By Randy Kidd, DVM, PhD
Compare the actions and functions of the musculoskeletal system to a finely tuned symphonic orchestra. Each and every instrument (each of the dozens of independent muscles in the dog) contributes to the whole. For one instrument to be heard above the rest, the sounds from other instruments must be softened – and for one set of muscles to flex or contract a joint, that joint’s muscles of extension must relax, and vice versa. The music of the muscles is this month’s stop on the Tour of the Dog.
Anyone who loves the symphony knows that the individual instruments resonate or play off one another to create an apparent experience of one musical “organism.” This same resonance exists within an animal’s musculoskeletal kinesthetic system. (Kinesthesia is the sense by which movement, weight, position, etc., are perceived; commonly used to refer specifically to the perception of changes in the angles of the joints.) Thanks to his constant kinesthetic feedback mechanism, a healthy dog automatically knows where his legs are, and his nervous system tells him exactly where to place each foot as he travels down the road.
But enough of this analogy; let’s see how the real muscle tissues work (and don’t work) in our canine friends. And, let’s see if there are some ways we can help keep our dogs’ muscles fit, to keep them from becoming soft and under-productive.
Note: The musculoskeletal system is generally considered as one unified system because that’s the way it operates. The musculoskeletal system includes bones, muscles, tendons, ligaments, nerves, and the biochemicals and nutrients that are required to energize and move the system. The system’s functions include propelling the dog from one site to another, providing architectural stability, and the protection of internal organ systems.
In this article we will stick with the muscular portion of the system, and cover bones and joints in later editions.
Anatomy and physiology
A single muscle fiber is a cylindrical, elongated cell, and may vary in length from very short (for example, the muscles controlling eye movement) to the length of a dog’s thigh (the sartorius muscle extends across the anterior thigh, wrapping around from the outer hip to below the inner knee, thus it is actually longer than the dog’s thigh).
Between and within muscle cells is a complex latticework of connective tissue, resembling struts and crossbeams that help to maintain the integrity of the muscle during contractions and straining. Crossbeams within muscles can be seen using a microscope and are responsible for the muscles appearing “striated.” Other muscles in the body, such as those found in the intestine, bladder, and blood vessels, are non-striated. Cardiac muscles are striated, but they have a slightly different microscopic appearance from skeletal muscles.
Organizationally, thousands of muscle fibers are wrapped by a thin layer of connective tissue (called the perimysium) to form a muscle bundle (or “fascicle”) that is surrounded by another sheath of connective tissue (the epimysium). Muscle bundles (which, when taken together, are named – the biceps for example) are joined at each end into tendons, which attach to bones. The muscles function by contracting across joints and using the joint as a fulcrum, causing movement.
The contractile machinery of the muscle fiber is organized into structural units called sarcomeres, with several sarcomeres lining up to form into a contractile unit of myofibrils. Muscle length is determined by how many sarcomeres are lined up in series, one next to the other. Muscle thickness ultimately depends on how many sarcomeres line up in parallel (one on top of the other).
The sarcomere structures consist of two important proteins, actin and myosin (comprising about 85 percent of the muscle cell’s volume); other proteins such as troponin and tropomyosin are also present. Today’s accepted theory for how muscles contract involves the action of actin and myosin sliding over each other, increasing the amount of overlap and thus shortening the length of the muscle cell. Muscle fibers can be excited into shortening by chemical, electrical, or physical means. Muscular activity requires energy input, and prolonged activity necessitates the oxygenated “burning” of metabolizable nutrients.
Muscle cells contain an abundance of interconnected mitochondrial powerhouses, the organelles that contain the enzymes that utilize oxygen during exercise (via the conversion of fats, carbohydrates, and proteins to ATP). This entire network is supplied with zillions of capillaries – a cross section of one square inch of muscle contains 125,000 to 250,000 capillaries.
Exercise is the vital component for improving overall muscular function. It enhances the efficiency of the mitochondrial power-packs (and over time increases their numbers), and increases the number of functional capillaries per square inch of muscle for better overall energy supply and enhanced waste product removal.
Muscle fibers are innervated and activated by motor neurons. Each motor neuron activates, on average, some 600 muscle fibers. Large muscles may have as many as 2,000 fibers per motor unit; the tiny eye muscles may have only 10 or so fibers per motor unit. When stimulated, a motor unit will fire all the fibers it is in contact with, not just a few. In addition, there are several mechanisms that insure muscle-use efficiency by firing only those muscles needed for the task at hand and by using the more efficient, oxygen-consuming energy pathways whenever possible.
