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).

Factors affecting ejection risk in rollover crashes.

Ejection greatly increases the risk of injury and fatality in a rollover crash. The purpose of this study was to determine the crash, vehicle, and occupant characteristics that affect the risk of ejection in rollovers. Information from real world rollover crashes occurring from 2000 - 2010 was obtained from the National Automotive Sampling System (NASS) in order to analyze the effect of the following parameters on ejection risk: seatbelt use, rollover severity, vehicle type, seating position, roof crush, side curtain airbag deployment, glazing type, and occupant age, gender, and size. Seatbelt use was found to reduce the risk of partial ejection and virtually eliminate the risk of complete ejection. For belted occupants, the risk of partial ejection risk was significantly increased in rollover crashes involving more roof inversions, light trucks and vans (LTVs), and larger occupants. For unbelted occupants, the risk of complete ejection was significantly increased in rollover crashes involving more roof inversions, LTVs, far side occupants, and higher levels of roof crush. Roof crush was not a significant predictor of ejection after normalizing for rollover severity. Curtain airbag deployment was associated with reduced rates of partial and complete ejection, but the effect was not statistically significant, perhaps due to the small sample size (n = 89 raw cases with curtain deployments). A much greater proportion of occupants who were ejected in spite of curtain airbag deployment passed through the sunroof and other portals as opposed to the adjacent side window compared to occupants who were ejected in rollovers without a curtain airbag deployment. The primary factors that reduce ejection risk in rollover crashes are, in generally decreasing order of importance: seatbelt use, fewer roof inversions, passenger car body type, curtain airbag deployment, near side seating position, and small occupant size.

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.