Human Response to Low Speed Impact

One of the most baffling problems with low speed rear end collisions is that the response of the automobile occupant is totally unpredictable. Some patients can experience sizable accelerations from such collisions and report no symptoms; others can be in lower speed impacts and suffer long-term pain and disability.

In an effort to determine what factors may be involved in causing injury, a recent study examined the effects of such collisions in a group of 42 test subjects—21 men and 21 women. The study had three goals:

1. Assess the difference in response between men and women. Many previous studies have reported that women are more likely to suffer whiplash injuries than men are.
2. Which individual subject variables would predict symptoms after the collision. These factors included the ratio of head to neck size, patient neck strength, and the distance between the head and the head restraint.
3. Assess the long-term clinical outcome for those with symptoms.

All of the test subjects were given a thorough physical examination that included cervical ROM (range of motion), measures of tenderness, muscle strength testing, and reflex grading. Each subject was then exposed to a collision with a ?V of 4 km/hr (2.5 mph) and 8 km/hr (5.0 mph). The two separate test collisions were performed seven days apart (or seven days after the end of symptoms from the first collision.

All subjects were evaluated 30 minutes after the test collision, and were also required to fill out a symptom questionnaire four times within the following 24 hours. At 24 hours, the subjects were again physically examined.

The following symptom distribution was found in the test occupants. As you can see, the most common complaints are identical to those experienced by all whiplash patients:

Very few of the test subjects reported symptoms at the 30-minute posttest evaluation—no women did at 4 km/hr and only one woman did at 8 km/hr. Most of the symptoms were reported within the 24 hours after the collision. In total, 29% of the subjects reported symptoms from the 4 km/hr collision and 38% did from the 8 km/hr collision.

Contrary to previous studies, this report found no difference in whiplash-related symptom occurrence between the men and the women. They did, however, find a difference in the duration of symptoms between men and women, as the following chart illustrates:

Duration of Symptoms, In Hours

The study reported that, statistically speaking, only the 4-km/hr collision showed any gender differences—women reported symptoms lasting six times longer than did men. The averages only tell part of the story. Looking at the data in the chart above, we see that at least one male experienced symptoms from the 4 km/hr collision that lasted 111 hours, or 4.6 days. And that's from a 2.5-mph change in velocity.

The fact that occupants in low speed collisions suffer symptoms for a few days is not especially new. The most interesting finding from this study is related to the range of motion measurements taken before and after the collision on each subject:

"Analysis of the effect of the 4-km/hr impact severity on ROM measurements over time revealed that, at both postimpact examinations, subjects with and without symptoms had a significant decrease in cervical flexion, extension, retraction, and right lateral flexion, with left lateral flexion ROM approaching statistical significance."

The same results were found in the 8-km/hr collisions.

The key here is that all of the test subjects had reduced ROM—even if they did not have symptoms. The authors suggest that these changes in ROM are physiological:

"Most interesting from a clinical perspective was the fact that the cervical ROM deficits were characteristically similar to the subjective reports of symptoms in that both were transient. In most cases, the ROM deficits were present immediately postimpact, but by the 24-hour post impact physical examination, cervical ROM had returned to normal, consistent with most subjects' short duration of symptoms."

Even though these results suggest some injury mechanism at work in low speed collisions, most test subjects had short-term symptoms, if any. Why do some people in real world collisions experience long-term pain from the same type of collisions?

Because they are not the same type of collisions. The following details illustrate the difference between these test collisions and real-world collisions.

1. All of the subjects in these tests had no history of neck or back pain or any soft-tissue condition. In the real world, many accident victims have some kind of pre-existing soft-tissue problem.
2. They were all 20 to 40-years-old and were in good health.
3. They were all sitting in the car seat relaxed, with hands on laps, and with head in the proper face-forward position.
4. All had properly adjusted head restraints.

Brault JR, Wheeler JB, Siegmund GP, Brault EJ. Clinical response of human subjects to rear-end automobile collisions. Archives of Physical Medicine and Rehabilitation 1998;79:72-80.