Smaller muscles have fast-twitch fibers – muscle fibers that are activated early, require more energy to function, and thus tire faster. Larger muscles contain more slow-twitch fibers – more energy-efficient fibers that are recruited when a heavier load of work is required. As an animal continues to exercise, more and more slow-twitch fibers are recruited into action. Eventually even the larger muscles tire, and ultimately the dog becomes so tired he can go no farther.
Anytime a dog moves he relies on all the systems working in healthy unison. Whenever dogs need to extend their activity range, they rely on their previous exercise history and their current nutrient status to provide them with healthy muscles. Exercise not only enlarges muscle mass by stacking muscle cells on top of one another, it increases the blood supply to the muscles, and it adds more mitochondria to the cells – thus enhancing cellular metabolism and function.
If there is any one medicine especially made for muscles, it is exercise, liberally applied, several times daily.
Diseases of the muscles
There are several diseases that are muscular in origin, and several more that affect the muscles secondarily via the nervous system, nutrient deficiencies, and other sources. Following are some of the more important of these diseases.
• Pain. While pain may not be a specific disease, it deserves a brief note here, especially since much of the pain animals contend with comes from the musculoskeletal system, and since many of our dog’s conditions of the muscular system involve nothing more than mild strains that cause variable amounts of pain. As prevalent as pain is, it is curious how little we really know about it in animals – most of what we think we know is conjectural or theoretical.
One thing we do know, however, is that pain perception is a purely subjective phenomenon, whether we are talking about humans or other animals. An important concept to keep in mind is that, although pain perception appears to be quite constant across species, actual tolerance of a painful stimulus may vary widely even within a single species – that is, all of us (animal and human) have similar pain thresholds, but some individuals can tolerate a higher level of pain than others without showing clinical signs. Thus, one dog may scream and howl with nothing more than a pinched toe; another will seemingly have no pain after severe injuries that would leave most of us debilitated.
From a practical standpoint, this means to me that I consider the treatment of pain an important component of anything that seems like it could cause pain, even if the animal isn’t showing obvious outward signs of experiencing pain. Mild pain relievers contained in herbal remedies, acupuncture, chiropractic, trigger point therapy, massage, and homeopathic remedies are my treatments of choice for pain. Western medicine relies on analgesic drugs (narcotics and nonsteroidal anti-inflammatories), but I’d just as soon avoid the adverse side effects seen with these.
There are two basic kinds of pain, superficial and deep. Superficial pain or fast (first) pain can be described as pricking, bright, or sharp. Descriptors for deep or slow (second) pain include dullness, burning, aching, or throbbing. To get a sense for the two types of pain, consider how it feels when you cut yourself with a sharp knife. Initially you feel a sharp first or fast pain, and then a few seconds later you feel the throbbing, dull, slow, or second pain. The combination of pains is referred to as double pain and is common after tissue injury.
Before you (or your dog) are able to feel any pain, pain receptors (nociceptors) in the affected organ or structure must first be stimulated above their thresholds. In both first (acute) and second (chronic) pain, a naturally occurring inflammatory process causes firing of the pain receptors. Pain mediators such as histamine, serotonin, and prostaglandins are released from various cellular structures during the course of the inflammatory process, and they cause the initiation and continuation of pain.
Pain receptors found in joints, muscles, and tendons combine with proprioceptors (nerve bundles that detect the position of body structures) to help keep limbs and joints within a normal range of motion. If they detect pain or a joint that is approaching its normal limits of motion, the receptors fire and cause a tightening or loosening of the muscle masses that control the structure, thus preventing structural damage. If structural damage occurs, the pain receptors will fire and instigate the inflammatory process.
Most (about 80 percent) of a joint’s protection comes from the stabilizing ability of the muscles and tendons; ligaments contribute only about 20 percent – yet another reason to keep your dog’s muscles in shape with exercise.
• Hereditary conditions. There are several hereditary conditions that affect dogs’ muscles. Muscular dystrophy (MD) refers to a group of genetic disorders associated with a deficiency or dysfunction of the muscle protein, dystrophin. A primary interest in canine MD is its potential similarity to the human disease. The disease complex has been reported in several breeds of dogs; the one that has been best characterized is a sex-linked disease of Golden Retrievers.
Muscular dystrophy’s major symptom is muscle wasting, and clinical signs in Goldens include generalized weakness, enlargement of the tongue, and difficulty eating. Most affected dogs die while still young, but some survive to three to five years of age, eventually dying from cardiac disease. Blood chemistries and muscle biopsies are necessary for accurate diagnosis.
Scientists are currently working on “gene surgery” to repair the affected DNA segment of MD, and the technique seems to hold some promise for Goldens and potentially for humans.
