Lecture – The Muscular System
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The Muscular System
EQS 110
Lecture – The Muscular System
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Table of Contents
Function of Muscles ...................................................................................................................... 3
Types of Muscle Tissues ............................................................................................................... 3
Smooth Muscle ............................................................................................................................ 3
Cardiac Muscle ............................................................................................................................ 3
Skeletal Muscle ........................................................................................................................... 4
Deep & Superficial ........................................................................................................................ 4
Muscle Movement ......................................................................................................................... 5
Muscle Fiber Types ..................................................................................................................... 5
Slow Twitch ........................................................................................................................ 5
Fast Twitch.......................................................................................................................... 5
Flexors & Extensors .................................................................................................................... 6
Sphincters .................................................................................................................................... 6
Cutaneous .................................................................................................................................... 6
Management of Muscles – Tying Up ........................................................................................... 7
What is Tying Up? ...................................................................................................................... 7
Types of Tying Up ...................................................................................................................... 7
Signs of Tying Up ....................................................................................................................... 8
Causes.......................................................................................................................................... 8
Treatment of Tying Up ................................................................................................................ 8
Management of Tying Up ........................................................................................................... 9
Dietary Management ..................................................................................................................... 9
Exercise Management ................................................................................................................... 9
Effect on Performance ................................................................................................................. 9
Self-Knowledge Checks .............................................................................................................. 10
Answers ..................................................................................................................................... 11
Glossary ....................................................................................................................................... 12
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Lecture – The Muscular System
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Function of Muscles
The muscular system of the horse forms the largest tissue
mass in the body and is responsible for controlling every
aspect of movement, both internal and external. In addition
to movement, muscles also function to:
Produce heat
Maintain posture
Types of Muscle Tissues
The horse’s muscular system can be broken down into three types of tissue:
Smooth
Cardiac
Skeletal
We will look at each of these muscle types individually, placing the largest focus on the last
category, skeletal muscle.
Smooth Muscle
Smooth muscle is an involuntary muscle which functions
automatically. It surrounds all internal tissues and organs
and responds to stimuli from the nervous system. Smooth
muscle can be found in areas of the body that do not
require conscious control, such as the urinary tract and
digestive system.
Cardiac Muscle
Cardiac muscle is also an involuntary muscle but its function is to
coordinate the propulsion of blood in and out of the heart. With the
heart beating around 100,000 times a day throughout a horse’s lifetime,
this muscle is fatigue resistant. For a racehorse, proper development of
cardiac muscle through training is crucial for performance!
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Skeletal Muscle
Skeletal muscle is a voluntary muscle responsible for external
movement as well as:
Maintaining posture
Controlling range of movement
Protecting from trauma
Thermoregulation
A horse has over 700 different skeletal muscles! Muscles are
able to produce movement via tendons, a type of connective
tissue that attaches muscles to bones. Muscles are then signaled to contract via nerve impulses.
Relaxation occurs when the nerve impulses cease. Due to their immense role in creating movement,
we will take a closer look at the categorization and functions of skeletal muscle.
Deep & Superficial
Skeletal muscles can be categorized based on their depth of placement in the body. For
purposes of this course, you will not be required to know the names of each individual deep
and superficial muscle.
Deep Muscles
Deep muscles are for posture and stability. They attach
directly to the bone and are located close to the joints.
Superficial Muscles
Superficial muscles are located between the deep muscles and the skin. Although they vary
in size and shape, they are generally known as movement muscles and are greatly responsible
for a horse’s athletic performance. As movement muscles, superficial muscles can become
tense and sore – no different than a human athlete. This, in turn, can affect performance as
well as mental attitude. Since these muscles are the outermost layer, they can be easily felt for
tension, heat, and swelling and be readily influenced by complementary therapies, such as
massage, magnetic therapy, and stretching.
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Muscle Movement
Muscle is made up of thousands of fascicles, which are bundles of fibers running parallel to each
other – this is what gives muscle its striated appearance. Skeletal muscle, which is the muscle
category primarily responsible for movement, can consist of two muscle fiber types. The type
and proportion of muscle fibers a horse has can determine what activity he/she may excel at.
Slow Twitch
Slow twitch, also called Type I, are muscle fibers that contract
slowly for long periods of time in the presence of oxygen; they use
aerobic metabolism to produce the energy needed for muscle
contraction. Since these are “slow-burning” muscle fibers, they do
not create toxic end products of muscle contraction, such as
lactate, which can cause a horse to cramp. This makes them ideal
for non-strenuous work.
