A muscle strain occurs when the fibres that constitute the muscle are stretched or torn. This trauma is caused when the muscle contracts too vigorously or if it is stretched beyond its current capability. Causes include twisting suddenly, like a dog bounding off a flyball box to not having enough strength in the muscles for the exercise or activity being performed. This is a risk when bringing dogs back to work in any discipline following a period of rest or injury. 

There are three grades of strain which are defined based on the severity of the injury. The Munich consensus statement on terminology and classification of muscle injuries in sport define strain severity as:

Grade I (Mild)

In this grade of strain, up to 10% muscle fibres are stretched or torn. Although the injured muscle is tender and painful, it doesn’t incur a reduction in strength or function. 

Grade II (Moderate)

This is a moderate strain, with a tear of up to 50% of the muscle fibres and more severe muscle pain and tenderness. There is also mild swelling, noticeable loss of strength and sometimes bruising. There is typically a “palpable defect in muscle structure, often haematoma, fascial injury [and] stretch induced pain aggravation.” (Mueller-Wohlfahrt, H-W. et al.,2012)

Grade III (Severe)

This strain tears the “complete muscle diameter” (Mueller-Wohlfahrt, H-W. et al.,2012) or is a “tendinous injury involving the bone-tendon junction.” (Mueller-Wohlfahrt, H-W. et al.,2012) There is often a popping sensation as the muscle rips into two separate pieces or shears away from its tendon.

As a grade III strain is a complete tear of the muscle, the gap where the ripped pieces of muscle have come apart are often visible by the naked eye. This gap is also apparent upon palpation. This grade of strain is a significant injury and can “cause complete loss of muscle function, as well as considerable pain, swelling, tenderness and discoloration.” (Harvard Medical School, 2020) 

Laumonier and Menetrey’s 2016 paper on muscle injury and repair informs us that due to the presence of adult muscle stem cells called satellite cells, skeletal muscle has a natural ability to heal itself following an injury. Injury-induced disruption of the homeostasis of the muscle tissue results in a healing process that follows three main phases.

Stage 1 – Degeneration/inflammation phase (Days 1-3)

Immediately following the injury inflammation occurs “due to bleeding into the site of the injury and is characterised by pain, heat, swelling, bruising and tenderness to the touch.” (Lenton,2020)

Laumonier and Menetrey go on to state that during the initial days following the injury, necrosis of the muscle fibres takes along with the emergence of a bruise, or haematoma. 

Signs that the dog has torn a muscle are the inability or unwillingness to weight bear on the affected limb, the dog may not want to eat, or they may appear stiff or sore when going about their daily activities. The affected area may also be expelling more heat than other areas around it due to the influx of blood. 

Cryotherapy should be used to reduce the inflammation as the colder temperature restricts the capillaries and reduces the blood flow to the area. 

During this stage, the dog should be rested. This means exercise should be ceased and the dog should be kept quiet at home. E.g., Not running up and down the stairs or racing around the garden.

Stage 2 - The Regeneration/proliferation phase (Day 4 – up to 4 Weeks)

“Muscle regeneration usually starts during the first 4-5 days after injury, peaks at 2 weeks, and then gradually diminishes 3-4 weeks after injury.” (Laumonier & Menetrey,2016) 

As Laumonier & Menetrey,2016 go on to discuss, muscle regeneration is a multi-step process which includes proliferation of satellite cells, repair and maturation of damaged muscle fibres and formation of connective tissues. In order for the full recovery of the contractile muscle function to take place, these mechanisms must reach a fine balance. 

During this stage the muscle is more prone to re-injury as the tissue is not fully repaired, rendering it weak. As there may appear to be improvement to the injury to the owner, due to less pronounced visible symptoms, it can be easy to return to normal activities too early and as such get stuck in the strain, re-strain cycle.

The repaired tissue in the affected muscle is known as scar tissue. Scar tissue is formed primarily of collagen, which is less flexible than healthy muscle and as such restricts the elongation of the muscle. 

The foundations of the scar tissue are also much different to that of the healthy tissue. In healthy tissue the fibres will run in the same direction or parallel to each other whereas scar tissue fibres are formed in a more chaotic pattern, this further compounds the muscles’ ability to lengthen as it had prior to the injury. 

Once scar tissue has formed it will always be present, massage can assist with breaking down the scar tissue and re-aligning the collagen fibres, but it is unable to break down the entirety of the tissue. With regular treatment scar tissue can be remodelled and “made more supple and flexible by taking into account the other tissue to which it is adhered, helping to release the associated fascial net, make[s] the tissue more supple, pliable and therefore less prone to re-injury.” (Lenton,2020)

Stage 3: Remodelling phase (3 Weeks – Weeks, Months or Years)

At this point in the process the scar tissue has reached a stage where it can bond the damaged muscle tissue together. As the scar tissue has hardened, the muscle’s elasticity can be severely affected. Due to this it can cause a feeling of restriction and stiffness accompanied by a dull ache. Untreated scar tissue will restrict the range of motion the dog has in the affected area and as such hinder everyday activities. 

Often an owner may become aware of an area of scar tissue as their dog will be reluctant to have this zone groomed or even petted or will be reluctant to undertake activities that were previously well received, e.g., jumping into the car, going for a walk, or climbing the stairs. 

Sources

Harvard Medical School (2020) Muscle strain, Harvard Health Publishing. Available at: https://www.health.harvard.edu/a_to_z/muscle-strain-a-to-z 

Laumonier, T. and Menetrey, J. (2016) “Muscle injuries and strategies for improving their repair,” Journal of Experimental Orthopaedics, 3(1). Available at: https://doi.org/10.1186/s40634-016-0051-7. 

Lenton, N. (2020) How does a muscle repair (strains part 2), Canine Massage Guild. Available at: https://www.k9-massageguild.co.uk/muscle-repair-strains-part-2/ 

Mueller-Wohlfahrt, H.-W. et al. (2012) “Terminology and classification of muscle injuries in sport: The Munich Consensus statement,” British Journal of Sports Medicine, 47(6), pp. 342–350. Available at: https://doi.org/10.1136/bjsports-2012-091448.