Among the different myths surrounding extended rear-facing car seats, there is one that is especially worrying because it does not concern comfort but safety. It is the fear that travelling backwards and with less space to accommodate the legs, these may break in case of an accident. How much is true in this widespread fear?

What can break the child´s legs in an impact?

This is the first point that we should clarify. In case of suffering a fracture, how would it occur? Well, in the most obvious way: by the impact of one’s legs against a hard surface.

In an accident or abrupt braking, all the elements inside the vehicle are propelled forward with the inertia that it had at the time of the impact, both the objects that we can carry in the car and our bodies.

Therefore, in a collision, the legs will always move in the same direction as the head: towards the front of the car. We already know that the back of the head is secured and protected by the back of the chair and does not suffer any traction but what happens to the legs?

When affected by the same inertia as the other elements of the vehicle, the legs are flexed towards the torso and stretched again. If we watch some videos of impact tests or crash tests, we can see that the legs do not impact, at first, against any surface riding rear-facing. In the return movement, the legs would only break if the chair also suffered a setback, something that does not happen in rear-facing seats as they have specific security elements that prevent this from happening, such as the anti-rebound bar or lower tethers (straps that hold the chair to the bottom of the vehicle).

But isn’t it frequent when riding rear-facing?

A fracture in the lower limb is much more likely and frequent when riding front-facing than in the opposite direction, and the reason is simple: in this case, the legs DO hit the front seat when thrown violently forward in the collision. As we are most concerned with cervical lesions, we tend to focus only on what happens in the head and neck but if we look at a crash test in a front-facing car seat we can see how the legs greatly exceed the impact line. What is the impact line? It is the point that defines where the vehicle´s front seat is. That is, everything that goes beyond that red line will directly impact on the front seat. It is important to know that in the homologation tests it is only the head what does not exceed the impact line. The legs, as we can see, do.

So, would the traction force break the legs riding rear-facing as well as it breaks the neck riding front-facing?

No, it wouldn`t, because there is no distal “pull” force. In front-facing car seats, the weight of the head turns it into a projectile that is shot forward with the inertia of the vehicle causing strong traction on the neck, like a plumb line that pulls a rope and tightens it under the risk of harming it. That pulling is what can cause cervical and spinal injuries. When rear-facing, however, the legs ‘shrink’ towards the rest of the body. They are not pulled from the end (the feet) applying force on the pelvis; on the contrary, they flex and then stretch again, with a much lower kinetic energy in the backward movement. In addition to this, the differences between one structure and another are decisive. The head of a two-year-old child weighs about 2.5 kg and the cervical area is immature and weak. In lower limbs, the more distal the joint, the less weight and size it has so that it hardly generates traction on the proximal, stronger, larger and heavier joints. However, even if it were so, an impact with forces of inertia so high as to cause an injury to the lower limb riding rear-facing would result in much more serious neck injuries in a front-facing position, generally irreversible and even fatal in some cases. Between a possible broken leg or an internal decapitation, what would we prioritize when it comes to protecting our children?

But, there is less space between the legs and the seat, isn’t it dangerous?

The small space between legs and the back seat of the car would only be significant or dangerous if the inertia violently pushed the legs in that direction, hitting us while driving backwards at a certain speed.

What happens if the car is crushed?

In case the impact causes the vehicle to collapse or there is an intrusion, the space between the legs and the rear seat could be compressed (as is the case between the space between our legs and the glove box of the vehicle when we travel in the passenger seat) and cause lower limb injuries. However, the same situation would be even more serious if riding front-facing as it would move the entire chair forward, making the distance between the body and the front seat disappear and causing more serious injuries.

Car seat guide

Source: Marta Erill, Physiotherapist in collaboration with Ana Irene Urbieta Quiroga Professor of physical sciences, Universidad Complutense de Madrid