Topic 2.1 Musculoskeletal System

The musculoskeletal system forms the structural framework of the body and enables movement, stability and physical function. It consists of the bones, joints, muscles, tendons, ligaments and connective tissues that work together to support body weight, protect internal organs and generate movement. Every physical task performed by an assistance animal, from walking calmly through a shopping centre to retrieving dropped objects or assisting with mobility tasks, relies on the coordinated function of this system.

For assistance animals, musculoskeletal health is fundamental to both welfare and occupational performance. These animals are expected to undertake repetitive physical activities throughout their working lives, often while carrying specialised equipment or performing physically demanding tasks. Unlike companion animals, assistance animals cannot simply avoid activities that cause discomfort if they remain motivated to assist their handler. Consequently, subtle musculoskeletal disease may progress unnoticed unless handlers and professionals recognise the early signs of discomfort or reduced function.

Occupational therapists are not responsible for diagnosing orthopaedic disease or prescribing veterinary treatment. However, because they frequently observe assistance animals performing functional activities within natural environments, they are often among the first professionals to notice changes in gait, posture, endurance or willingness to work. Understanding the normal structure and function of the musculoskeletal system enables occupational therapists to recognise potential concerns, adapt activities where appropriate and facilitate timely referral to veterinary professionals.

Bones

Bones provide the rigid framework upon which the body is built. In addition to supporting body weight, they protect vital organs such as the brain, heart and lungs while providing attachment sites for muscles that generate movement. Bones also serve important physiological functions by storing minerals including calcium and phosphorus and housing bone marrow, where blood cells are produced.

Dogs possess approximately 319 bones, although the exact number varies depending on breed characteristics such as tail length and the presence or absence of dewclaws. Cats have approximately 230 bones, contributing to their remarkable flexibility and agility. Despite appearing static, bone is a dynamic living tissue that continually remodels throughout life in response to mechanical loading, nutrition, hormonal influences and ageing. Regular weight-bearing exercise promotes healthy bone maintenance, while prolonged inactivity or disease can reduce bone strength over time.

Joints

Joints are the points at which two or more bones meet and permit movement while maintaining stability. Most of the major joints involved in assistance animal work—including the shoulders, elbows, hips, stifles (knees), carpi (wrists) and tarsi (ankles)—are synovial joints. These joints are lined with smooth articular cartilage and lubricated by synovial fluid, allowing movement with minimal friction.

Joint stability depends not only on the bones themselves but also on surrounding ligaments, joint capsules and muscles. Damage to any of these structures may result in pain, inflammation, instability or reduced range of motion. Because assistance animals repeatedly load these joints during walking, turning, retrieving and mobility-related tasks, maintaining joint health is essential for long-term working longevity.

Muscles

Muscles are responsible for generating movement by contracting and relaxing in a coordinated manner. They also provide dynamic stability to joints, absorb shock during locomotion and assist with posture and balance. In working assistance animals, muscular endurance is often just as important as strength. A guide dog, for example, may spend many hours walking each day, while a mobility assistance dog may repeatedly perform physically demanding retrieval or opening tasks.

Healthy muscles require regular exercise, adequate nutrition and sufficient recovery time. Muscle weakness, fatigue or imbalance may alter an animal’s gait and increase the mechanical load placed on joints and tendons, predisposing the animal to secondary injuries. Conversely, excessive exercise without appropriate conditioning or recovery can also result in muscular strain and overuse injuries.

Tendons and Ligaments

Although tendons and ligaments are both composed of dense connective tissue, they perform distinct functions. Tendons connect muscles to bones, transmitting the force generated by muscle contraction to produce movement. Ligaments connect bone to bone, providing passive stability that prevents excessive movement within joints.

These tissues are subjected to considerable forces during everyday movement. Sudden changes in direction, repetitive loading, slips, falls or excessive workload may result in strains, sprains or complete rupture. Because tendons generally have a poorer blood supply than muscles, healing following tendon injury is often prolonged and requires careful rehabilitation.

