The human head/neck system is a rotational, biological model that can be represented as an inverted pendulum and approximated as a second order system. The central nervous system responds reflexively to external perturbations by activating the neck muscles to return the head to neutral position. This study consisted of 3 parts. A device to examine the reflexive response of the human head/neck system was built and tested. An inverted pendulum was tested to verify that the methods will identify the system parameters of an inverted pendulum. Finally, 2 human subjects were tested with the device to determine the muscle onset latencies and kinematics for the head/neck system following a perturbation to the head.

The motion in the frame designed for this experiment was less than 1% of the motion of the force applicator during trial runs and thus proved sufficiently stiff to meet our requirements.

The inverted pendulum’s system parameters were identified by a second order model. The estimated stiffness averaged a 6% error when compared with the known spring stiffness. The damping coefficient was consistent throughout the trials.

The human subject results showed that, for our experiment, the head/neck system should not be approximated as a second order system. It demonstrated a brief period of second order vibrations but did not oscillate about its initial position. The maximum head motions showed different results between subjects and perturbation directions. Subject 1 moved more when their head was pushed forward while subject 2 moved more when their head was pushed back.

The muscle onset latencies (time between force onset and muscle activation) averaged between 20 and 90 ms. These were within the range of other studies’ results. Subject 1 showed no statistically significant difference in onset latencies between forward and backward perturbations. Subject 2 showed differences in most of their muscle onset latencies between directions. This suggests a different response in forward versus backward motion.

Overall, we achieved our goals. This study allows other investigators to continue the testing later with more subjects under different conditions. This will lead to a better understanding of the control of the human head/neck system.