Access Type

Open Access Dissertation

Date of Award

January 2011

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Biomedical Engineering

First Advisor

John M. Cavanaugh

Abstract

In 2000 and 2001 an estimated 150,000 children between the ages of 0 and 8 years old were injured or killed in a motor vehicle accident. Despite advances in child safety restraints and vehicle restraints, automobile accidents remain the primary cause of death for children in the 0-8 year old age group. In 1982, in an attempt to reduce the number of deaths and injuries of children, the first child crash test dummy was developed. The responses of this dummy were scaled from the adult response data based on the assumption that children were similar to adults both anatomically and physiologically, only children were smaller. It was also assumed that the soft tissue response, such as muscle force, was the same as for an adult.

Recent studies have shown that not only are children different from adults due to the development of their skeleton, but that their ability to develop muscle force for a given cross-sectional area of muscle is also different. This difference calls into question not only the relationship that was used to develop the child crash test dummies but also the ability of these crash dummies to predict child injuries due to automobile impact.

METHODS

The aim of this study was to determine if the assumption of equivalent stress is appropriate. The muscle response of the neck muscles in 50th percentile adult male was compared to the neck muscle response of the 10-year-old boy under static and dynamic loading conditions. Magnetic resonance imaging was used to measure the muscle length, moment arm and cross-sectional area of the superficial flexor and extensor muscles of the neck.

Two EMG studies were used to analyze the muscle force generated in the neck in response to static and dynamic loads. In the static study, subjects were asked to generate a maximal voluntary contraction (MVC) in four bending directions - flexion, extension and lateral left and right bending. Using and EMG-assisted optimization model, the forces and stresses in the superficial flexor/extensor muscles was calculated. The second EMG study was conducted during a low speed frontal impact in two test conditions - aware and unaware of the up-coming impact. The dynamic moment and displacement of the head were calculated. Latency of muscle activation in response to the onset of swing acceleration and peak swing acceleration were also examined.

RESULTS

Results of the MRI study confirmed the relationship between age and muscle moment arm (r=0.855, p=0.05 for the SCM), and age and muscle cross-sectional area (r=0.741, p=0.05 for the SCM) used in the Wolanin et al. scaling relationship.

Both EMG studies showed that adults were able to generate higher applied moments (p<0.05) and muscle forces and moments (p<0.05) than 10-year old children in the same testing conditions. There was no difference in the stress generated during static loading of the neck muscles. The results of the study did show, however that the neuromuscular efficiency [Grosset, 2008] was significantly higher in adults than in children, suggesting that due to an immature neuromuscular system, children are unable to fully recruit their muscles during a contraction. These results are further supported by the latency results which show that children, in spite of early muscle activation when aware of an impact are unable to generate sufficient muscle tension to reduce the moment of the head or its maximum displacement.

CONCLUSION

The results of this study are unable to contradict the scaling model put forth by Wolanin et al. [1982] since there was no difference found between the adult and child muscle stress under various loading conditions. The results do however suggest that scaling may be more accurate at low speeds if an additional factor were added to the model which takes into account the inefficiencies of the pediatric neuromuscular system.