From: matthews@ajsh.colorado.edu (Alex Matthews)
Subject: Re: Fatal motorcycle accident in Untersiggenthal
Originator: matthews@ajsh
Organization: JILA, University of Colorado at Boulder
Date: Thu, 10 Sep 1992 17:50:55 GMT

In article <9SEP199222121286@erich.triumf.ca> ivan@erich.triumf.ca (Ivan D. Reid) writes:
>
>The bike...hit the kerbstone and crashed onto the footpath, sliding 50
>metres into the edge of a corn-field.  Castellazi, unfortunately, hit a
>lamp-post and suffered severe head and chest injuries.  

My condolences to his relatives.

>...given a postulated speed of 80 km/h
>(the norm in a 60 zone) I would be surprised that he would sustain severe head
>injuries if he was wearing a good full-face helmet.  

Some biophysics may clear this up.  What follows is gleaned from textbooks
and articles in medical journals.  I started researching this because
it seemed like the FAQ needed an answer to the question "What good
does a helmet do?"  In this case, though, I have written an answer to
the question "What does a helmet not do?"  The answer is that a helmet
cannot prevent all injuries or, in some circumstances, death.

Injuries result from acceleration, and the two types of acceleration
(linear and rotational) in general cause two types of brain injury.
Linear acceleration causes the brain to slosh forward in the skull,
forming a high-pressure area (coup) under the point of impact;  the
brain then rebounds to the opposite side and forms another high-pressure
area (contrecoup).  The brain basically turns to jelly at these points,
and the damage is fairly localized.  As a consequence, the person 
typically survives, but with reduced function according to which 
parts of the brain were damaged.

Rotational acceleration of the skull creates shearing motion that is
propagated inward to the center.  As an illustration of this, consider 
a bucket of water at rest; spin the bucket around its vertical axis using 
the handle, and watch the stationary water at the center begin to spin up 
as viscosity transfers momentum from the bucket to the water near the 
edge, from that layer to the next, etc., until the whole bucketful
is in motion.  With rotational acceleration, each part of the brain
shears against its neighbors, and the brain suffers "diffuse axonal
injury" (DAI).  As the term implies, each nerve cell is separated
from its neighbor, and the damage is widespread.  This type of injury
is frequently fatal, and if a person survives the recovery is much
more difficult.

Our knowledge of the accelerations needed to produce a certain level
of injury is limited to poorly-measured after-the-fact accident
analysis and tests on animals.  Unless a victim's head was CT-scanned
at the hospital it is unlikely that we would even be able to determine
whether he suffered from coup/contrecoup or DAI, let alone find out 
what happened to cause the injury.  

To apply this to a specific accident, first remember that a properly-
fitted helmet is designed to prevent significant coup/contrecoup injury 
in a fall from about two meters onto an unyielding surface.  (For
more information about helmet engineering I refer you to other peoples'
articles - I think that Chuck Rogers wrote a good one earlier this
spring.)  As you describe it, the accident included a 2-meter fall 
to the ground, *plus* fast horizontal impact with an oddly-shaped 
object, and perhaps also multiple impacts from tumbling prior to the 
final impact.  This is more than enough to overwhelm the design of 
the helmet, especially if the head was subjected to rotational forces, 
which helmets cannot yet guard against. 
-- 
-Alex Matthews (matthews@ajsh.colorado.edu)
DoD #0010   
"It's too pure, too unrefined."  - Lance Holst, August '91
Newsgroups: rec.motorcycles