Cartilage

Articular cartilage covers the ends of bones within synovial joints and provides a smooth, resilient surface that allows bones to glide over one another with minimal friction. It also distributes forces evenly across the joint and acts as an important shock absorber during weight-bearing activities.

Unlike many other tissues, cartilage has very limited capacity for repair because it lacks its own blood supply. Once damaged, cartilage degeneration is often progressive and contributes to the development of osteoarthritis. Maintaining healthy body weight, appropriate conditioning and early treatment of joint disease are therefore important strategies for preserving cartilage health.

Movement and Gait

Normal movement depends upon the coordinated interaction of the musculoskeletal and nervous systems. Efficient locomotion requires muscles to contract in precise sequences while joints move smoothly through their normal range of motion. Healthy animals should demonstrate symmetrical weight-bearing, coordinated limb placement, smooth transitions between gaits and a willingness to move without hesitation.

Because assistance animals perform highly repetitive functional tasks, even subtle alterations in gait may indicate developing pathology. A shortened stride, reluctance to bear weight, stiffness after rest or changes in posture may represent early signs of pain long before obvious lameness develops. For occupational therapists, observing the animal moving in natural environments often provides valuable information about its physical wellbeing.

Common Musculoskeletal Conditions

Musculoskeletal disorders are among the leading causes of reduced working capacity and early retirement in assistance animals. Osteoarthritis is particularly common, especially in older animals and larger breeds. Progressive degeneration of joint cartilage results in pain, stiffness and reduced mobility, which may initially present as reluctance to perform previously routine tasks, slower movement or decreased enthusiasm for work.

Developmental orthopaedic disorders, including hip and elbow dysplasia, are also significant concerns in many breeds commonly selected for assistance work. These conditions alter joint development, resulting in instability, chronic pain and premature arthritis. Responsible breeding programmes routinely screen breeding animals to reduce the incidence of inherited orthopaedic disease.

Soft tissue injuries involving muscles, tendons and ligaments are also frequently encountered in working animals. Repetitive loading, sudden acceleration, slips or falls may result in strains or sprains that limit an animal’s ability to work safely. Cranial cruciate ligament disease, one of the most common orthopaedic conditions in dogs, often requires surgical intervention followed by extensive rehabilitation before an animal can return to normal activity.

Intervertebral disc disease (IVDD) represents another important condition affecting the spine. Degeneration or rupture of the intervertebral discs may compress the spinal cord, resulting in pain, weakness, incoordination or paralysis. Because neurological deficits may progress rapidly, prompt veterinary assessment is essential whenever spinal pain or altered neurological function is suspected.

Recognising Pain

One of the greatest challenges in working animals is that many continue performing tasks despite experiencing pain. Assistance animals are often highly motivated to work and may suppress obvious signs of discomfort, making early recognition particularly difficult.

Rather than looking solely for limping, occupational therapists should consider more subtle behavioural and functional changes. Reduced enthusiasm for work, hesitation before performing familiar tasks, altered posture, shortened stride length, slower movement, difficulty rising after rest, reluctance to negotiate stairs or jump into vehicles, muscle loss and changes in facial expression may all indicate underlying pain. Persistent behavioural changes, particularly when accompanied by reduced task performance, warrant veterinary assessment.

Occupational Therapy Considerations

For occupational therapists, understanding the musculoskeletal system extends beyond recognising anatomy. It provides the foundation for evaluating how physical health influences an assistance animal’s ability to participate safely and effectively in its occupational role. Functional observations made during therapy sessions often provide valuable insight into an animal’s physical condition and may identify problems before they become clinically obvious.

Occupational therapists should consider the physical demands placed upon assistance animals when recommending tasks, equipment or environmental modifications. Activities should be appropriate for the animal’s age, size, breed, physical condition and overall workload. Collaboration with veterinarians, rehabilitation professionals and qualified trainers ensures that interventions remain safe, evidence-informed and centred on both client outcomes and animal welfare. This interdisciplinary approach supports the long-term health, wellbeing and working longevity of assistance animal teams.