Fast Twitch
Fast twitch, also called Type II, are powerful and fast
contracting muscle fibers that produce quick energy without
the presence of oxygen; they use anaerobic metabolism to
produce the energy needed for muscle contraction. While
these muscle fibers produce speed, they also tire quickly and
create a by-product during muscular contraction known as
lactic acid. Lactic acid can result in cramping if allowed to
build up in the body.
Every horse has a mix of fast and slow twitch muscle fibers. The proportions are determined by
breed and genetics. Horses with a higher proportion of slow twitch fibers have more stamina,
tire less easily, and make good endurance or leisure horses. Those with a higher proportion of
fast twitch fibers have less stamina but more explosive energy. Arabians are a breed of horse
that show higher proportions of slow twitch muscle fibers while Quarter Horses and
Thoroughbreds show higher levels of fast twitch muscle fibers. Appropriate training can have an
immense impact on performance and bring each type of horse to its peak level of fitness. For
example, a Thoroughbred must have stamina as well as speed – adapting the muscular system
to exercise can improve the muscle’s ability to store oxygen and increase its aerobic and
anaerobic capacity. While training and conditioning can have a significant impact on the
muscular system, it cannot entirely rewrite what type of muscle fibers the horse was given; this
is why we don’t see certain breeds racing competitively!
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Flexors & Extensors
Flexors and extensors are a group of muscles that lay on the limbs to
either decrease or increase the angle. Flexor muscles lay on the back
of the limb to decrease the angle (axes of the limb are brought closer
together) – think about when you flex your arm, you are bringing the
axes of your bone closer together. Extensor muscles do the opposite;
they lay on the front of the limb to increase the angle (axes of the
limb are brought farther apart).
Sphincters
A sphincter is a ringlike muscle that contracts (closes)
a passage or opening. Commonly found in the
digestive tract, the strength of the lower esophageal
sphincter is one reason why a horse cannot vomit!
Cutaneous
Cutaneous muscles allow for movement of the skin. Horses
use this muscle to twitch and remove irritants like flies.
Flexing
Extending
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Management of Muscles – Tying Up Taken in part from “What is Tying up in Horses” by Tania Cubitt, PhD, January 17th, 2004
Muscle disorders are a frequent cause of poor performance in horses, and for a racehorse, can be performance-limiting or even career ending. Tying up, also referred to as Exertional Rhabdomyolysis, Azoturia, or Monday Morning Sickness, is the most common muscle disorder seen in the athletic horse. Understanding this disorder is vital if you wish to be involved in the care and training of horses, no matter the discipline.
What is Tying Up?
The scientific name for Tying up is rhabdomyolysis, which simply translated means muscle (myo) breakdown (lysis). The causes of Tying up vary but the result is the same, muscle cramping. In this condition, there is a massive contraction of the muscle groups along the back and rump of the horse. The muscles contract and do not relax. The horse will appear to be almost nailed to the floor in the hindquarters and unwilling to move. The horse will appear to be significant pain and show signs of distress, including increased heart rate and sweating. If you place your hands along the muscles on either side of the spine on a horse that has tied up, they feel rock hard. While cases will range in severity, horses that are showing signs of tying up require immediate medical attention from a veterinarian.
Types of Tying Up
Horses experiencing tying up can be placed in one of two broad categories:
1. Sporadic (acute) exertional rhabdomyolysis – this classification applies to horses which, on rare occasion, experiencing an episode of generalized tying up. This will generally involve muscle stiffness and cramps
2. Chronic exertional rhabdomyolysis – when a horse experiences repeated episodes of ER, with the first episode usually occurring at a young age. This type is much more problematic for an athletic horse and can result in irreversible muscle damage
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Signs of Tying Up
The signs of tying up vary with the severity of the condition and the speed of onset. In mild cases, the horse’s gait gradually stiffens and he becomes reluctant to move. The muscles are actually cramping and feel very much to the horse like muscle cramps do to human athletes. The large, heavy muscles of the hind legs are most affected, but the triceps muscles above the elbow of the front legs may also appear stiff and painful. Additional signs include sweating, elevated heart and respiratory rate, and dark red/brown urine due to muscle damage. Most cases of tying up are diagnosed by these clinical signs, but bloodwork can be taken to evaluate the muscle enzyme levels, which can help develop a specific treatment plan.
