Victor Forge
The muscular system is one of the largest systems in a dog's body, with muscles making up about half of a dog's weight. Skeletal muscles are attached to bones, other muscles, and skin, and they are responsible for movement, including walking, eating, tail wagging, and eye movement. Smooth muscles maintain bladder control and bowel movement, and they also help generate heat by shivering. Dogs have both voluntary muscles, which can be controlled by thought, and involuntary muscles, which are controlled by the nervous system. Voluntary muscles include extensor muscles, which straighten the limbs, and flexor muscles, which bend the joints. The supraspinatus and infraspinatus muscles control the shoulder and support the shoulder joint, while the pectoral and trapezius muscles control the inward and outward movement of the legs, respectively.
| Characteristics | Values |
|---|---|
| Purpose of muscles | Facilitate movement of the limbs, head, neck, and joints |
| Provide stability to the joints of the body, making it easier for them to function under pressure | |
| Muscles used during swimming | Pectoral muscles, supraspinatus, infraspinatus, gluteal muscles, biceps femoris, semitendinosus, semimembranosus, quadriceps femoris, cranial tibial muscles, and Achilles tendon |
| Muscles used during running | Supraspinatus, infraspinatus, gluteal muscles, biceps femoris, semitendinosus, semimembranosus, quadriceps femoris, cranial tibial muscles |
| Muscle groups | Intrinsic and extrinsic muscles of the thoracic limb |
| Muscle attachments | Bones via tendons |
| Muscle innervation | Controlled by the nervous system |
| Muscle blood supply | Delivers oxygen and nutrients to the muscle cells |
| Muscle fibre types | Slow twitch (aerobic) and fast twitch (anaerobic) |
| Muscle spasms | Localized twitches caused by an interruption in normal muscle contraction; can be due to injury or over-exertion |
| Muscle atrophy | Muscle loss, commonly in dogs with hip dysplasia or cruciate damage |
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What You'll Learn
- Hind limb muscles: seven major muscles or groups control hip movement
- Front legs: stronger than hind legs, with 60% of body mass
- Pectoral and trapezius muscles: control inward and outward leg movement
- Smooth muscles: maintain bladder continence and bowel movement
- Cardiac muscle: striated and involuntary, found in the heart
Hind limb muscles: seven major muscles or groups control hip movement
The hip joint in dogs, as in other animals, is a ball-and-socket joint that connects the pelvic girdle to the lower limb. It is a multiaxial joint, permitting a wide range of motion, including flexion, extension, abduction, adduction, external rotation, internal rotation, and circumduction. The hip joint serves as a central pivot point for the body, allowing simultaneous, triplanar movements of the femur relative to the pelvis, as well as the trunk and pelvis relative to the femur.
The gluteal muscles are one of the seven major muscle groups that control hip movement in dogs. These muscles help the dog extend and move the hip joint outward from the body. In addition to the gluteal muscles, the biceps femoris, semitendinosus, and semimembranosus support the extending and flexing of the hip, stifle, and tarsus. The stifle is the equivalent of the human knee.
The quadriceps femoris is another powerful muscle that not only extends the stifle but also flexes the hip. The cranial tibial muscles and Achilles tendon help dogs flex the tarsus, extend digits, and flex the stifle. These muscles work together in a delicate balance to create stable and fluid movement. For example, when a dog is running or jumping, multiple muscle groups and individual muscles are working together to make movement possible.
Additionally, the abdominal muscles play a crucial role in hip movement. During rapid flexion of the hip, the abdominal muscles are activated first, followed by the hip flexor muscles. This anticipatory activation helps stabilize the lumbopelvic region by increasing intra-abdominal pressure and tension in the thoracolumbar fascia. Without sufficient activation of the abdominal muscles, strong contraction of the hip flexor muscles can inadvertently cause an anterior tilt of the pelvis.
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Front legs: stronger than hind legs, with 60% of body mass
The anatomy of dogs varies tremendously from breed to breed, more than in any other animal species, wild or domesticated. Dogs have been bred for different sizes and functions, which can affect their locomotor biomechanics. As quadrupeds, dogs must distribute their mass between their front and hind legs when standing.
A study of 552 dogs from 123 breeds ranging in size from Chihuahuas to Mastiffs found that the overall mean proportion of mass on the front legs to the total body mass was 60.4%. This indicates that dogs' front legs bear a significantly greater proportion of their body mass than their hind legs. The study also found that there was no significant change in the ratio with total body mass, but there was a significant difference by sex.
The greater mass on the front legs may be due to the larger and stronger shoulders and proportionally larger heads of certain breeds, such as European Mastiffs. The mass distribution between the front and hind legs can also be influenced by differences in skeletal structure, muscle composition, and the placement of internal organs and feet.
The front legs of dogs contain several important muscles, including the descending superficial pectoral muscle, which originates on the first sternebrae and inserts on the greater tubercle of the humerus. The supraspinatus and infraspinatus muscles are also located in the front legs and are responsible for controlling the shoulder and supporting the shoulder joint. The supraspinatus allows the dog to extend its shoulder during movement, while the infraspinatus flexes the shoulder joint.