Myasthenia gravis is another disease that has been reported (rarely) in dogs as well as in humans. It occurs as both an acquired, autoimmune disorder and a congenital, familial one. The consistent symptom is severe fatigue that is relieved with rest and anticholinesterase therapy. It is thought that both the acquired and congenital forms are due to a diminished number of cholinesterase-responsive chemical receptors on the muscle cell walls (acetylcholine receptors).
While the exact cause of the immune-mediated condition has not been defined, one can suspect the usual factors: excess use of corticosteroids, diminishing the immune capacity; vaccine-related; and excess exposure to toxins (especially suspect here would be pesticides).
Treatment is challenging, and in Western medicine it would include the use of anticholinesterase drugs, followed up with corticosteroids or other immune-suppressors. Alternative therapies would ultimately attempt to enhance the balance of the immune system rather than trying to shut it down.
Fibromyalgia (literally meaning pain in the muscles, ligaments, and tendons) is another complex of diseases seen in humans that has so far eluded attempts to define or treat it. Of interest for dogs is the fact that fibromyalgia’s symptoms – chronic fatigue, muscle and joint pain and stiffness, and cognitive dysfunction – are the same as those seen in some chronic cases of Lyme disease. In fact, Lyme disease may progress to a chronic multi-symptom form that appears much like fibromyalgia. What we ultimately learn about how to treat Lyme disease may thus be a good indicator for how we will eventually treat fibromyalgia in humans.
Most treatments for fibromyalgia are geared toward improving the quality of sleep and reducing pain. Massage and trigger point therapy may be helpful.
Interestingly, fibromyalgia in humans is one of many diseases that responds very well to canine therapy: fibromyalgia sufferers who have companion dogs seem to fare better – in terms of pain reduction, depression, and the ability to continue exercising – than those who go it alone.
Other rare hereditary diseases affecting dogs include myotonia (a condition in which muscle contraction persists after stimulation), a number of disorders of glycogen metabolism that influence the metabolic processes within muscles, and several diseases of the muscles that are found in one or only a few breeds.
Hypothyroidism has been associated with myopathy and neuropathy in humans, and there is some evidence it may also be a contributing factor in some dogs. Hyper-adrenocorticism (Cushings disease) is a rare disorder that may create a variety of clinical signs of muscular dysfunction that ultimately result in muscle weakness and/or stiffness.
Myositis (inflammation of the muscles)
Generalized inflammatory myopathy of dogs has been associated with toxoplasmosis, systemic lupus, and leptospirosis. Clinical signs include weakness, pain, fever, muscular atrophy, and stiffness. Almost all cases of myositis have an autoimmune component, and some can be diagnosed using immunological testing and/or muscle biopsy. A disease of juvenile Collies has been described that is characterized by concurrent dermatitis and myositis, symptoms that often wax and wane over time.
There are several inflammatory conditions reported that affect the masticatory (jaw) muscles, collectively referred to as masticatory muscle myositis (MMM) or masticatory muscle disorders (MMD). Some of these involve an eosinophilic infiltrate, and most are thought to be immune-system related. The dog has difficulty opening his jaw, and as the disease progresses, eating becomes progressively more difficult, and the jaw muscles atrophy. Corticosteroids may be helpful, although symptoms tend to recur. Alternatively, acupuncture or homeopathy may be effective.
Parasites of the muscles
Sarcocystosis is an invasion of muscles and other soft tissues by protozoans of the genus Sarcocystis, a coccidia-like organism. The life cycle of the organism requires two hosts. Predator animals (dogs) are typically the final host; prey animals are the intermediate hosts where the organism forms cysts within the muscle mass. Dogs (and other predators) then pick up the cysts by eating infected meat; cattle (and other “prey” animals) are infected by sniffing predator feces.
Most often the cysts are asymptomatic, but occasionally they become so numerous and large that they cause muscle soreness. In some animals the cysts become large enough that they can be seen as white spots scattered throughout the muscle tissues.
There have been a few reports of serious illness and death in dogs (as the final host), associated with sarcocystosis, and under some rare conditions, dogs can become the secondary host with invasion of cysts into muscle tissues. However, the real significance of the disease is that one mode of transmission is via ingestion of raw, infected meat.
Primary skeletal muscle tumors can be benign (rhabdomyoma) or malignant (rhabdomyosarcoma). Tumors may also spread or metastasize secondarily from other tissues, and local tumors may invade adjacent muscles. Treatment is surgical incision, chemotherapy, or radiation, depending on the tumor type. Acupuncture or homeopathic remedies may be tried.
