Muscle activity influence on the kinematics of the cervical spine in frontal tests.

Objective: The question of muscle activity influence on the cervical spine kinematics during rear-end and frontal crash events has been discussed. Less data are available concerning frontal collisions. Therefore, the objective of this study was to investigate the influence of the ventral and dorsal neck muscles on the cervical spine kinematics during simulated frontal sled collisions.

Methods: A frontal collision with a velocity change (delta V) of 10.2 km/h was simulated in a sled test with 10 healthy subjects (7 female; 3 male). A high-speed camera and accelerometers recorded the motion and acceleration data. The activity of the sternocleidomastoid muscles was recorded with surface electrodes. To avoid cross-talk, an intramuscular recording of the semispinalis capitis muscles was performed with fine-wire electrodes.

Results: The sequence of both muscle activities was reproducible in all subjects. The maximal force of the sternocleidomastoid muscle was observed after a median of 152 ms, with 0 defining the time of the trigger signal. With earlier onset of muscle force, maximal dorsal horizontal acceleration of the head (r = -0.600) was reached later and the ventral translation (r = -0.733) and flexion movement (r = -0.755) set in earlier. The maximal force of the semispinalis capitis muscle was observed after a median of 160 ms. If the duration of muscle force was longer, the maximal head flexion (r = 0.685) and the maximal ventral head translation (r = 0.738) were reached later.

Conclusions: The sternocleidomastoid muscle force is mainly associated with the horizontal head acceleration and influences the onset of the flexion and translation motion. To summarize, these temporal correlations allow the conclusion that the semispinalis capitis muscle force is mainly associated with the angular head acceleration and influences the duration of the flexion and translation motion.