Mathematical Modeling of the Muscle Effects on the Human Body Responses under Transient Loads - Example of the Head-Neck Complex
Abstract: The goal of this study was to develop and apply a method that overcomes limitations of the currently available solutions for finite-element modeling of skeletal muscles and their effects on dynamic responses of the human body under transient loads.One of such limitations is that finite element codes commercially available in the field impact biomechanics do not contain any specific procedures for simulation of skeletal muscles.Therefore,a new type of finite element for such simulation was developed and implemented in the PAM-SAFE code in this thesis.In this implementation a concept of Hill-type multi-bar muscle element was invented.This concept exhibited sufficient accuracy and numerical stability in the prediction of muscle force under transient loads. Therefore,it was applied in the investigation of the muscle effects on dynamic responses of the head-neck complex in frontal and rear-end impacts. The results of the investigation of the muscle effects on the kinematics of the head- neck complex in a frontal impact at an acceleration of around 140 m/s 2 (15 g)suggest the following. When the cervical muscles are activated at around 25-50 ms after the start of such an impact,they are likely to appreciably reduce the peak values of the displacement and acceleration of the head-gravity center. When the activation of cervical muscles is assumed to start at around 100 ms or later,their effects seem to be negligible. The modeling of the muscle effects on the cervical spine responses in rear-end impacts at low speed improved the understanding of the mechanisms of injury to cervical facet joints. The results suggest that the action of the cervical muscles may limit the stretch of the capsules of such joints,which is a protective effect. The reflex times of the cervical muscles,which constitute the necessary input data for modeling of the muscle effects on the head-neck complex responses under transient loads, were obtained from electromyographic signals (EMG)of these muscles. The present method for determination of the reflex times complemented the EMG analyses conducted so far in the field of impact biomechanics. Its application yielded a reflex time of the sternocleidomastoidmuscle of around 80 ms in a rear-end impact at a speed of 6 km/h.
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