AQHA November / December 2019 Magazine AQHA NOV-DEC 2019 PRINT (2) | Page 20

PG.18
HORSE HEALTH
Locking
STIFLES...
WE KNOW HORSES HAVE MANY VERY UNIQUE, WONDERFUL TALENTS
AND ABILITIES. ONE WHICH IS COMMON TO ALL HORSES IS
THE ABILITY TO SLEEP STANDING UP.
T
HE forces of gravity normally cause
flexion of the stifle and unless there
is a passive ‘locking’ mechanism, the
extension muscles (quadriceps and
biceps) must work constantly to counteract
this. The passive locking mechanism is the
‘stay apparatus’ - a complex evolutionary
adaptation of an enlarged patella (knee cap),
it’s three ligaments and an enlargement of
the end of the femur (thigh bone). When the
back leg is in extension, the medial (inside)
ligament hooks over the enlargement at the
end of the femur preventing the leg from
collapsing during sleep. Together these
elements allow a passive locking of the stifle
in an extended position, without continued
high levels of muscular activity from the
quadriceps. Fossils from early horses around
12 million years ago have the enlarged end of
their femur. About 5 million years ago, horses
also developed a locking mechanism in their
shoulder.
A little knowledge of the anatomy helps
to understand the mechanism underlying
‘locking stifles’. In the standing position, when
the hindleg is straight (extended), the medial
patella ligament ‘hooks’ over the enlarged
ridge at the end of the femur (Figure 1).
When the horse steps forward there is a
reflex tightening of the quadriceps and biceps
that lifts the patella upwards and sideways
off of this ridge so it slides easily back into its
moving position. If the muscles are weak or
the ligaments loose or stretched, this may not
happen, resulting in locking patella.
AUSTRALIAN QUARTER HORSE ASSOCIATION - WWW.AQHA.COM.AU