The Dog's Internal Anatomy: Part 2

Muscles, Tendons, Ligaments, and Joints

For the next topic of the dog's musculo-skeletal system, I will be talking about four things. I will go over the muscles, tendons, ligaments, and joints. These four tissues help tie together the skeletal system, which supports the animal, and allows movement.


Muscles are what gives your dog movement. There are a lot of other systems that also contribute, but it is the contraction and relaxation of muscle that gives mechanical movement to the skeletal system, allowing dogs to walk, run, wiggle their noses, etc.

There are three different types of muscle in the mammalian body. There is Skeletal muscle, Cardiac muscle, and Smooth muscle. Cardiac muscle is just what it sounds like. This is heart muscle which has well-developed cross-striations throughout the muscle. Heart muscle beats rhythmically on its own due to Pacemaker Cells in the Myocardium which discharge and cause the involuntary heartbeat.

The Myocardium is just the thickest middle layer of the heart wall and Pacemaker Cells are just cells that set and keep a pace within the heart.

Smooth muscle also contains these pacemaker cells, but these beat at an irregular rhythm. Smooth muscle does not have any cross- striations, and is found in the internal organs of mammals, and are also involuntary.

Next comes the skeletal muscle. This is what I will be concentrating on. Skeletal muscle requires voluntary thought processes, which triggers nerve impulses, which then triggers muscle movement.

Skeletal muscle is attached to the skeleton by tendons. I will talk about these in a minute. Muscles allow movement of the skeleton to occur by contracting and relaxing. Muscles do not expand and push, they can only contract and pull. This is why there are muscles on each side of a bone. They are called Antagonistic Muscle Pairs. For example, one muscle contracts and pulls to bend the leg, the other contracts and pulls to straighten it out.

These are the basics of muscle, and from here on out things get very complicated. I will keep it simple, so here are just a few other things to know about muscle.

Skeletal muscle is made up of individual muscle fibers, that taken together, form the muscle structure. Each muscle fiber runs in a parallel line between the tendons, and most muscle fibers both begin and end at the tendons. The way that these muscle fibers are arranged makes the contractile force additive, that is, when contraction occurs, the force along the line of muscle adds up as more force occurs.

There are three main proteins that make up the contractile mechanism in skeletal muscle. They are Myosin-II, Actin, and Tropomyosin. Tropomyosin is further made up of three subunits.

I will try to generalize what happens in muscle when contraction occurs. This process is very complex, so remember, this is just a layman's explanation.

Skeletal muscle is made up of thin filaments and thick filaments. The contraction of muscle occurs when the thin filaments slide past the thick filaments. The so called "power stroke" occurs by what I would call a lever action. A lever from the thin filament detaches from the thick filament, moves down the thick strand, re-attaches, then flexes and pulls the strand shorter. The distance moved is just a few nanometers, but when hundreds or thousands of these lever actions occur all along the muscle filaments, considerable shortening of the muscle occurs.

I hope I explained that in a way that you can understand, because you can spend months studying just the mechanism of contraction and relaxation of muscle.

There are two types of muscle fibers in the dog's body. These are Type I and Type II fibers. The different fiber types are found in different types of muscle. Red muscle is mostly made up of Type I fibers and are darker, respond slowly and have long latency, and are responsible for maintaining posture. These are long, slow contractions.

White muscles contain mostly Type II fibers and are responsible for fine, skilled movements. They have short twitch durations.

One last tidbit about muscle is that dog skeletal muscle has the ability to exert 3 to 4 kilograms of tension per square centimeter of cross-sectional area. This is also true for human skeletal muscle.

Tendons and Ligaments

The information that I have about tendons and ligaments will be brief. Tendons are what attach the muscles to the bone, and Ligaments are what attach bone to bone. They are both very tough and fibrous. They can take a lot of stress, but once they sustain an injury, tendons and ligaments heal slowly. If they are allowed to rest, they can regain their strength and motion. But if the injury is ignored, and more damage continues to occur, it will result in obvious pain. Without rest and treatment, tendons and ligaments will never regain their original strength or range of motion.

Tendons are made up mostly of collagen protein, along with other proteins. They have a poor blood supply, and when they suffer injury, they are difficult to treat, and often do not heal well.

Ligaments are similar to tendons in structure, but attach bone to bone.

Changes in the mechanical motion of a limb also affects changes the tendons and ligaments. When tendons and ligaments are gradually placed under increasing stress, they can adapt and become stronger and more flexible. This occurs in athletic dogs and dogs that perform daily work such as herding, pulling sleds, and other activities. When dogs gradually work up to a certain work level, the tendons and ligaments adapt as well.

When your dog lays around all day, and all week, then you take them to the park to play fetch on the weekend, your dog is placing great stress on their tendons and ligaments when they are suddenly running hard, making quick turns, and putting compressional stress on their entire skeletal system.

Stretching out your dog's muscles before any type of exercise, and making sure that they are warmed up before any activity will help decrease any chances of injury or damage on the cellular level. Take time to stretch out your dog's legs by taking each limb, moving it in all directions, and holding it in place for a few seconds.

For example, grab one of your dog's front legs and stretch it all the way out, and hold for a few seconds. Then stretch it all the way in the other direction and hold. Repeat this a few times. Make sure to stretch his leg at the elbow and the wrist as well. Repeat this for all limbs. Then walk your dog for at least 10 minutes before any activity to make sure he is warmed up and will not be working with "cold" muscles.

Another stressful situation placed on your dog is when you have them jump in or out of your vehicle with cold muscles. As described above, this places stress on your dog's entire skeletal system. Over time, this will cause problems such as arthritis and hip dysplasia. Young puppies should never, ever be allowed to jump or run before they are at least one year old. To see the reason why, refer to part 1 of this series, The Dog's Internal Anatomy: Part 1- Skeletal to read it now.

In order to reduce stress on your dog from jumping in and out of your vehicle, or on and off of your bed or couch, read about how to increase your dog's longevity and keep him youthful.


A joint is simply the place where two bones meet. Each end of the bone is covered by a layer of cartilage called articular cartilage. The entire joint structure is covered by a joint capsule called the articular capsule. This capsule is lined on the inside by the synovial membrane, which produces joint fluid, called synovial fluid. The synovial fluid provides lubrication for the bone, muscle, ligament, and tendon at that joint.

This is a generalization, because there are many different types of joints depending on their location in the dog body, and their structure and purpose. What I have described above is called a synovial joint, which is a freely moving joint. Note that the two bones are separated by a cavity.

Synovial joints are further divided into two different types. A Hinge Joint is a freely moving joint that can only bend in one direction, such as the knee. A Ball-and-Socket Joint will allow rotaional movement and can move in all planes. An example of this would be the hipbone joint.

There are also two other types of joints. One, called Fibrous Joints are immovable joints. These joints hold bone together tightly, such as in the skull. The skull consists of many bones that do not move.

The other type of joint is called a Cartilaginous Joint. These joints allow some movement and are found mostly in the vertebral column. Each vertebrae is divided by a cartilaginous joint, which provides protection and cushion between vertebrae. Continual jumping up and down from high places can cause damage to these joints, and lead to back problems in dogs.

Author: Richard Delgado

Copyright 2003-2005

CESS Enterprises

Pet Care 4 Our Animals

About the Author: Richard Delgado brings his 15 years of pet care experience to the internet community to teach all dog lovers to properly care for their dogs.

Note: This section of the Canada's Guide to Dogs website is intended as a source of information only. It is not intended as a substitute for professional care. Always consult with your Veterinarian about health related matters.