seating position with duration of one second or longer was assessed and categorized according to the defined seating positions (Table 1 and Table 2).

The total duration of each position was summed up. Ultimately, the sagittal seating positions where the entire back or the head was in contact with the BPB back (AA, AB, AC and BA) were grouped and the duration of these seating positions were summed up by child and seat type. The duration of each seating position was assumed to be normally distributed. A paired t-test was conducted to show if the durations of the AA, AB, AC and BA positions in seat X minus the corresponding durations in seat Y were significantly greater than zero. If so, there was a significant difference between the two seats.
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RESULTS

The ages of the six participating children (two males and four females) are shown in Table 3. Most of the children were accompanied by passengers besides the driver (Table 3). The children were numbered by age.
Table 3

Demographics of the participants in the study.
No Gender Age (y) Length/sitting height (cm) Seat back angle (°) Seat X/Y Seat cushion angle (°) Seat X/Y Other passengers
1 F 3 102/58 75/74 25/20 Infant: infant seat; rear left.
2 M 3 94/53 69/70 27/16 Child: BPB, rear left. Infant: rear facing seat, rear center.
3 F 4 112/61 65/71 26/20 Adult: front passenger seat.
4 F 5 115/64 66/68 29/27 Toddler: CRS, rear left.
5 F 5 123/66 68/65 29/20 None.
6 M 6 114/59 75/80 24/15 None.

The total duration of each seating position is presented as a percentage of the total riding duration. Figure 2, Figure 4 and Figure 5 show the averages of all children and confidence intervals (95%). The detailed data are shown in Appendix 1 to Appendix 4.

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Figure 2

The distribution of sagittal seating position durations, shown as a percentage of the total ride duration. The averages of all children are presented by BPB seat type with corresponding confidence intervals (95%). The sagittal seating positions are defined in Table 1.

Figure 4

The distribution of lateral seating position durations shown as a percentage of the total ride duration. The averages of all children are presented by BPB seat type with corresponding confidence intervals (95%). The lateral seating positions are defined in Table 2. The plus indicates inboard, the minus indicates toward the side window.

Figure 5

The distribution of face and gaze orientation durations shown as a percentage of the total ride duration. The averages of all children are presented by BPB seat type with corresponding confidence intervals (95%).
Sagittal seating position

The averages of the children’s sagittal seating position durations are shown in Figure 2 and the distributions of the sagittal positions by child are shown in Appendix 1. In general, the most common seating positions in seat X were those with the entire back or head in contact with the BPB back (AA, BA, AB and AC, Figure 2). In seat Y, the most common seating positions were those with the head in an upright or forward leaning position and the back with complete or partial back contact (AB, AC, BB, or BC, Figure 2). The duration of the A torso positions (AA, AB and AC), i.e. with the entire back in contact with the BPB back and the head in any position, comprised an average of 75% of the total duration in seat X. The corresponding figure for seat Y was 45% (Appendix 1). More specifically, the two most common sagittal seating positions in seat X were AA and AB, both adding up to 72% (definitions in Table 1, data in Appendix 1). Notably, the two oldest children, numbers 5 and 6, had proportionally longer durations in the AA seating positions. In seat Y, the two most common positions were AB and BB, adding up to 64%. Some of the seating positions are illustrated in Figure 3.

Furthermore, the durations of the seating positions AA, AB, AC and BA were an average of 32% greater in seat X compared to seat Y (P=0.05). The two samples were tested for normality using both the Kolmogorov-Smirnov and Anderson-Darling tests. Neither of the tests were significant at the alpha=0.05 level, thus indicating that the data were drawn from normal distributions (P_X,KS=0.12, P_X,AD=0.098, P_Y,KS=0.15, P_Y,AD=0.25).

The positions without torso contact with the seat back (torso position C and D) were rare in both of the seats; 4% in seat X and 11% in seat Y (Figure 2). These positions were initiated by specific activities such as reaching for something, looking out the windscreen or communicating with someone.

