Muscle Injury After Whiplash

Neck pain and headache are the most common symptoms after rear end collisions, and most of the symptoms that are experienced immediately after a crash are due to muscle injury. A new study1 by leading whiplash researchers has examined the role that muscle plays in auto injuries; specifically, they looked at when the muscles of the neck became activated during a crash and what role that activation would play in spinal injury.

The researchers subjected test subjects to controlled rear-end collisions in which they measured muscle activity through EMG and recorded the crashes using high-speed video. With this data, they were able to identify at what point the various muscles of the neck became active and, using the video record, were able to determine the length of the muscles throughout the test impact. They were able to combine this data to determine the amount of strain that the individual muscle groups experienced during the crash.

0 milliseconds Before impact	75 milliseconds Ligament injury occurs	150 milliseconds Maximum head extension	300 milliseconds Maximum head flexion

The study found the following:

• None of the muscles of the neck became activated until about 100 milliseconds – 25 milliseconds after the point where the majority of the ligament injury occurs.
• The muscles of the front of the neck reach their maximum stretch at around 150 milliseconds, but the muscles do not reach full contraction until a bit later. The authors determined that injury to the muscles of the front of the neck is not as severe.
• The muscles of the back of the neck, however, experience a different pattern. They also begin to activate at about 100 milliseconds, so that they are fully contracted just when they reach their maximum stretch at 250-300 milliseconds.

The authors write that it is a combination of two factors that make the muscles of the back of the neck susceptible to injury: they are contracting while being lengthened, and they are contracted into their shortest length (at the 150 millisecond extension phase) and then rapidly stretched (during the 300 millisecond rebound phase):

“Whether short or long initial lengths lead to greater injury, we found that both the fascicle strain and strain rate calculated for the neck muscles during whiplash exposures exceed those which have been found previously to cause injury in maximally activated muscle.”

Muscle Activity Occurs Too Late to Prevent Spinal Injury

One issue of contention in the whiplash literature is that of muscle contraction and whether it can help prevent injury. Many previous studies have shown that occupants who are unaware of the impending crash are more likely to be injured, because they do not have a chance to tense the neck musculature in anticipation.

In fact, a previous study by Siegmund et al.2 found that in live crash tests, occupants who were exposed to more than one

crash exhibited a dramatic reduction in neck movement, since they were tensing their necks in anticipation of the crash.

This study found that the neck muscles did not begin to tense until 100 ms – 25 milliseconds after the primary spinal injury has occurred, showing that an unaware occupant will experience little protection from neck muscle contraction. When interviewing whiplash patients, it is important to ask whether they were aware of the impending crash.

1. Vasavada AN, Brault JR, Siegmund GP. Musculotendon and fascicle strains in anterior and posterior neck muscles during whiplash injury. Spine 2007;32(7):756-765.
2. Siegmund GP, Sanderson DJ, Myers BS, Inglis JT. Rapid neck muscle adaptation alters the head kinematics of aware and unaware subjects undergoing multiple whiplash-like perturbations. Journal of Biomechanics 2003;36(4):473-482.