Written by Holly Pollard-Wright DVM, CCRP

The basic goals of veterinary physical rehabilitation are to implement a therapeutic program that remedies musculoskeletal injuries, any injury that affects the bones, muscles, ligaments, nerves, or tendons resulting in pain. This rehabilitation process includes addressing musculoskeletal imbalances of the entire body, where tissues are allowed to heal before they are reconditioned to accept predetermined workloads.1  As described by physics, kinetic energy is the work needed to accelerate a body of a given mass from rest to its stated velocity.  Exercise physiology is a discipline that examines how exercise affects the body.1 Conditioning refers to preparing the body to perform physical activity and where muscles are used for a particular task.  Training refers to the skill of the animal that includes behavioral aspects such as jumping over an obstacle in a sporting event. The physiology of physical exercise includes the study of the acute responses and chronic adaptations to exercise and applies to veterinary physical rehabilitation. Importantly the level of conditioning of an animal constantly changes any time the body performs at a specific metabolic level and is proportionate to the animal’s current adaption to physical stress.1  The ability of an animal to perform maximal physical work is termed physical work capacity. It is a function of the intensity and duration of work. Each animal has many different capacities and, in the context of physiology, relates to:

  • Anaerobic exercise – is an exercise that breaks down glucose in the body without using oxygen.
  • Aerobic exercise – is exercise “with oxygen” in which breathing controls the amount of oxygen to the muscles.  
  • Endurance capacity – is the ability to sustain an activity for extended periods, including when the exercise is unpleasant.  

Each of these capacities has limiting factors and where the body’s systems are conditioned to maintain a level of homeostasis, the state of steady internal, physical, and chemical conditions. Homeostasis comes from the Greek words for “same” and “steady” and refers to processes necessary for the survival of living organisms maintained by physiological processes. Related to the dog, more energy is required for a dog to trot 13 miles than sprint 100 yards because energy requirements for movement are more proportional to the distance traveled than travel speed.1Importantly, environmental considerations related to dogs include knowing that they rely almost exclusively on panting for heat loss, especially when ambient temperatures are high.2   Dogs are homeotherms, organisms that maintain their body temperature at a constant level with an average body temperature between 37.9 (100.2°F) to 39.9°C (103.8°F).2  Canine sweat glands are poorly developed and largely non-functional. In addition, fluid loss from panting can be substantial, and thus proper hydration is paramount.  In contrast, cold environments require the dog to produce more heat via catabolism, the process where energy is released via the breakdown of complex molecules to form simpler ones. Vasoconstriction and countercurrent heat exchange represent how heat loss is conserved in the dog.1 Vasoconstriction is a narrowing of blood vessels, and countercurrent heat exchange is a mechanism of heat exchanged between the warmer arterial blood to the cooler venous blood. 

The Role of the Cardiovascular or Circulatory System 

The cardiovascular or circulatory system is designed to ensure the survival of all body cells at every moment, assists with thermal regulation, and is an essential mediator of acid-base balance.3  Part of this process includes maintaining the immediate chemical environment of each cell in the body (i.e., the interstitial fluid, fluid found in the spaces around cells) at a composition appropriate for that cell’s normal function.3  The cardiovascular or circulatory system plays a role in this process by carrying various oxygen, substrates, and chemical messengers and removing carbon dioxide and metabolic byproducts.2,4   Blood consists of liquid and solids or fluid and cellular components.2   Blood circulates through the body via arteries and veins and contains many types of cells. Blood cells include red blood cells (or erythrocytes), platelets (or thrombocytes), and white blood cells (or leukocytes) such as monocytes, lymphocytes, neutrophils, eosinophils, basophils, and macrophages.  Blood plasma and serum are different preparations of blood. Blood plasma is the liquid portion of blood without blood cells. It contains the proteins fibrinogen (that contributes to blood clotting) and albumin (which keeps fluid in the bloodstream and prevents leaking into other tissues). It plays a role in transporting protein, nutrients, antibodies, and hormones throughout the body and waste removal as cells deposit their waste into plasma. In contrast, blood serum is the fluid component of blood obtained after clotting and can be defined as blood plasma without the clotting factors and blood cells.   

