Big bone vs small bone

[Niccolai Murphy <niccolai_m@yahoo.com>]

Tom and Heidi mentioned, in passing, big bone and small bone, with Tom
referring to studies indicating that young small boned horses had
denser stronger bone than big boned horses. Now don't you all go
having dirty thoughts about big boned stallions, let's pretend we are
talking mares here. Doing some arm chair equine design, skip to the
last paragraph if you don't want the logic....

BENDING LOADS***********************************
A purpose of the muscles in a track horse is to move the legs back and
forth as quickly as possible. This movement will cause inertial loads
which contribute to bending moment and axial tensile loads on the
bones, both second order and disregarded.

A purpose of the muscles on an endurance horse is mostly to carry it
up and down hills are through deep sand, etc as well as to rotate the
legs. Also we would think that endurance horses do more of this than
track horse do accelerating.

A common purpose for muscles in both sports is to contribute to
support while moving, with bones doing most ok the work, second order,
disregard.

This being the case, the longitudinal forces that cause bending
moments on the leg bones would be dominated by hill climb and decent
in the endurance horse and starting acceleration in a track horse with
mechanical loading (moving the legs back and forth) and drag being
second order for both sports. 

Load normal to the bone applied by hill climbing being a function of
the product of mass, acceleration due to gravity and  the sine of the
angle of the hill.

Load normal to the bone applied by acceleration being a function of
the product of mass and acceleration.

A quick look at the terms and performance of the horses indicates that
they are of the same order, with the exception that the number of
cycles is much greater in endurance than track thus implying an
increase fatigue strength requirement.

The acceleration due to going around corners on the track do not
contribute to bending loads on the bone as the horse compensates by
leaning into the turn, so axial loads increase but not bending.

Conclusion, bending strength requirement of bones in both sports is
that same, but endurance horses need this strength over more
repetitions.

AXIAL LOADS********************
If we make the tacit assumption that axial loads are proportional to
the square of the speed of the horse (typical phenomenon observed in
nature but not proven correct in this case) then we can see that the
bones of a track horse travelling at 30 mph will be nine times higher
than an endurance horse travelling at 10mph. 

Conclusion, the endurance horse has a much lower requirement but must
withstand this loading for many more cycles than a track horse.

STRESSES IN THE BONE*******************************
Axial stresses are simply a function of the load divided by the cross
area of the load carrying portion of the bone. More area less stress
translates to big bone less stress but don't draw any conclusions from
this yet.

The maximum bending stress is given by the product of the moment and
the square of the distance of the extreme fiber of bone from it's
neutral axis (approximately the center of the bone cross section)
divided by the moment of inertia of the cross section of the bone
(itself a square function of the radius of the bone). Again this
implies that the larger the cross-section of the bone the lower the
stress for a given bending load.

ADAPTATION OF BONE TO STRESS*****************************
It has been shown that bone gets stronger the more we stress it and
that it takes time. That's why we condition young ones slowly, even
though they are willing and their muscles can take them faster, their
bones cannot.

BIG BONE BETTER THAN SMALL BONE?*******************
Maybe maybe not. Consider. If you have big bone you have a jolly
difficult time stressing that bone so that it can become strong bone,
and most probably big bone takes longer than small bone to develop the
same strength, if it ever does. On the other hand you don't need such
strong bone if you have big bone. However if you give that big
-weaker- bone a sudden whack it will fail more readily than small
-strong- bone. To visualize this consider an aluminum tube like pipe
corral and whack it mentally with a hammer. You just put a good dent
in it. Now consider a narrower steel pipe and whack that one, your
mental hammer just bouced back and hit you on the fore head, no dent.

If I were a track guy I would put my money on strong small, efficient
bone. In endurance I could be convinced either way, but would tend to
think that small bone would be best for flat Florida and big bone
would be best for the Sierras. I could also be persuaded to believe
that big bone is better for providing longer moment arms for tendons
and ligaments, thus reduing stresses in these components too. However
we know that we have to stress these to strengthen them too. if I were
to bet on it I'd go big bone in general for endurance since systems
with lower operating stresses tend to survive more load cycles than
highly stresses systems.
==
Nicco Murphy - Poway, San Diego, CA


_________________________________________________________
DO YOU YAHOO!?
Get your free @yahoo.com address at http://mail.yahoo.com