BOO! Did that startle you? Of course not. But if you received a sharp blow from behind, the chances are that almost instantly your eyes would close, your heart rate shoot up, your breathing quicken, your neck and shoulders tense, the muscles in your chest and stomach stiffen, your elbows stick out and your hands clench.
These are the classic signs of the startle response, which protects us from bodily harm. The response (or reflex) may have evolved to limit the damage caused by predators attacking us from behind, or from falling coconuts or jackfruit. Unfortunately, even the best evolutionary mechanism can come unstuck in our modern world, and researchers have found that in car crashes, rather than protecting us, the startle response could contribute to or even exacerbate whiplash injuries.
The startle reflex is the most extensive of all reflexes, involving hundreds of muscles rather than just local ones as in the knee-jerk reflex. It occurs when people are exposed to unexpected stimuli such as loud noise, touch or blows. All three occurring together 鈥� as in car crashes 鈥� produces the greatest effect.
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The response protects by causing hundreds of competing muscles to contract suddenly all over the body, making it stiffer and less likely to be penetrated or damaged. By making our elbows point outwards while keeping the arms close to the body with fists clenched, sensitive areas such as soft tissue around the waist are as protected as possible. The fast, hard breathing and rapid heartbeat makes us ready to respond to events.
鈥淭he startle reflex involves hundreds of muscles鈥�
Where does whiplash come in? It鈥檚 an injury that can cause a variety of musculoskeletal complaints, such as stiffness, dizziness and neck pain ranging in severity from mild discomfort to persistent, chronic and debilitating pain. It costs the world billions every year in insurance claims, hospital fees and days off sick.
For years, whiplash was thought to be caused by a sudden acceleration of the head relative to the body, thereby damaging the neck鈥檚 joints and muscles. But because being unprepared for a rear-end collision was known to be one of the few factors associated with greater risk of whiplash, researchers wondered if the startle response was involved.
They wired up the muscles of different groups of volunteers, and put some through a single rear-end collision and others through repeated collisions. Sure enough, they spotted the telltale muscle signature of the startle response 鈥� but only in those who were taken by surprise (The Journal of Physiology, vol 573, p 857).
Now it looks as if the exaggerated and inappropriate muscle contractions produced by the startle response explain the higher incidence of whiplash in people who are not prepared for a rear-end car crash.
The good news is this research might help reduce or prevent whiplash. Watch out for novel seats and headrests. Alternatively, take up walking.