Causes
Horses often tie up when exercise exceeds the horse’s underlying state of training. We may see this occur with young horses when they first go to the track and do too much too soon or with older horses returning from a rest (also called a layoff) and exercising beyond their current level of fitness. Tying up may also occur due to excitability and stress (traveling, breeding, showing). Tying up may also have a nutritional basis, such as an electrolyte imbalance, abnormal carbohydrate metabolism, or mineral/vitamin deficiencies. Unfortunately, for some horses, the cause of Tying up may be unknown. Working with your veterinarian is going to be key in the treatment and management of horses that tie up.
Treatment of Tying Up
Horses showing signs of tying up should cease exercise immediately – do not force a horse to walk if it cannot as this can increase muscle damage. If it is cool, cover the horse, especially its hindquarters with a cooler or blanket to keep warm. Horses that are able to walk should continue to do so unless otherwise directed. Your veterinarian should be contacted but in the meantime, determine if the horse is dehydrated and provide fluids via small frequent sips of water. In order to relax the muscles and allow the horse to move, your veterinarian may need to administer a non-steroidal anti-inflammatory drug (NSAID) and/or a sedative depending on the severity. Horses that suffer from an episode of Tying up should be rested for 3-5 days with a gradual increase in exercise.
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Management of Tying Up
Diet and exercise are closely linked to Tying up and the most important preventative strategies are based upon diet and exercise management.
Dietary Management
The most common dietary change made to horses that tie up involves a reduction in grain. High cereal grain diets can overload the normal digestive processes of the small intestine and overflow to the hindgut, which is not meant to digest grain products. Too much grain in the horse’s hindgut can increase lactic acid production, fluctuate blood glucose levels, and increase fluid and electrolytes losses due to increased heat production. Checking the diet for balanced levels of minerals and vitamins will also be key. Incorporating the assistance of an equine nutritionist is beneficial for horses suffering from Tying up.
Exercise Management
An exercise protocol is going to vary horse to horse but several basic principles can help horses that suffer from Tying up:
Consistent, structured program Grading the level of exercise with a slow increase in intensity as fitness develops Following hard work with a slow trot or canter to stimulate the delivery of oxygen and removal
of lactic acid from the muscles Adequate warming up and cooling down procedures
Effect on Performance
There is no cure for horses with chronic forms of Tying up. Severe and recurrent episodes can result in irreversible muscle damage and impact the ability for a horse to perform. Even if a horse does not show repeated bouts of this disorder, the appearance of these signs needs to be taken seriously to not only prevent muscle damage but prevent future episodes. Horses that tie up due to excitability and nervousness will need to managed carefully – avoiding stress and excitement will be key, but that is difficult for a racehorse. Working closely with your veterinarian, as well as an equine nutritionist if needed, will be vital for your horse’s health and success.
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Self-Knowledge Checks
1. Muscle that is involuntary and surrounds all
internal tissues and organs is characterized as:
a. Cardiac b. Smooth c. Skeletal
2. Muscles are able to produce movement via
this structure, which attaches muscles to bones
a. Tendon b. Ligament c. Joint d. Fascia
3. Which type of skeletal muscle will be most
readily influenced by therapies such as massage
and stretching?
a. Deep b. Superficial
4. _____ twitch are muscle fibers that contract
slowly for long periods of time in the presence of
oxygen
a. Slow b. Fast
6. A buildup of lactic acid in the body can result in:
a. Heart failure b. Muscle cramping c. Inability to breathe d. Inability to defecate
8. The image below is engaging which group
of muscles?
a. Extensors b. Sphincters c. Flexors d. Cutaneous
9. When a horse twitches his skin to remove an irritant,
like a fly, he is engaging which muscle group?
a. Flexors b. Cutaneous c. Sphincters d. Extensors
10. Severe muscle cramping that involves distress,
inability to move, and increased heart rate is known as:
a. Stocking Up b. Bucked Shins c. Tying Up d. EPM
5. Fast twitch muscles use anaerobic metabolism
to produce quick energy – which statement
correctly describes this process?
a. Fast twitch muscles produce quick energy without oxygen b. Fast twitch muscles produce quick energy with oxygen
7. Appropriate training can improve muscle fiber ability
a. True b. False
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Answers
1. Muscle that is involuntary and surrounds all
internal tissues and organs is characterized as:
b. Smooth
Smooth muscle in an involuntary muscle that functions automatically. It surrounds all internal tissues and organs
2. Muscles are able to produce movement via this
structure, which attaches muscles to bones
a. Tendon
Tendons are a connective tissue that attaches muscles to bones, allowing for movement
3. Which type of skeletal muscle will be most readily
influenced by therapies such as massage and stretching?