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Pectoral and trapezius muscles: control inward and outward leg movement
The dog's anatomy varies tremendously from breed to breed, more than in any other animal species, wild or domesticated. Dogs have several muscles that play different roles in posture and movement.
The pectoral and trapezius muscles in a dog control the flex and movement of the legs. The pectoralis muscles, also known as the superficial pectorals, are located in the chest area, between the forelimbs. They are responsible for the inward movement of the legs, towards the midline of the dog's body. This movement is important for activities such as turning, where the dog brings its limbs inward and across its body to rotate. The pectoralis muscles also provide stability to the forelimbs and are crucial for many sporting activities such as weave poles in agility sports. Long walks are an excellent way to engage the pectoral muscles without overworking the muscles along the back.
The trapezius muscle, on the other hand, controls the outward movement of the legs, away from the midline of the dog's body. It also allows dogs to elevate their legs. The trapezius is part of the shoulder "girdle" and joint muscles, which act as postural stabilizers during walking. These muscles work together with the biceps and triceps, which control the flex and movement of elbows and assist in the extension of shoulders.
It is important to maintain proper muscle health and function in dogs through targeted exercises and physiotherapy. Conditions such as hip dysplasia, cruciate ligament disease, and elbow dysplasia can interfere with a dog's movement, causing discomfort and lameness. Working with professionals like vets and therapists can help effectively assess muscle function and develop therapeutic exercises to prevent injuries and enhance performance.
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Smooth muscles: maintain bladder continence and bowel movement
The dog's anatomy varies tremendously from breed to breed, more than in any other animal species, wild or domesticated. Dogs are highly variable in height and weight, with the smallest known adult dog being a Yorkshire Terrier and the heaviest being an English Mastiff.
Smooth muscles are essential for maintaining bladder continence and bowel movement in dogs. The bladder and bowel management in dogs with spinal cord injuries is a complex process that involves the coordination of smooth and striated muscles in the bladder and urethral outlet. The regulation of urination is controlled by a complex neural control system in the brain, spinal cord, and peripheral autonomic ganglia. During urine storage, as the bladder fills up, bladder afferent signals increase until they exceed a threshold in the brainstem, activating the pontine micturition centre and triggering reflex voiding.
The bladder is a smooth muscle, which means it relaxes slowly. This is in contrast to the external urethral sphincter, which is composed of striated muscles that relax quickly. When the bladder reaches its stretch threshold, a motor response is triggered via the pudendal nerve, activating the external urethral sphincter and leading to sphincter closure. This prevents urine leakage and maintains bladder control.
Bowel incontinence in dogs can be classified into two types: reservoir incontinence and sphincter incontinence. Reservoir incontinence occurs when the rectum is unable to hold or store stool normally due to diseases such as diarrhoea, inflammatory bowel disease, or cancer. Sphincter incontinence happens when the anal sphincter muscle cannot remain closed, resulting in involuntary bowel movements.
In cases of spinal cord injuries, bladder and bowel management can be improved through the use of Brindley's neuroprosthesis. This involves placing implants along the sacral nerves and connecting them to a subcutaneous receiver. A remote control then delivers electrical currents to activate the system, providing nearly complete bladder emptying and restoring continence.
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Cardiac muscle: striated and involuntary, found in the heart
The heart is a vital organ in the human body, and its function is dependent on cardiac muscle. Also known as myocardium, it is one of three major categories of muscles found within the human body, the other two being smooth muscle and skeletal muscle.
Cardiac muscle is an involuntary, striated muscle, meaning that its movement is not under conscious control. It constitutes the main tissue of the walls of the heart and is responsible for the contractility of the heart, enabling the pumping action. The heart itself is made up of three layers: the pericardium, myocardium, and endocardium. The cardiac muscle, or myocardium, forms a thick middle layer between the outer layer of the heart wall (pericardium) and the inner layer (endocardium).
The contractile forces of the cardiac muscle and the frequency at which they are activated determine the cardiac output, which is defined as heart rate multiplied by stroke volume. The contractility of the heart is influenced by metabolic demand, and the cardiac muscle must contract with enough force to supply blood to the entire body.
Cardiac muscle cells, or cardiomyocytes, are rectangular in shape and are joined at their ends by intercalated discs to form long fibers. These cells contain many mitochondria, which provide energy in the form of adenosine triphosphate (ATP), making them highly resistant to fatigue. The presence of numerous mitochondria is a key difference between cardiac muscle cells and skeletal muscle cells, which have fewer mitochondria. The intercalated discs that connect cardiac muscle cells contain gap junctions and desmosomes, which enable cardiomyocytes to contract together synchronously, allowing the heart to function as a pump.
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Frequently asked questions
The primary function of a dog's muscles is to bring about movement to all or part of its body.
The front legs carry about 60% of a dog's body weight.
There are skeletal muscles, smooth muscles, and cardiac muscles in a dog's body.
Long walks engage the pectoral muscles but not the muscles along the back.
The muscles on a dog's hind legs are larger and therefore stronger than the muscles on the front legs.
