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Should You Really Be Concerned about Harness and Seat Belt Geometry?

A common question parents ask is whether or not the buckle should be sitting so high on their child after he’s buckled in the carseat. Caregivers may have heard that the buckle should fit low on the hips, like a seat belt, or have seen that the harness fits their child differently than their friend’s child who rides in a different carseat.

The simple answer is that harness geometry differs by manufacturer and may even differ within that manufacturer’s line. The lap portion of the harness may ride higher on your child’s belly in your rear-facing infant seat whereas it sits lower across his thighs when he rides in his convertible seat made by the same manufacturer. Any insert that comes with the carseat will also affect whether the straps cross your child’s thighs or his hips.

    

The harness may also sit quite differently on a newborn than it does on a larger child because a newborn has skinnier legs. Manufacturers certify their carseats to fit a wide range of weights and all children are proportionally built differently.

The harness also serves several purposes: it restrains a child in a crash, contacts the strongest parts of the body (the bones), spreads out the crash forces over a large part of the body, helps the body ride down crash forces, and protects the head, neck, and spinal cord. The direction the carseat faces plays a role too; a buckle sitting high on a child’s belly plays less of an injury role on a rear-facing carseat because the carseat itself is bearing the brunt of the crash force as are the shoulder straps.

One feature being provided by manufacturers that can greatly change the way the harness sits on your child’s legs is crotch strap length adjustment. Clek’s innovative way of adjusting crotch strap length is to provide two straps of different lengths on one buckle (see pics below). Baby Trend infant seats have adjustable crotch straps and require the buckle latches to be ½”-1″ from the baby’s thighs. Other manufacturers allow the crotch strap to be routed back through the outside slot, which greatly reduces the length and brings the harness down on the thighs.

  

The one place where harness fit counts is in the shoulder slot height. When your child is rear-facing, the straps must be at or below her shoulders. Why? The majority of crashes are frontal crashes and in all crashes, everything, including your child, will move toward the point of impact. Your rear-facing carseat will dip down and allow your child to slide up the seat. If the harness is positioned above her shoulders, she’ll slide up until she reaches the harness. Then on rebound—after the crash happens and everything that moved toward the impact comes back and settles—your child will slide back down in her seat. That’s a lot of movement and potential for injury, so keeping her down in her seat in the first place is important.

When forward-facing, you want the straps to be at or above her shoulders to hold her back in a crash. The carseat will again move down and toward the front of the vehicle. If the slots used are below her shoulders, possible spinal compression can occur as she’s held down and her shoulders roll forward.

  

So, for harness use, as long as the shoulder slots are properly positioned, everything else is good (as an end-user, you can’t really redesign where the lap straps are coming out of the carseat anyway). What about belt-positioning boosters and seat belts? Doesn’t using a belt-positioning booster automatically mean proper seat belt positioning?

No.

With boosters, you want the lap portion of the seat belt to fit low on the hips, touching the thighs—and there’s quite a range of what that means. Clothing can get in the way of seeing where the belt actually lies on the lap, so when assessing lap belt fit, it’s good for the child to be wearing snug-fitting clothing. The shoulder belt should be squarely on the shoulder—closer to the neck than the edge of the arm where it may slide off. It should also be noted here that since your child won’t always be wearing snug-fitting clothing, when she’s wearing jeans or other bulky clothing, she should be taught to pull the lap belt snug and low on her hips after buckling.

Younger children tend to have narrow torsos, so getting proper shoulder belt fit on them is more difficult because there’s no room for the seat belt. For a narrow kid like this, it’s better to have the shoulder belt closer to the neck—but not over the throat—so that in a crash, the child can’t slip out of the shoulder belt. Adjusting the headrest up on the booster sets the shoulder belt closer to the neck. The child below shows that she’s too small for a backless booster; she should ideally be in a harnessed seat or at the very least a highback booster so the shoulder belt is positioned better on her. A shoulder belt positioner attached to a backless booster can help too, but not as well as a highback booster. The seat belt is also not adjusted securely on her.

The lap belt portion of the seat belt should be touching or partially laying on the thighs. It can be too far forward on the thighs, which can lead to sliding (submarining) under the seat belt in a crash, and of course, it can sit too high on the soft belly, where injury to internal organs can occur in a crash.

 

Of course we can’t forget about the big kids who fit into seat belts. How do you know if your child is big enough to fit into a seat belt without needing a booster seat? There’s an easy test, called the 5-Step Test which allows you to tell if your kid still needs a booster.

5-Step Test
  1. Does the child sit all the way back on the vehicle seat?
  2. Are his knees bent comfortably at the edge of the vehicle seat?
  3. Does the seat belt cross the shoulder properly on the center of the collar bone?
  4. Is the lap belt low, touching the thighs?
  5. Can the child stay this way for the entire ride?

Vehicle seat belts are designed to fit adult males, not children. It’s not safe for them to sit in one unless it fits them well; they’re at risk for seat belt syndrome, serious head injuries, broken bones, and death. Most kids won’t fit in a seat belt without a booster until around ages 11-12.