• How and Why Boosters Work

Kids need to use boosters because the seat belt does not fit them correctly without the booster. If the seat belt doesn’t fit the child, he is at risk for injury; in fact a child between ages 4 and 8 riding in a booster is 59% less likely to be injured in a crash than a 4 to 8 year old wearing a safety belt alone. Read on to learn what seat belts do and why they don’t automatically work for kids.

IMG_9522 (1)

The Problem with Safety Belts for Children

In order for the seat belt to work–to prevent you from being ejected and to distribute the crash forces–it needs to fit like it fits the man in the picture below. Unfortunately, seat belts don’t fit kids under ages 10 to 12 this way because of three main problems, discussed below. These three factors combine together in a crash to create an extremely unsafe situation which can be corrected only by the use of a booster. 


Problem 1: The shoulder belt is anchored too high

The shoulder belt is usually anchored so high up in the vehicle (despite adjustable shoulder belt anchors in many newer vehicles) that instead of crossing the center of a child’s chest and resting between the neck and the shoulder, it crosses only a small part of the chest and rests against the neck. This causes the child discomfort and most kids respond by putting the shoulder belt behind their backs or under their arms. If the shoulder belt isn’t going across the child’s chest and shoulder, it can’t hold the child’s upper body back in a crash–allowing the possibility of severe head, neck or spinal injury.

Problem 2: The vehicle seat is too big and too deep

The vehicle seat is too deep for most kids to be able to sit comfortably without slouching – since their thigh bones (femurs) are too short. Slouching worsens the already poor belt fit by placing the lap belt further up on the abdomen and moving the shoulder belt even further off the chest and onto the neck. By creating a big gap between the child’s back and the back of the vehicle seat, slouching introduces a large amount of slack in the safety belt, which can allow a child’s head and body to move dangerously far forward in a crash.

Problem 3: The child’s body is too small and underdeveloped

A child’s pelvis (hip bones) is relatively small and cartilaginous and lacks the prominent anterior superior iliac spines of an adult’s. What does this mean? Try to feel your hip bone. What you’re touching is the anterior superior iliac spine–the part that sticks out the farthest. If you try to feel a child’s hip bones, it’s much more difficult (even on skinny kids) because the iliac spine isn’t fully formed until about age 8-10.

An adult’s iliac spine keeps the seat belt on his hips and prevents the lap belt from going into the abdomen. Unfortunately, a child’s small pelvis, lacking the fully developed iliac spine, is often unable to prevent the lap belt from riding up into the abdomen. The lap belt may start out in the proper position but moves up into the abdomen during the crash, leading to a pattern of injuries to the abdominal organs and lower spinal cord known as “seat belt syndrome. These injuries are severe and life-changing.


What does the difference in seat belt fit look like?

CHOP drawing of lap belt riding too high into abdomen and over hip bonesThis diagram, created by the Children’s Hospital of Philadelphia, shows how a safety belt should fit (green belt) and how it typically fits on a 4-8 year old child (orange belt).

The correctly placed green belt rests on the hips, over a bony area of the body.

The incorrectly placed orange belt has moved up around the child’s abdomen. It’s easy to see how the spinal cord is in danger as the orange belt tightens against the soft abdomen and pushes on the spinal cord. You can also see how the soft organs inside the abdominal area would be at great risk for injury.

How boosters solve this problem

Better shoulder belt fit: A booster raises a child up to allow a proper shoulder belt placement — so the shoulder belt rests across the center of the chest and between the shoulder and neck. This placement is comfortable for a child (or adult) and the child is therefore much less likely to place the shoulder belt in an unsafe position, such as behind their back or under their arm. With the seat belt now in the right place, it can do its job in a crash.

Better lap belt fit: This is a complicated one—boosters make a child more comfortable in an upright position, which eliminates the child’s desire to slouch, which therefore means a better lap belt fit. Boosters do this by providing two features not found on the regular vehicle seat: support for a child’s knees, and seat belt guides.

Support for a child’s knees: Boosters are shallow (short in the leg area) and thus allow a child’s knees to bend comfortably over the edge of the seat. This is a comfortable position for a kid, and they’re likely to remain in this supported correct position throughout the car ride.

Without a booster the regular vehicle seat is too long to let a child’s knees bend, and to get comfortable on the long vehicle seat the child would likely slouch. When a child slouches, the seat belt both moves over their stomach and becomes loose, both of which can cause injury in a crash.

In a booster, the child’s back rests against the back of the vehicle seat (in a backless booster) or booster seat itself (in a high-backed booster), allowing both the shoulder belt to tighten appropriately against the child’s upper body and the lap belt to remain safely over their hips . 

Seat belt guides: Boosters also include seat belt guides to prevent the vehicle seat belt from approaching the child’s abdomen. The booster will have small handles, armrests, guides or slots to position the lap portion of the seat belt low and flat across a child’s upper thighs. These guides both position the belt in the proper area and also prevent the belt from then sliding up onto the child’s abdomen. 

Boosters in Action: Comparing Booster vs. No Booster

Booster seats work in conjunction with lap and shoulder belts to restrict head movement during a crash. The farther forward the head moves, the more likely it is to be injured. These diagrams, which are courtesy of the Children’s Hospital of Philadelphia, show the difference in head movement in three different scenarios.

Lap and Shoulder Belt worn incorrectly, No Booster

In the first diagram, a child who should be using a booster seat is instead using just a lap-and-shoulder belt. However, he has put the shoulder portion of his seat belt behind his back, which means his upper body is not restrained. Look how far his head moves in a crash!

Lap and Shoulder belt worn correctly, No Booster

In the second diagram, a child who should be using a booster is instead using just a lap-and-shoulder belt, albeit correctly. His head still moves quite far forward during a crash.

Lap and Shoulder Belt worn correctly WITH Booster

In the third diagram, the child uses a booster correctly. His head barely moves forward and he will sustain much less serious injuries than the other two children in the above diagrams.