Biodynamic models

Biodynamic models of the human musculoskeletal system have direct implications on device tool design and use and the modeling of normal and/or abnormal (or undesired) movements or movement patterns (the techniques with which a device or tool is used). Applications of the models can provide a better understanding for soft and hard tissue injuries, such as repetitive strain injuries (RSI), and can be used to identify and predict the extent of a musculoskeletal injury (Peterson, 1999). 

Biodynamically Modeling the Upper or Lower Extremity by the Table Method... 

Equinoxious frequency contours may be estimated from epidemiological studies of health effects, or from the response of human subjects, animals, cadavers, or biodynamic models to the stimuli of interest Human subjects cannot be subjected to injurious accelerations and forces for ethical reasons, and so little direct information is available from this source. Some information has been obtained from studies of accidents, though in most cases the input acceleration-time histories are poorly known. 

Frequency Weighting. The inverse frequency contour (i.e., reciprocal) to an equinoxious contour should be applied to a stimulus containing many frequencies to produce an overall magnitude that appropriately combines the contributions from each fr uency. The frequency weightings most commonly employed for whole-body and hand-transmitted vibration are shown in Fig. 10.1 (ISO 2631-1,1997 ISO 5349-1, 2001). The range of frequencies is from 1 to 80 Hz for whole-body vibration, and from 8 to 1250 Hz for vibration entering the hand. A frequency weighting for shocks may also be derived from a biodynamic model (see Dynamic Response Index (DRI) in Sec. 10.3.1). 

FIGURE 10.5 Idealized values for the modulus and phase of the seat-to-head transmissibility of seated persons subjected to vertical vibration. The envelopes of the maximum and minimum mean values of studies included in the analysis are shown by thick continuous lines, and the mean of all data sets is shown by the thin line. The response of a biodynamic model (see text and Fig. 10.8) is plotted as a dash-dot line. (ISO 5982, 2001.)... 

Knowledge of tolerable limits for human exposure to vibration, shock, and impact is essential for maintaining health and performance in the many environments in which man is subjected to dynamic forces and accelerations. As already noted, humans cannot be subjected to injurious stimuli for etbical reasons, and so little direct information is available fixim this source. In these circumstances, the simulation of human response to potentially life-threatening dynamic forces and accelerations is desirable, and is commonly undertaken using biodynamic models, and anthropometric or anthropomorphic manikins. They are also used in the development of vehicle seats and, in the case of hand-arm models, of powered hand-held tools. 

Simple Lumped Models. At frequencies up to several hundred hertz, the biodynamic response of the human body can be represented theoretically by point masses, springs, and dampers, which constitute the elements of lumped biodynamic models. The simplest one-dimensional model consists of a mass supported by a spring and damper, as sketched in Fig. 10.6, where the system is excited at its base. The equation of motion of a mass m when a spring with stiffness k and damper with resistance proportional to velocity, c, are base driven with a displacement x it) is ... 

FIGURE 10.6 Smgle.degieeK>f-fi elumped-parameter biodynamic model. The mass m is supported by a spring with stiffness k and viscous damper with resistance c. The transmis-sibility of motion to the mass is shown as a function of die fiequency ratio r (=aj/oib) when the base is subjected to a displacement (After Griffin, 1990.)... 

Whole-Body Apparent Mass for Vertical (z-Direction) Vibration. The apparent mass of the seated human body may be described by a variant of the simple biodynamic model of Fig. 10.6. A satisfactory prediction is obtained with the addition of a second mass, / o, on the seat platform to represent the mass of the torso and legs that does not move relative to the seat (i.e., to the base of... 

FIGURE 10.7 CompaTison between predicted and observed apparent mass. The mean, nonnalized apparent masses of 60 subjects, 1 standard deviation, are shown by the continuous lines, and the prettictions of a sinj e-d iee-of-hreedom, lumped parameter biodynamic model by the dashed line. Fairley et at., 1989.)... 

FIGURE 10.8 Three-degree-of-fteedom, lumped-parameter biodynamic model of the seated human body for estimating mechanical impedance, apparent mass, and transmissibility, for vertical vil tion. The model is driven at the base (x ), and the transmissibility is calculated to the head, mass m2. (ISO 5982,2001.)... 

That concern about additives leaching from microplastics was addressed in a study examining the fate of plasticizers in ingested plastics. Using a biodynamic model, researchers explored the potential of leaching of nonylphenol (NP) and bisphenol A (BPA) from plastics in the intestinal tracts of the lug-worm Arenicola marina) and North Sea cod Gadus morhua). They concluded that this was "not likely to constitute a relevant exposure pathway" [155]. 

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