Kinematics and kinetics of vigorous head shaking.

Previous studies on neck muscle strength and motion have assumed or imposed varying constraints on the heads and bodies of the subjects. In this study, we asked 20 subjects to vigorously shake their heads 5-10 times in a completely unconstrained manner. The kinematics and kinetics of the head and neck were measured from video analysis and instrumentation mounted inside the mouth. Subjects shook their heads at self-selected tempos ranging from 1.9-4.7 Hz over a 20-91° range of motion. The motion of each subject's head could be approximated by a fixed center of rotation that was typically located in the midcervical spine, but varied widely among subjects. Significant differences between men and women were observed. Peak head accelerations were low (4.3 ± 1.1 g and 250 ± 103 rad/s2 for men, 3.0 ± 0.9 g and 182 ± 58 rad/s2 for women) and estimated peak generated neck moments at C7/T1 were comparable to values reported in isometric neck strength studies (47 ± 14 N·m in extension and 22 ± 9 N·m in flexion for men, 25 ± 8 N·m in extension and 9 ± 7 N·m in flexion for women).

Head and neck loading in everyday and vigorous activities.

The purpose of this study was to document head and neck loading in a group of ordinary people engaged in non-injurious everyday and more vigorous physical activities. Twenty (20) volunteers that were representative of the general population were subjected to seven test scenarios: a soccer ball impact to the forehead, a self-imposed hand strike to the forehead, vigorous head shaking, plopping down in a chair, jumping off a step, a seated drop onto the buttocks, and a vertical drop while seated supine in a chair. Some scenarios involved prescribed and well-controlled stimuli, while others allowed the volunteers to perform common activities at a self-selected level of intensity. Head accelerations up to 31 g and 2888 rad/s(2) and neck loads up to 268 N in posterior shear, 526 N in compression, and 36 Nm in extension were recorded. Most head and neck injury criteria predicted a low risk of injury in all activities. However, rotational head accelerations and Neck Injury Criterion (NIC) values were much higher than some proposed tolerance limits in a large number of tests, all of which were non-injurious. The data from this study help us to establish an envelope of head and neck loading that is commonly encountered and presents a minimal risk of injury.

Relationship between linear and rotational head acceleration in various activities.

Head injury is typically predicted using linear or rotational acceleration-based injury criteria. In many cases, the linear components of head acceleration can be determined more easily than the rotational components. Peak rotational head acceleration (apeak) can be calculated from the peak linear head acceleration (apeak) by assuming a value for the effective radius of rotation (r) of the head (apeak = apeak / r). Empirical values for the effective radius of rotation were calculated using linear and angular head acceleration data from 20 human volunteers subjected to a wide variety of test scenarios, including a soccer ball impact to the forehead, a voluntary hand strike to the forehead, voluntary shaking of the head, plopping down in a chair, and a vertical drop while seated supine in a chair. In addition, values for the effective radius of rotation of the head were calculated for American football, boxing, and frontal and rear end automotive impacts using data from the literature. The range of values for the effective radius of rotation of the head for each activity was characterized statistically by calculating median, middle 50%, and middle 95% values from the cumulative distribution. Median values for the effective radius of rotation of the head ranged from 84 mm for boxing to 376 mm for voluntary hand strikes to the forehead. It is important to note that the concept of an effective radius of rotation of the head is simply a convenient method for expressing an empirical relationship between peak linear and peak rotational head acceleration, and does not represent an accurate model of the kinematics of the head. In most of the activities studied, the head kinematics were complex, and the center of gravity of the head did not rotate at a constant radius about a fixed point.