Relationship between HIC15, HIC36, Peak Acceleration and Pulse duration
A ssume acceleration to be in the form of a half sine pulse () where A is the peak acceleration and T is the pulse duration, as shown in the figure 1.

Figure 1. Acceleration pulse in form of a half sine with peak A and duration T.
For a given peak value of HIC15 and pulse duration, T, the peak acceleration can be computed by the following formulas (Chou, C. C., Nyquist, G. W., “Analytical Studies of the Head Injury Criterion (HIC),” SAE Paper No. 740082, Society of Automotive Engineers, 1974.)

Similarly, for a given peak value of HIC36 and pulse duration, T, the peak acceleration can be computed by the following formulas (Equation 2)

Using Equations 1 and 2, contour plots of HIC15 and HIC36 levels for different peak acceleration and pulse durations can be made as shown in Figure 2. For an 80g peak pulse, a HIC15 level of 700 is achieved with a pulse duration of 35 msec.

Figure 2: Peak acceleration A versus pulse duration T for HIC15=700, 1000, 1700 and HIC36=1000, 1700.
Figure 2 suggests that for a pulse duration of say 10 msec, the peak acceleration allowed for a HIC15=700 threshold level is 120 gs while the peak allowable acceleration for HIC15=1000 is 140 gs. The HIC15=1000 and the HIC36=1000 contour are identical for a pulse duration up to 22.4 msec after which the HIC15 curve is less conservative than the HIC36 curve. Therefore if the head acceleration pulse duration is longer than 22.4 msec, utilizing the HIC36=1000 is more protective than using HIC15=1000.