stay apparatus Flashcards
stay apparatus is for
for standing for a long time while minimising fatigue, with minimal muscular activity
Horse have 2 mechanisms to reduce fatigue & prolong
ability to monitor for danger (remain alert)
- Nuchal ligament: Enables head to be repeatedly raised without much muscular effort
- Stay apparatus (so-called passive stay apparatus): Permits standing for a long time while minimising
fatigue, with minimal muscular activity: involves muscle, ligaments & tendons
3 parts of the stay apparatus
- Stay apparatus of the forelimb (proximal to the carpus)
- Stay apparatus of hind limb (proximal to the tarsus)
- Suspensory apparatus (distal to the carpus/tarsus, identical fore & hind)
basic principle of stay apparatus
Basic principle is that if the horse relaxes most of its muscles, the legs will remain in a standing position.
sleep in horses; 2 types
slow wave sleep (SWS) &
paradoxical sleep (PS - includes REM sleep)
describe muscle tone in slow wave sleep and paradoxical sleep
Some muscle tone in SWS but none in PS
what position is horse in slow wave sleep
can stand, usually sternal recumbancy
what position is horse in paradoxical sleep
lateral recumbency
how long does paradoxical sleep last
only a few minutes at a time
total daily sleep required
3 or 4hr but large individual variation
the stay apparatus in part is a ____ support mechanism
fibrous support mechanism (FSM)
describe the fibrous support mechainsm of the stay apparatus
The fibrous support mechanism includes those connective tissue structures found in the thoracic & pelvic limbs of the horse that mechanically FIX the respective joint of the limbs in order to support the weight of the animal in a relatively relaxed, standing position. The same structures are necessary for normal locomotor function.
stay apparatus of forelimb
If the muscles of the forelimb relax,
ligaments & tendons should hold the leg
in normal resting position:
* Neutral shoulder & elbow
* Extended carpus
* Over-extended fetlock
* Extended pastern & coffin joints
___% of BW is on thoracic limbs
60
If left unsupported by opposing
ligaments & tendons, what
would happen to the jointed
bony column of the thoracic
limb?
- Flexion (elbow joint)
- marked flexion of shoulder joint
- Overextension (dorsiflexion) of
the carpus
-Marked overextension of the fetlock - result; joints would collapse under weight of animal
shoulder stability
- Weight of body tries to flex (collapse) the shoulder
- Prevented by internal (short) tendon of biceps whose RADIAL insertion effectively
acts as a fixed point. - ALSO via lacertus fibrosus (long tendon of biceps brachii) & the extensor carpi radialis
Elbow stability
Body weight (& tension on biceps tendon) tries to flex elbow.
Flexion prevented by:
* Tension of triceps (medial head)
* Collateral ligaments
* Origins of digital flexor muscles (medial humerus)
Carpus stability
- Body weight could either flex or hyperextend carpus
- Flexion prevented by tension on ECR from biceps tendon via lacertus fibrosus
- Overextension prevented by superior check lig. (Accessory lig. SDFT) (NOT IN HINDLIMBS)
Fetlock, pastern, coffin joint
stability
- Suspensory Apparatus: Suspensory ligament, Proximal sesamoid bones (PSB), Ligament from PSB to cannon bone, and phalanxes
- Accessory ligament (check) DDFT
- Flexor tendons (DDFT, SDFT)
Suspensory Apparatus:
- Suspensory ligament
- Proximal sesamoid bones (PSB)
- Ligament from PSB to cannon bone, and phalanxes
Hindlimb passive stay apparatus Prevents leg from collapsing by preventing:
- flexion of stifle & hock
- overextension of fetlock & phalangeal joints
important part of hindlimb passive stay apparatus is ____ mechanism
reciprocal: links stifle and hock
hindlimb reciprocal mechanism links ____ and ____
stifle and hock
locking of their patellas
(stabilising the limb with minimal muscular
effort). However, some muscular effort is needed, which is why horses tire
& alternate their resting hindlimbs
check ligaments on hind limb
NO superior check ligament, SMALL inferior check ligament
Normal locking of the patella; 3 steps
1) Stifle extended & patella lifted by quadriceps
2) Patella rotated medially approx 15 degrees by vastus medialis so parapatellar cartilage & medial patellar ligament sit over medial
trochlear ridge
3) Quadriceps relax (?) to lower fibrocartilage onto medial trochlear ridge where it is hooked/locked in place
3 steps of unlocking patella
1) Lift patella (extend stifle)
2) Rotate patella laterally with gluteofemoris & tensor fascia late
3) Relax quadriceps
when unlocking patella; can get stuck, whats this called and why does it occur
“INTERMITTENT/PERMANENT UPPER
FIXATION OF THE PATELLA”)
- young horses/off work
- poor conformation
- neuromuscular problem??
***Surgery (desmotomy-transection medial patellar lig.) only on severe permanent cases
reciprocal apparatus comprised of 2 cords:
peroneus tertius & superficial digital flexor
reciprocal apparatus flexes
stifle and hock together
reciprocal apparatus is mostly
tendinous
reciprocal apparatus
Aids in both (movement & stay apparatus) functions
reciprocal apparatus when stifle is locked
When stifle is locked, the weight on the limb pressures the hock joint to flex. However, tension in the superficial digital flexor resists
this pressure, and the hock does not flex
Horse’s musculoskeletal system evolved to ____ metabolic cost of locomotion
reduce
at high speed; increase stride length ____ energetic cost of limb protraction (bringing limb forward)
decreases
how did horse evolve to reduce metabolic cost
- Bulk of muscles proximally in the limb, more distal aspect tendons provide
energy for locomotion - Distal part limb elongated & number bones reduced to single digit
- Cannon bone longer & lighter
- Musculotendinous units in the forelimb: support body mass, generate forces used for propulsion, breaking & turning, stabilise joints during stance phase of the stride & flex joints during the swing phase
For high speeds achieved at the gallop rapid protraction of the forelimbs is required, how is this achieved
special catapult action of biceps muscle
describe catapult action in forelimb
- In horses, instead of hight output contraction of muscles, rapid protraction is achieved by the special catapult action of biceps muscle
- In a catapult system energy is stored slowly with a large force & then released quickly to accelerate a small mass
- During stance, the elastic biceps muscle is stretched, while carpus is locked in extension
- Before limb lifts off, the carpus buckles forward, this releases the catapult, bringing
the leg forward for the next stride - The power output of this catapult action of the biceps muscle is about 100 times its
mass of a non-elastic muscle
SDF and DDF have ____ muscle bellies and ___ tendons which is energy efficient
small
long
describe spring and coil mechanism of flecor tendonds
- During stance phase, flexor tendons are loaded under tension (this stress may be as high as twice of the bodyweight at the gallop)
- During the first half of stance phase, flexor tendons store great amount of elastic energy, which is released in second half of stance
phase –> propulsion - The muscles bellies of these tendons damp this spring-like action
spring and coil mechanism: in thoroughbred horses the SDFT elongates by ___% its original length as response to sustained strength
12-16%
