Author: Richard H. Adler

The nature of cervical hyperflexion/hyperextension injuries and their importance in the medical arena and field of highway safety is well documented. Despite the mass of clinical references and increasing numbers of experimental papers in the literature, however, there has been a scarcity of information on the basic response characteristics of the human neck structure and musculature to acceleration/deceleration injury.

Recently, a study was published by researchers at the University of Michigan defining the normal range of motion, muscle reflex time, and muscle strength in the neck for a group of subjects representative of the United States adult population.¹ The study population was chosen based on three variables: sex, age, and body size. Sex was chosen as a primary variable because of indications that females more often incur whiplash injury than males.² Age was considered an important variable as well, since it is generally believed that the aging process adversely affects both joint range of motion and muscle reflexes. Body size or stature was included as the third primary variable on the assumption that neck characteristics could be predicted based on a person's overall height, sitting height, and neck length.

The large amount of data collected during the study produced many results never before available in the literature. These include:

1. Cervical range of motion of females was found to be greater than that of males, depending upon age; a definite degradation in range-of-motion was observed with increasing age.


2. Average neck muscle reflex times ranged from 56 to 92 milliseconds for flexors and 54 to 87 milliseconds for extensors, with males being generally slightly slower reacting. This finding is significant because it means that cervical muscles generally cannot be activated in sufficient time to mitigate the hyperextension effects of a surprise rear-end collision. As the authors state:
   
   
   ...the worst of the crash may be over before the muscles can become fully effective. Therefore, in the situation of complete surprise, it is doubtful that the cervical muscles can be of much help in the high speed rear-end collision. (page 1034)
   


3. There is a definite aging effect noticed with reflex time -- a pronounced degradation of reflex time between middle and elderly age groups.


4. On average, the stretch reflex was elicited with head accelerations at the c. g. of approximately 0.25 g.


5. The time required to stop the head after initial detection of the muscle reflex was slightly longer for neck extensors.


6. Neck flexor strength tests revealed that males are, on the average, stronger than females in both flexor and extensor strengths in every age and stature group. Sex also seems to influence the effects of age. Females tend to decrease gradually in neck strength, but only slightly throughout their lives, while males are often stronger at middle age than when they were young.


7. Strikingly similar results were obtained from the extensor muscle tests as noted above in paragraph #6.


8. Males and females exhibit different aging characteristics.


9. Age and sex were found to be important factors in cervical flexibility and response characteristics of the neck.


10. Extensor muscles, even though they had a faster stretch reflex, take longer to activate than do the flexors. The authors suggest:
    
    
    A possible physiological explanation for this lies in the anatomy of the muscles. The primary neck flexor is the sternomastoid, a single muscle, whereas the neck extensors are the semispinalis capitis, the splenius capitis, a various occipito-spinal and interspinal muscles -- all of which are relatively short and layered successively deeper in the neck. It is probable that the neural control system takes longer to activate the complicated system of extensor muscles sufficiently to stop forward head motion than it does to activate the single muscle necessary to stop rearward motion. (page 1033)
    

We hope you find this information useful in the evaluation and treatment of acceleration/deceleration type of injury.

Very truly yours,
ADLER GIERSCH, P.S.

Richard H. Adler
Attorney at Law

¹ David R. Foust, et al., "Cervical Range of Motion and Dynamic Response and Strength of Cervical Muscles," in Stanley H. Backaitis (ed.), Biomechanics of Impact Injury and Injury Tolerances of the Head-Neck Complex, Society of Automotive Engineers, Warrendale, Penn., pp. 1023 - 1034.
² B. O'Neill, et al., "Automobile Head Restraints -- Frequency of Neck Injury Claims in Relation to the Presence of Head Restraints," American Journal of Public Health, Vol. 62, March 1972, pp. 399 - 406.