On average, the head was in contact with the BPB back 33% of the time in seat X, and 11% in seat Y (Figure 3a and b). Most commonly, the head was in an upright position (head position B, Figure 3c, d, e and f): 59% of the time for seat X and 75% for seat Y. The head was leaning forward (head position C) for 8 – 13% of the time and this position was often associated with activities such as reading, playing with something, looking at something in the lap or beside, and eating. The children usually took this position to look down, except for child 3. He sat like this in the Y seat while looking left or right.
Lateral seating position

The lateral position aa with the torso within the BPB and the head within the side supports was the most common lateral seating position, with 85% of the time in seat X and 77% in seat Y (Figure 4). The position with the head contacting the side supports was the second most common position; with 12% for seat X and 17% for seat Y. Three children had a greater proportion of the head contacting the right support vs. the left support. All were tired and rested in this position, either by leaning the side of the head or the back of the head towards the head side support. The one child that spent 20% (Appendix 3) of the time with the head against the left side support in seat X did this to rest in a position where the safety belt did not rub on the skin of her neck. She frequently moved the belt from her neck or put her hand under the safety belt for protection.

Positions with the torso partly or completely outside the BPB together with the head partly or completely outside the side supports were uncommon. They seldom occurred and had short duration
Orientation of the face and gaze direction

The face and gaze direction was similar in the two seat designs (Figure 5). About half the time, the children looked forward, followed by looking right about a fourth of the time. The children looked either left or right on an average of 35 to 43% of the time. The children looked down for 14 to 21% of the time, and this position was associated with activities such as reading, eating, playing with a toy etc.
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DISCUSSION

This study was designed to investigate the effect of the high back BPB design on the choice of seating position of children during naturalistic car riding.
Seating positions

The most striking difference in sagittal seating positions between the two BPBs was that the six children spent an average of 75% of the time with their entire backs against the BPB back in the seat with small side supports, but only 45% in the seat with large side supports. The position with the entire back and head against the BPB back (position AA, Figure 3a and b) was three times more common in the seat with small side supports. The positions without shoulder and head contact (BB and BC) were more than twice as common in seat Y. The difference between the two seats was statistically significant (P=0.03) for the group of seating positions with most BPB back contact (AA, AB, AC and BA). A plausible reason is that the head side supports in seat X did not obstruct visibility even in the position with torso, shoulder and head against the BPB back, while the children needed to be at least in a head upright position to achieve equal visibility in the seat with large side supports (Figure 3d and f). However, the results of the study showed that the children looked left or right slightly less in the Y seat. It may be that the head side supports worked as blinders, preventing the temptation to look sideways. Looking left or right was associated with many different sagittal seating positions, but the positions with the head in contact with the BPB back were rare with the Y design (Appendix 2). The looking left or right activity was one of the most common activities (34 to 43%) and should be considered a normal position (Figure 3).

During the second half of each ride, there were noticeable signs of tiredness or discomfort for most of the children in both seat designs. Common signs were to take an upright position without seat back contact (C) or a forward leaning torso position (D), to support the upper body with the arms (hands on belt guides, elbows on side supports, or elbows on legs), to more frequently change the seating position or to “readjust”, i.e. to briefly move about and then return to the initial position. These findings are in line with the theory by Bentsen (1971), which states that children vary their seating positions often to avoid tiring the muscles and use legs and feet to support themselves while sitting. Based on the knowledge of children’s physical development and sitting behavior, it can be assumed that the adequate type of sitting support is age dependent, i.e. younger children need more support than older. The data suggests that rides with duration of more than 15 to 20 minutes require additional comfort features than the two studied seats offered.

Fortunately, the extreme seating positions were unusual in both seats and did not seem to be related to the design of the side supports. On the contrary, they were related either to a specific activity of the child or to discomfort. Discomfort may be related to parameters in the seat designs, e.g. the seat stiffness or seat angles. The seat stiffness was not measured in this study, but the foam layer on seat X was thinner than the foam layer of seat Y, suggesting that the children should experience discomfort sooner in seat X. The booster back angles correlated with the car seat back angles and the average difference in back angle between seat X and Y for one child was 2.7 degrees. Such small angle differences may be hard to perceive. However, the cushion angle differed 7 degrees on average. The cushion angle was always greater for seat X. This may have contributed to the difference in seating positions between the two seats.

