Cervical electromyographic activity during low-speed rear impact

Abstract

IntroductionRear-end collisions typically occur in dense traffic at very low speeds. The vehicle is subjected to a forward acceleration during which the occupants are pushed forward by the seatbacks. The head lags behind forcing the neck into extension. This head motion continues until the neck hits the headrest or reaches its maximum range of motion, or is counteracted by the muscles. The head then reacts by moving forward into a flexed neck posture. This is the typically described injurious extension-flexion motion of the neck called "whiplash motion" [12]. Hypertranslation of the head was suggested by Penning as being the primary mechanism of whiplash injury [16]. The translatory motion takes place in the upper cervical spine followed by an angular extension in the lower part, forming as S-shape of the cervical spine. This means that the upper cervical spine is undergoing a flexion motion while the lower cervical spine is undergoing an extension motion. When the upper cervical spine reaches its limit for maximum flexion, the lower cervical spine reaches its limit for full extension. The cervical spine then goes into full extension and stops before the motion is reversed [22].The hyperextension of the cervical spine results in compressive forces on the posterior structures, such as the Abstract Whiplash motion of the neck is characterized by having an extension-flexion motion of the neck. It has been previously assumed that muscles do not play a role in the injury. Eight healthy males were seated in a car seat mounted on a sled. The sled was accelerated by a spring mechanism. Muscle electromyographic (EMG) activity was measured by wire electrodes in semispinalis capitis, splenius capitis, and levator scapulae. Surface EMG activity was measured over trapezius and sternocleidomastoideus. Wavelet analysis was used to establish the onset of muscle activity with respect to sled movement. Shorter reaction times were found to be as low as 13.2 ms from head acceleration and 65.6 ms from sled acceleration. Thus the muscles could influence the injury pattern. It is of interest that clinical symptoms are often attributed to muscle tendon injuries.

The role of sternocleidomastoid muscle in simulated low velocity rear-end impacts

“…Magnusson et al [11] did not report significant differences in cervical muscle reaction times between expected and unexpected conditions. Kumar et al [9] reported lower SCM EMG activity in expected impacts.…”

“…Among the cervical muscles, the sternocleidomastoid (SCM) muscle might play a significant role in whiplash injury [2,9,11,13,16]. Magnusson et al [11] did not report significant differences in cervical muscle reaction times between expected and unexpected conditions.…”

Cervical muscle response to whiplash-type right anterolateral impacts

“…Given ethical concerns with subjecting volunteers to injurious neck perturbations, most volunteer experiments have, for example, been conducted with military personnel and members of the research team. A few other experiments have been done with other volunteer groups but were necessarily limited to low-or very low-velocity collisions [1,3,4,7,9,10,14].…”

Turning away from whiplash. An EMG study of head rotation in whiplash impact

“…[4, p. 331). Yet, on the basis of studies showing that the neck does not move beyond the physiological range for lowvelocity impacts [9,10,15,16] and that an eccentric muscle contraction occurs in response to impacts [9], muscle injury remains the most likely candidate for site of tissue injury. Only through more dedicated imaging protocols is the muscle sprain likely to be captured, but part of the process of understanding where to look for the whiplash injury may begin with whiplash experiments pointing to the tissues at greatest risk for injury.…”

“…This body of research has been primarily motivated by a desire to prevent or lessen the extent of whiplash injury, and is, for example, the basis for the introduction in 1969, of the head restraint [13]. Based on human research [5][6][7][8][9][10][11][12]15,16], it has been suggested that for rear-end impacts we can understand the injury mechanism to be a whip's lash, in the very sense that Crowe first postulated when he coined the term whiplash in 1928 [2].…”