THE TOLERANCE OF MAN TO IMPACT

Within the past few years there has been a tremendous increase of interest in the effects of crashes on man. This is due, of course, to the increasing concern with automobile and airplane crash casualties, and to the associated problems of shock protection packaging of man for the astronaut programs. This concern is also reflected within the armed services where, in addition, problems caused by air and underwater blast are present. Any rigorous attack on this problem requires an answer to the question, “At what level of impact will a man be injured?” One would expect such an answer to be readily available. The fact is that, as in so many apparently simple questions, there is no direct answer; it all depends on how it happened and to whom. Even when narrowed down to specifics, there are no answers in which we can have a high degree of confidence. The reason for this is apparent. Since subjecting volunteers to injury-producing impacts is unthinkable, we must resort to extrapolation from animal tests, deductions from biomechanical modeling, and cadaver tests, and reconstruction of accident situations, as well as noninjurious human testing. While none of these techniques alone can give a complete answer, the results from a combination can be useful in setting tolerance limits. I shall show some of the methods used and results obtained in our efforts to assimilate the variety of impact data. At first examination, tolerance figures are confusing. We know, for example, that our men in centrifuge tests will suffer considerable distress when subjected to accelerations as low as log. We know that Stapp’ and Beeding and Mosley? in their rocket sled, when subjected momentarily to accelerations of 40g were severely shaken. Yet DeHaven’s3 study of falls indicates that some people suffered only slight injury from accelerations of 200g, and men in our laboratory4 are routinely subjected to 100 to 200g without complaint. How can we explain this inconsistency? It would seem that specification of a so-called g load is not sufficient for defining a tolerance level. In 1961, this problem was discussed at some length by Kornhauser and Lawton.’ They pointed out that man, like any other combination of masses and springs, has some mechanical response characteristics to a blow, and that the nature of the response is sensitive to both the amplitude and time characteristics of the blow. In a simplified view, one could expect that man would react to the force directly if it were applied for a long duration, or he would react to a forcetime component or impulse when the force application was of short duration. How long is “long” depends upon the frequency response of the system damaged. Kornhauser applied this idea to mice subjected to drop tests, and the results are shown in FIGURE 1. He also attempted from the then existing data on humans to draw a similar curve for man’s tolerance. We have generated some new information and reevaluated some of the oId (FIGURE 2). For example, the two sled rides in Stapp’s experiments which resulted in what we considered injuries reached plateau values of about 45g and 39g and were maintained at those levels for more than 100 msec. Beeding lost consciousness in a ride terminating in a deceleration of about 40g sustained for about