SPINAL INJURY CONTROL - Injury Biomechanics and Control: Optimal Protection from Impact - page 179

TABLE 5.1 Threshold Values of DRI

DRI Value Risk of Spinal Injury

15.2 Low

18.0 Medium

22.8 High

3

2

1

0

0 50 100 150

Time (ms)

FIGURE 5.5 Optimal time history of the compression of the vertebral column.

This function, plotted in Fig. 5.5, monotonically increases from zero and
tends to a value of P/K as t increases. As is apparent from the figure,
the limiting value of the spine compression P/K is practically attained by
the time t T . Therefore, this quantity can be used to evaluate the DRI

criterion defined by Eq. (5.34):

=

P
m 1 g =

DRI = 7 . (5.36)

According to Table 5.1, this value is below the threshold of low risk of
injuries.

5.2.3 Trade-Off Curve: Reciprocal Problem

In the previous section, the minimum excursion for an occupant of a seat
subjected to a vertical shock pulse was determined, provided that the spinal
compressive force is constrained, and the optimal control to ensure this
excursion was constructed. Problem 5.1 was solved for the parameters of
the model specified by Eqs. (5.8), (5.18), and (5.19). These parameters
are typical of the spine injury model and the impact pulse characteristic
of a helicopter crash landing. In addition, the solution of the problem for
these specific parameters allowed all stages of the construction of the opti-
mal control using the technique of Chapter 4 to be reproduced. Of course,