All children spent the majority (80%) of the time in the torso and head mid lateral position in both seats.

This position was the most common when the children were alert, either looking down or forward, which they did about 60% of the time. The next two most common lateral seating positions were ab+ and ab–, i.e. with the child resting its head against the head side supports. This seating position was most commonly chosen for rest. The two children with longest duration in the ab positions were tired and resting, but did not fall asleep (eyes closed only short periods). Even the small head side supports seemed to give the children adequate support for rest when awake; the head position was stable while leaning against the supports, but further studies are needed to determine an optimal head support size. However, there was one notable difference in the duration with head resting against the side supports (ab positions) between the X and Y seats. The outboard head side support was more frequently used on the Y seat. It was not clear whether the children preferred the extra support given by the head side support, or if the contact between the head and the support was just a result of the head position that was chosen because of the desire to look out the window.

In general, the lateral movements outside of the side supports were short and temporary. Often, these movements were associated with communication, either with the driver or with the adjacent occupant, or achieving a better view. Child nr. 6 – the oldest child – spent 18% of the time leaning inboard to look out through the windscreen (the view was otherwise obstructed by the front seat) and to talk to the driver. This behavior did not seem to be primarily related to the seat design, and could be expected to occur with either of the two designs. On the other hand, the large side and head supports of the seat supported the leaning posture. The supports may thus have enticed the child to take this posture as well as enabled long duration of the posture. Other reasons for the positions outside the side supports were to reach something on the floor or on the seat beside the child or to play, for example, by swinging from side to side. There was no difference in the time spent in the extreme lateral position between the seats; even in the seat with the large side supports, the children were able to make substantial lateral movements, which was unexpected. It is important to note that none of the children in the study fell asleep during the rides. It is possible that there is a difference in lateral seating position between the two seat designs for sleeping children.

The study showed that the seating position was influenced by the activity the child carried out. Often, the children came up with something to do when not resting. Activities were: talking with the driver, reading, playing with toys or something similar, looking out, eating or drinking, or playing with a sibling. None of the children watched DVDs or played games. Since this was a naturalistic study, there were no restrictions for the activities. A greater sample would decrease any bias of the actual activities chosen by the children in the study. Although there were differences in the duration of the different activities in the two seats, it appeared that the activity the children chose was independent of the seat. However, how the activity was performed may have differed, e.g. looking out left or right was not possible with the head against the BPB back with the large head side supports. Other activities, such as eating or playing, could be performed in the same seating positions in either seat.
Shoulder belt position

The shoulder belt position was not studied specifically, but there were observations in the study of shoulder belt misuse and shoulder belt discomfort. For example, the lateral inboard and/or forward movement occasionally resulted in a shoulder belt routed under the arm. In some cases, the children adjusted the belt back in position, but in other cases, the belt stayed under the arm until the driver noticed and asked for correction. The misuse durations varied between 2 and 21 minutes (until the ride ended).

Another child experienced discomfort from the shoulder belt. She stated that it made her neck hurt and she intentionally pulled the belt away from her neck several times during the ride. This child had three strategies to avoid discomfort: moving the torso and neck away from the safety belt, moving the safety belt away from the neck, or placing the hand in between the safety belt and neck. The first strategy thus affected the lateral seating position, resulting in head contact with the left or right side support in 42% of the ride. A third child also experienced discomfort to the neck from the shoulder belt. She put her hand under the belt on several occasions or put her toy between the shoulder and the belt.
Safety implications of the seating positions

We hypothesize that some of the observed seating positions may result in less effective crash protection. The children generally adopted seating positions that resulted in head positions further forward in the seat with large head side supports, compared to the seat with small head side supports. The protective effect of the head side support in side impact is negligible when the head is positioned in front of it, which was the case 52% of the time in the seat with large side supports. As a comparison, the standard anthropomorphic test device (ATD) position is comparable to the AB seating position (FMVSS 213). The forward positions also decreased the residual distance to vehicle interior surfaces in front of the child, potentially increasing the risk of head impact in frontal crashes. The identified forward positions of children’s heads compared to the ATD’s are in line with the findings of a study on rear-seated adults’ head positions relative to adult ATDs [Reed, Ebert-Hamilton and Schneider, 2005].

