Delayed Onset Muscle Soreness Part 2: Do animals experience DOMS?

Do animals experience DOMS?

DOMS is not often described in equine or canine literature. In fact, the terms ‘horse’, ‘dog’, ‘equine’ or ‘canine’ and ‘DOMS‘ did not reveal any specific research papers when entered into several research databases recently. There are however several studies evaluating the effect of exercise on the muscle health of various species, the findings of which I have summarised below.

We do know that horses can suffer from the effects of muscle soreness after undergoing sudden or unaccustomed exercise; for example, when young horses are being started, introducing new exercise routines too quickly, starting jumping or even after a bolting episode. Similarly, dogs can suffer the effects of muscle soreness; weekend warriors who run all day on the beach (after a very quiet week) or who suddenly increase their exercise levels by participating in a lure or agility event, when the dog has not completed a graduated training program in the lead up.

There are many other muscular disorders that may mimic post exercise muscle soreness (or DOMS if you like) in the horse, so it is important to consult your vet to rule out other conditions such as metabolic issues or myopathies, which may be affecting your animal and would need timely veterinary care and attention to manage appropriately.

Signs of DOMS like symptoms in animals may include:

  • General hyperalgesia – that is general muscular pain, tenderness on palpation – especially the large movement producing muscles of the body
  • A stiff gait pattern
  • Dogs may have difficulty with lie to sit to stand movements
  • Dogs may not do their usual ‘upward dog’ or ‘downward dog’ stretch after getting up from a period of lying down
  • Difficulty negotiating slopes (or steps for dogs)
  • Poor performance
  • Back pain – horses may be reluctant to accept the saddle

Mechanisms of DOMS Summary (here comes the science…)

Lactic Acid:

Lactic acid (a toxic waste product) is produced when exercise ceases. As it accumulates, it stimulates nociceptors in the body which leads to the pain which occurs after exercise. Previously this was the theory which was used to explain DOMS however it is now largely rejected as lactic acid levels tend to return to normal with 1-2 hours post exercise. Human studies have failed to show a relationship between lactic acid levels and perceived soreness ratings. Although lactic acid may explain pain up to a few hours post exercise, it cannot be attributed to delayed pain experienced 24 – 48 hours.

Muscle Spasm:

Increased levels of resting muscle activity measured by electromyography (EMG) is seen after eccentric exercise, that is, a localised spasm of muscle motor units. This sustained muscle activity results in compression of local blood vessels, leading to ischaemia and pain – an unpleasant cycle.

Connective Tissue Damage:

Connective tissue sheath surrounding muscle fibre bundles are damaged. Type 2 ‘fast twitch’ muscle fibres are less robust than the slow acting type 1 ‘slow twitch’ fibres and are vulnerable to stretch induced injury – excessive stain can lead to muscle soreness.

Muscle Damage:

The contractile part of muscle is disrupted by eccentric exercise. In eccentric exercise, less motor units in the muscle are recruited which means that more tension is placed on each unit, making those units are more susceptible to damage. Once damaged, nociceptors are activated in connective tissue, musculotendinous junction and blood vessels, all of which leads to pain. Blood enzymes (e.g. Creatine Kinase or CK) also levels become elevated. Studies have shown a clear link between CK levels in blood plasma and peak muscle soreness up to 5 days post exercise, and this is now a widely accepted theory for DOMS in humans.

Inflammatory Changes:

Changes such as oedema and inflammatory cell infiltration are seen after eccentric exercise. As a result of inflammation, muscle enzymes break down protein and lipid cell structure, leading to an accumulation of substances such as bradykinins, histamine and prostaglandins. These substances attract further inflammatory cells such as monocytes and neutrophils, which lead to further oedema. Oedema then has the effect of stimulating nociceptors – and pain is produced.

Enzyme Activity:

Calcium is normally stored in the sarcoplasmic retinaculum and accumulates around the cell after injury to the sarcolemma. This calcium affects cellular metabolism and ultimately stimulates nociceptors, again leading to further pain.

So what does the research tell us?

Munehiro and colleagues looked at establishment of an animal model for delayed-onset muscle soreness after high-intensity eccentric exercise. The researchers used the rat as its model, and found that muscle tenderness significantly increased and peaked at 48 hours after high-intensity eccentric exercise. The study also looked at how rats were affected by muscle soreness if they received a week of low-load eccentric training before performing high-intensity eccentric exercise. The study found that muscle tenderness was lower in rats who underwent low-load eccentric training for one week prior to performing high intensity exercise. This suggests that just one week of low intensity eccentric exercise training, before performing high intensity exercise training, can reduce the muscle soreness experienced by animals.

A 2015 study on Spanish Greyhounds by Lucas and colleagues in the American Journal of Veterinary Research, investigated the effect of exercise in hounds used for hunting, who had not undergone previous training. The authors studied 32 Spanish Greyhounds and 31 dogs of other breeding and found that the following blood and biochemical markers were significantly higher after exercise than at rest:

  • C-reactive protein (CRP) concentration and muscle enzyme (creatine kinase, lactate dehydrogenase, and aspartate aminotransferase) activities
  • Inflammatory cells such as white blood cells and neutrophil counts
  • Blood plasma alanine transaminase activity as well as total protein, calcium, and phosphorus concentrations

The authors concluded that exercise of Spanish Greyhounds without prior training activated an acute response indicative of a subclinical inflammatory state, seen at 24 hours post exercise. The blood and biochemical changes seen in this study on Greyhounds are similar to those seen in humans suffering from what we describe as DOMS.

A 2012 study by Horohov and others in the Journal of Equine Veterinary Science evaluated the effect of a nutritional supplement on the inflammatory response to exercise in a sample of 2-year-old Thoroughbred racehorses commencing initial race training. The authors of this study acknowledged that “although there is a great deal of information available on the inflammatory response to exercise in human athletes, less information is available regarding the inflammatory response to exercise in young horses undergoing training for racing careers”.  The study found that exercise intensity has a direct effect on lactate levels and pro-inflammatory cytokine gene expression, up to 2 hours post exercises. The study found that training itself was associated with an overall reduction in inflammatory markers, which were further reduced again with the use of a commercial anti-inflammatory nutritional supplement. Unfortunately, the study didn’t look at the time frame of 24 – 48 hours post exercise which is when we expect to see changes associated with DOMS. This study does support the recommendation that a graduated training program is important to limit the post exercise inflammatory response provoked by intense exercise.

 Josh - enjoying his cryotherapy session 

Josh - enjoying his cryotherapy session 

Some of the treatment strategies include:

  • Trying some of the interventions which are supported by research as discussed in part 1 of this DOMS blog series
  • Rest from intensive exercise
  • Horse turnout in a paddock – allowing the horse to gently move around at its own pace
  • Graded return to exercise
  • Addressing the cause of DOMS developing and ensure proper preparation for sporting endeavours