What Happens When Muscle Breaks Down During Exercise
Exertional rhabdomyolysis is a condition most people associate with human endurance athletes, but it is a genuine and serious risk in dogs that work hard, race, or compete in demanding sporting events. The condition occurs when muscle cells break down faster than the body can manage, releasing their contents — including a protein called myoglobin — directly into the bloodstream. What follows is a cascade of effects that can range from stiffness and pain to acute kidney failure if left unmanaged.
For handlers, trainers, and owners of working or sporting dogs, understanding this condition is not academic. It is a practical necessity. Greyhound racing, sled dog events, field trials, and even competitive agility can all produce the conditions in which rhabdomyolysis develops.
The Underlying Mechanism
Skeletal muscle is composed of fibres that contract through a complex interplay of calcium signalling, ATP energy expenditure, and protein interaction. During extreme or prolonged exertion, particularly when the dog is inadequately conditioned, poorly fed, overheated, or working in conditions beyond its current fitness level, the energy demands placed on muscle cells outstrip supply.
When ATP is depleted, calcium regulation inside muscle cells breaks down. Calcium floods the cell interior, activating destructive enzymes called proteases and lipases. These enzymes begin breaking down the muscle cell itself. The contents of ruptured muscle cells — including myoglobin, potassium, phosphate, and creatine kinase — spill into circulation.
Myoglobin is particularly dangerous to the kidneys. It is filtered through the glomeruli but precipitates in the renal tubules, causing mechanical obstruction and direct oxidative damage. In severe cases, acute kidney injury develops within 24 to 48 hours of the exercise event.
Dogs at Greatest Risk
Not every working dog faces equal risk. Certain populations and circumstances create a substantially higher likelihood of rhabdomyolysis occurring:
- Greyhounds returning to racing after a period of rest without graduated conditioning
- Sled dogs during multi-day endurance races, particularly in warm or unusually variable weather
- Dogs asked to perform at levels beyond their current fitness, such as a hunting dog used intensively at the start of a new season
- Dogs with pre-existing electrolyte imbalances or dehydration
- Dogs that have had previous episodes of rhabdomyolysis, which may indicate an underlying metabolic susceptibility
There is also a recognised hereditary component in some lines of racing greyhounds, sometimes called greyhound exertional myopathy, where affected individuals are particularly prone to repeated episodes regardless of conditioning.
Clinical Signs: What to Look For
The hallmark presentation of rhabdomyolysis occurs during or shortly after exertion, though signs can be delayed by several hours. Handlers should be alert to:
- Sudden reluctance or inability to continue working
- Muscle stiffness, particularly in the hindquarters and lumbar region
- Obvious pain on palpation of major muscle groups
- Muscle swelling, particularly in the thighs
- Trembling or apparent weakness
- Discoloured urine — ranging from dark orange to brown or port-wine coloured, indicating myoglobinuria
- Excessive panting disproportionate to conditions
The presence of dark urine is a particularly urgent sign. It indicates that myoglobin has reached the kidneys and that immediate veterinary attention is required. Dogs can deteriorate rapidly once kidney involvement begins.
Diagnosis and Veterinary Assessment
A veterinarian assessing a suspected rhabdomyolysis case will typically check serum creatine kinase (CK) levels, which rise dramatically following muscle breakdown. Levels in affected dogs can reach tens of thousands of international units per litre compared to a normal reference range of roughly 40 to 300 IU/L. Elevated aspartate aminotransferase (AST) is also common.
A urinalysis will assess for myoglobinuria, and blood chemistry panels will evaluate kidney function through creatinine and blood urea nitrogen values. In severe presentations, blood gas analysis may reveal metabolic acidosis from the release of intracellular contents.
Treatment Principles
Mild cases with no evidence of kidney involvement may be managed with rest, controlled hydration, and close monitoring. In more significant presentations, aggressive intravenous fluid therapy is the cornerstone of treatment. The goal is to dilute myoglobin in circulation, maintain urine output, and flush the renal tubules before irreversible damage occurs.
Alkalinising the urine using sodium bicarbonate may be used to prevent myoglobin precipitation in the tubules, though this is a decision made by the attending veterinarian based on the individual patient. Pain management with appropriate analgesics is also a priority, as affected dogs can be in considerable discomfort.
Dogs with severe kidney involvement may require more intensive support, including diuretics, and in extreme cases temporary dialysis, though this is rarely available in field settings and underscores the importance of early recognition and intervention.
Prevention: Where Effort Should Be Focused
The vast majority of exertional rhabdomyolysis cases are preventable. The following principles represent the standard of care for managing working and sporting dogs at risk:
- Gradual conditioning: never return a dog to peak workload after rest without a deliberate transition period of at least two to four weeks
- Hydration management: ensure the dog is well hydrated before, during, and after work
- Nutrition: ensure adequate caloric and protein intake and do not work dogs that are nutritionally deficient
- Heat management: avoid working dogs in conditions of high ambient temperature and humidity without appropriate precautions
- Know your individual dog: dogs with previous episodes require extra vigilance and may need metabolic screening
Exertional rhabdomyolysis sits at the intersection of physiology, management, and environment. Understanding the risk, recognising the signs early, and acting quickly separates a manageable event from a medical emergency with permanent consequences.