A large proportion of side impacts are angled [Arbogast, Ghati, Menon et al., 2005], resulting in occupant kinematics with a forward component added to the lateral kinematics. Henary, Sherwood, Crandall et al. (2007) showed 5 times higher efficacy of rearward sitting children compared to forward sitting children in side impacts. This difference was explained by the kinematics during the crash, where the forward component in the side impact would cause the forward facing child to move forward, out of the side supports. These kinematics, coupled with the child’s initial head position partly in front of the side supports, suggest that the additional protection by the side supports in an angled side impact is likely to be marginal.

These theories are supported by the findings of Arbogast et al. (2009). In their study, they could not detect any difference in injury risk between children (4 to 8 years) seated in a backless BPB compared to high back BPB in frontal and side impacts. The crashes occurred between 1998 and 2007 in vehicle model years 1990 or newer. This shows a need to differently design safety systems that provide head protection and that are more robust under a diverse set of seating postures and crash conditions.
Study design

This study could be regarded as a pilot study to increase the knowledge about children’s natural sitting behavior during everyday car rides, by identifying common seating positions. In the study, the children traveled in their own cars, driven by a parent. The film camera was positioned so that the children paid no attention to it. The ride duration was long enough for the children to become accustomed to the new BPB and thereby act in a more relaxed and natural way. The two rides occurred on different days, but at the same time of day to minimize variation in e.g. tiredness. It should be noted that the children with siblings were accompanied by the siblings on both rides. The study only comprised six children and no repeats were made. Six subjects were considered sufficient to provide increased knowledge and demonstrate common sitting behaviors. Due to the limited sample size, however, the extremes may not have been covered. According to theory about user studies, a sample size of six users covers nearly 70% of possible problems and behaviors [Nielsen, 1993].
Limitations

BPBs are intended for use by children from 3 up to 12 years of age. The European regulation stipulates a stature of 135 or 150 cm before the safety belt alone is allowed. In Sweden, BPBs are recommended up to an age of 10 to 12 years, while NHTSA’s recommendation for the US is 8 years. The current study only included 3 to 6-year old children, which corresponds to the smallest range for booster seats. It is possible that seating behavior of older children is different than observed in this study; the two oldest children in the study showed a slightly different behavior than the younger ones. Specifically, they maintained a more upright seating position for longer periods of time, which is in line with the theory of child development [Bentsen, 1971]. Still, it is reasonable to believe that some identified seating positions are relevant for the older age groups. For example, the older children would also need to move the head forward from the head side support to see out through the side window or to see, touch, talk to or hear the adjacent person in the car. However, these assumptions should be confirmed in a study involving older children.

The limitation of one film camera view resulted in limited direct visual confirmation of the lateral head and torso positions in the outboard direction. At times, the position had to be deduced by other indicators, e.g. movement of the head relative to the seat back indicated no contact (the BPB and child were in more or less constant motion due to road unevenness). Thus, the lateral seating position with head against the right side support (ab–) was hard to discriminate from head position between the side supports (aa), and may be underreported in the study.

The lack of means to take continuous numerical measurements of the head and torso positions in all three dimensions resulted in less detailed quantitative information on body positions. Further, the study lacks a formal evaluation of inter-observer reliability between the two observers.

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CONCLUSION

The design of the side supports of BPB influences the child’s seating posture. The time spent sitting with the entire back against the BPB back was nearly halved in the seat with large side supports compared with the seat with small side supports (45% vs. 75%). All in all, the children in the study were seated with the main part of the head in front of the front edge of the head side supports more than half the time, i.e. positioned forward to the standard crash dummy position. This finding applied to both of the seat designs.

Laterally, the children were almost constantly positioned between the side supports of the BPB in both seats; there was no difference due to seat design. Further, the extreme sagittal and lateral positions were limited in duration and independent of seat design. Nor did the face and gaze direction appear to be dependent on BPB design.