Notably, oxygen and carbon dioxide dissolves to a limited extent in blood plasma but is largely carried bound to hemoglobin, a protein in red blood cells that carries oxygen to organs and tissues and transports carbon dioxide from organs and tissue back to the lungs. The heart pumps blood to the pulmonary and peripheral circulation in response to exercise. Cardiac output (CO) is the amount of blood pumped by the heart per minute and is the mechanism whereby blood flows around the body.5  CO is the product of heart rate (HR) and stroke volume (SV), the volume of blood ejected by the heart with each beat, and is measured in liters per minute. The body’s demand for oxygen changes during exercise, and the CO is altered by modulating both HR and SV.5

There are three types of Muscle in the Body: Skeletal, Smooth, and Cardiac 

In general, muscle cells are specialized for contraction, and when muscles contract, they pull on joints facilitating movement such as walking and running. They also allow for motions such as those needed for the bodily processes of respiration and digestion:

  1. Skeletal muscle tissue forms skeletal muscles, which attach to bones or skin and control locomotion and any movement.6  In vertebrates, skeletal muscles are the most common of the three types of muscles. They are voluntary muscles that can be controlled by thought or consciously controlled. When viewed under a microscope, these muscles are long and cylindrical and described as having a striped or striated appearance. The striations are caused by the regular arrangement of contractile proteins (i.e., actin and myosin).6  Actin is a globular contractile protein that interacts with myosin. Together these proteins are found in every type of muscle cell and form the contractile filaments of muscle cells needed for muscle contraction. Skeletal muscle also has multiple nuclei present in a single cell.
  2. Smooth muscle tissue in which the contractile fibrils are not tightly ordered occurs in the walls of hollow organs such as the intestines, stomach, and urinary bladder, and around passages such as the respiratory tract and blood vessels.6  Smooth muscle has no striations and thus shows no cross stripes under microscopic magnification, has only one nucleus per cell, and is tapered at both ends.This muscle is not under voluntary control and thus is involuntary because it cannot be controlled by thought.
  3. Cardiac muscle tissue is only found in the heart. It is an involuntary strained muscle with one nucleus per cell, is branched, and is the main tissue of the wall of the heart. The cardiac muscle (also called myocardium) allows cardiac contractions to take place in which blood is pumped throughout the body to maintain blood pressure.

Muscle is a tissue greatly affected by exercise or disuse associated with injury in which reduced range of motion (ROM) occurs. ROM tends to reduce through injury because animals that are injured will compensate. Through musculoskeletal tissue, particular stress occurs that is too great for the body to overcome or when chronic stress is greater than its ability to repair itself and includes pain. In this process, ROM is reduced through the animal feeling discomfort and guarding against pain. The body adapts, and the animal alters its movement with an altered gait, the paces of an animal. Injury changes structure which alters how the body receives forces and the body starts neurologically developing movement patterns of the compensatory movements.1 According to limitations due to the injury and repeatedly performing altered gait, “abnormal” becomes the “new normal” that allows locomotion. This modified movement can create additional abnormal stressors, leading to secondary and sometimes tertiary injuries at locations other than the initial insult.1 Significantly, secondary injuries complicate the rehabilitation process and may prolong the recovery time. However, when physical veterinary rehabilitation is implemented soon after medical treatment is initiated for the initial injury, biomechanical alterations that may result in secondary problems might be prevented. 


  1. Millis, D. L., & Levine, D. (2014). Canine rehabilitation and physical therapy (2nd ed.). Saunders, Cop.
  2. Klein, B. G., & Cunningham, J. G. (2020). Cunningham’s textbook of veterinary physiology. Saunders.
  3. Pittman RN. Regulation of Tissue Oxygenation. San Rafael (CA): Morgan & Claypool Life Sciences; 2011. Chapter 2, The Circulatory System and Oxygen Transport. Available from: https://www.ncbi.nlm.nih.gov/books/NBK54112/Hill, R. C. (1998). 
  4. The Nutritional Requirements of Exercising Dogs.  The Journal of Nutrition128(12), 2686S2690S. https://doi.org/10.1093/jn/128.12.2686s 
  5. King J, Lowery DR. Physiology, Cardiac Output. [Updated 2021 Jul 23]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-.
  6. Hinic-Frlog, S. (n.d.). Introductory Animal Physiology. In ecampusontario.pressbooks.pub. Retrieved March 3, 2022, from https://ecampusontario.pressbooks.pub/animalphysiology/