Overexertion and everyday maladies
The most commonly seen muscle problems are those that occur with the normal use and abuse that comes with daily living: sprains, strains, contusions, bruises, abrasions, lacerations, and infections extending from cuts and scrapes. But some cases of overuse can be serious, including:
• Exertional myopathy (also known as Monday morning disease, rhabdomyolysis, or tying up), a disease that may produce muscle necrosis as a result of excess lactic acid buildup from overwork.
• Malignant hyperthermia, a condition most frequently seen in heavily muscled dogs, typically after anesthesia and stress.
• Severe muscular trauma that may cause muscular contraction and/or tendon rupture. Most of these conditions are limited to large or heavily muscled dogs or to working dogs such as racing greyhounds.
Most of the everyday maladies, however, if they need any treatment at all, respond well to alternative therapies: herbal antiseptics and antibiotics, alternative pain relievers, simple massage, or just an easy walk to enhance healing blood flow through the affected areas.
Remember that any strain of muscular tissues will cause the dog to compensate; four-legged animals compensate quite easily, quickly learning to walk on three legs. Whenever an animal alters his gait or posture, the compensation will extend into the spine, and a chiropractic adjustment will often be necessary to return the animal to normal function.
In my opinion, alternative medicines offer the perfect solution for most conditions of the musculoskeletal system – oftentimes creating more effective and longer-lasting healing than any Western medicine I ever tried. In fact, alternative medicines are so much better than Western medicines for most musculoskeletal problems I encounter, I almost always recommend them as my first and preferred treatment.
Acupuncture is good for almost any condition of the musculoskeletal system, because it decreases pain, allowing for enhanced mobility, which often speeds healing; enhances the flow of energy through areas where a blockage of natural flow has occurred; and enhances the immune system (many causes of musculoskeletal disease are immune-related).
Chiropractic is indicated for any condition that involves both the nerves and muscles. Almost all muscle and joint problems also involve input from nerves, thus I typically recommend the combination of acupuncture and chiropractic for any condition of the musculoskeletal system.
Herbal remedies can be added to enhance healing, to decrease pain, and to help relax the animal. Many herbs also have very high levels of antioxidant activity, to help speed healing. Check with your herbalist for herbs to use, dosages, and methods of use.
Homeopathic remedies are helpful, especially for pain. Arnica is the classic remedy for treating muscular pain and joint sprains. Rhus tox is good for what homeopaths call the “rusty gate” syndrome: sore muscles and joints that improve with movement. Bryonia alba may be beneficial for the animal that gets worse the longer he walks or exercises. Pain that comes from irritated or damaged nerve endings may respond to Hypericum, and Ruta graveoleus may work on deeper pain. Check with your homeopath for doses and dosage schedules.
Nutrient therapy can be used to enhance the growth and healing of muscle tissue. A deficiency of vitamin E and selenium, for example, has been shown to cause muscle damage in some animals, and these and other antioxidant vitamins are excellent supplements for muscle health.
Massage and physical therapy are vital in cases of muscular problems. Massage is the prescription for sore muscles, and physical therapy can be used to help return an ailing musculoskeletal system to normal function.
There are literally dozens of different “schools” of massage, ranging from very light touch to deep massage of inner connective tissues. There are also scads of massage practitioners who work on animals at every level of proficiency – from those who have seen a video on animal massage to those who have completed several hundred hours of training; the quality of the massage is thus highly variable. Ask about the prospective practitioner’s training, and ask for references. (See “Lay Your Hands On Dogs” and “How to Select a Canine Massage Provider” in WDJ July 2004.)
Physical therapy is often limited to animals recovering from surgery or severe injury, and should thus be a specific methodology aimed at the condition. Currently there are a few dozen people across the country who have studied physical therapy and are applying standardized techniques of the specialty to animals.
Massage, on the other hand, is a technique all of us can use on a routine basis. Simply rub and enjoy. Your dog will let you know what feels good and which body parts are sore – go gently on the sore areas. If you want to learn more about massage, there are several good books and videos on the subject, or you can attend one of several schools devoted to animal massage.
Finally, the best part about massage is that both the giver and the give benefit. Studies have shown that simply rubbing your pet slows both your and your pet’s heart and respiratory rates, enhances both your immune systems, and is whole-body calming for the two of you.
-Dr. Randy Kidd earned his DVM degree from Ohio State University and his PhD in Pathology/Clinical Pathology from Kansas State University. A past president of the American Holistic Veterinary Medical Association, he’s author of Dr. Kidd’s Guide to Herbal Dog Care and Dr. Kidd’s Guide to Herbal Cat Care.