b. Superficial
Superficial skeletal muscles are the outermost layer and can easily felt for tension, heat, and swelling. As a result, complementary therapies, such as massage and stretching, can help relax them
4. _____ twitch are muscle fibers that contract slowly for
long periods of time in the presence of oxygen
a. Slow
Slow twitch, also called Type I, are muscle fibers that contract slowly for long periods of time in the presence of oxygen
5. Fast twitch muscles use anaerobic metabolism to
produce quick energy – which statement correctly
describes this process?
a. Fast twitch muscles produce quick energy without oxygen
Anaerobic metabolism is not dependent on oxygen to break down fuel stores. This means that fast twitch muscles produce quick energy without the presence of oxygen
6. A buildup of lactic acid in the body can result in:
b. Muscle Cramping
Lactic acid is a by-product of fast twitch muscular contraction. When allowed to build up in the body, it can result in muscle cramping
7. Appropriate training can improve muscle fiber ability
a. True
Appropriate training and conditioning can help adapt the muscular system to physical demands by improving the ability of the muscle fibers to store oxygen or increasing its aerobic and anaerobic capacity. Training cannot rewrite the genetics and breed of a horse, which is how proportions of muscle fiber is determined
8. The image below is engaging which group of muscles?
c. Flexors
In the presented image, the axes of the hindlimb are being brought closer together. When the axes of a limb are brought closer together, the flexor muscles are being engaged
9. When a horse twitches his skin to remove an irritant,
like a fly, he is engaging which muscle group?
b. Cutaneous
Cutaneous muscles allow for movement of the skin. Horses use these muscles to twitch and remove irritants like flies
10. Severe muscle cramping that involves distress,
inability to move, and increased heart rate is known as:
c. Tying Up
Tying up is a muscular disorder characterized by severe muscle cramping, distress, inability to move, and increased heart rate
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Glossary
Acute – Sudden and severe, quick onset
Aerobic Metabolism – Is dependent on oxygen to break down fuel stores – inhaled oxygen
during aerobic exercise is carried to the muscles and liver to metabolize (utilize) carbohydrates
and fats for the production of ATP, the energy source for muscle contraction. Exercise is
classified as aerobic if the heart rate is less than 150 beats per minute. The waste products of
aerobic metabolism are carbon dioxide, water, and heat
Anaerobic Metabolism – Is not dependent on oxygen to break down fuel stores, provides a
rapid means of producing a limited supply of energy during anaerobic exercise. Exercise is
classified as anaerobic if the heart rate is greater than 150 beats per minute. In the absence of
oxygen, only carbohydrates can be metabolized for ATP production and the waste products of
anaerobic metabolism are heat and lactic acid
Cardiac Muscle – An involuntary muscle tissue that functions to coordinate the propulsion of
blood in and out of the heart
Chronic – Long term
Cutaneous – A muscle group that allows for the movement of the skin
Deep Muscles – A classification of skeletal muscle that provides the horse with posture and
stability
Extensors – A muscle group that lays on the front of the limb to allow for extension (increase
limb angle)
Fast Twitch – A type of muscle fiber that creates quick energy via anaerobic metabolism
Flexors – A muscle group that lays on the back of the limb to allow for flexion (decrease limb
angle)
Involuntary – Do not have conscious control over
Lactic Acid – A waste product of fast twitch muscle fiber use and anaerobic metabolism, can
result in cramping if allowed to build up
Layoff – A period of rest, may include turnout and/or stall rest
Skeletal Muscle – A voluntary muscle tissue responsible for the production and control of
external movement, maintaining posture, and thermoregulation
Slow Twitch – A type of muscle fiber that that contracts slowly for long periods via aerobic
metabolism
Smooth Muscle – An involuntary muscle tissue that surrounds all internal tissues and organs
Sphincter – A muscle group that contracts (closes) a passage or opening
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Superficial – A classification of skeletal muscle found between deep muscles and the skin,
generally known as movement muscles
Tendon – A type of connective tissue that connects muscle to bone
Tying Up – A muscular disorder that results in severe cramping of the large hindquarter muscles
Voluntary – Can be consciously controlled