13/10/2026
What lies beneath….
A depression in a muscle isn’t normal, it’s normally a sign that trauma has occurred to the muscle. Different injuries feel different and following through to dissection after the assessment allows us to expand our feel. Some injuries bubble
outwards and some injuries tighten to make an indentation so what can be happening below the skin?
When a horse tears a muscle, the body begins an extraordinary process of repair. But understanding how that muscle heals is important, because healing does not necessarily mean the tissue returns to its original structure or function.
Let’s look at the semitendinosus muscle that’s shown above, one of the large muscles forming part of the horse’s hindquarters. This powerful muscle plays an important role in hindlimb movement, helping to extend the hip, flex the stifle and contribute to the complex coordination required for locomotion.
When the semitendinosus muscle suffers a significant tear, muscle fibres are disrupted, blood vessels may rupture, and an inflammatory response begins. This inflammation is not simply something that needs to be eliminated. It is part of the body’s biological repair process, allowing immune cells to enter the damaged area, remove cellular debris and release chemical signals that initiate healing.
Within the damaged muscle, specialised cells called satellite cells become activated. These are the muscle’s resident stem cells, capable of producing new muscle cells and helping regenerate damaged muscle fibres. However, when the injury is extensive, the body cannot always replace all the damaged tissue with functional muscle.
Instead, another repair process takes place.
Cells called fibroblasts begin producing collagen, the structural protein that forms the foundation of connective tissue. This collagen is deposited within the damaged area, creating what we commonly refer to as scar tissue.
Initially, much of this repair tissue contains type III collagen, which forms a relatively immature framework. As healing progresses, the tissue undergoes remodelling, with increasing amounts of stronger type I collagen contributing to the developing scar.
But here’s where things become interesting.
Collagen is not muscle.
Healthy skeletal muscle consists of highly organised fibres capable of contracting, relaxing and generating force. These fibres are arranged to work together, allowing the muscle to lengthen and shorten while transferring forces through the surrounding connective tissues.
Scar tissue, however, is primarily connective tissue. It provides structural continuity and helps bridge the damaged area, but it cannot actively contract in the same way as the muscle fibres it replaces.
In the early stages of repair, newly deposited collagen is often relatively disorganised. Over time, as the tissue is exposed to appropriate mechanical loading, collagen fibres can reorganise and align more closely with the directions of force passing through the muscle.
This process is called remodelling, and it can continue for many months.
However, even after remodelling, a substantial area of fibrosis may retain different mechanical properties from the original muscle tissue. It may be less extensible, transmit forces differently and alter how strain is distributed between the scar and the surrounding healthy muscle fibres.
Imagine a muscle designed to stretch and contract as thousands of coordinated fibres, but now part of that muscle contains an area of collagen-rich connective tissue that cannot contribute to active contraction.
The surrounding muscle must continue functioning around this altered structure.
Depending on the size, location and severity of the injury, this may influence the muscle’s ability to lengthen, generate force and tolerate the mechanical demands placed upon it. Areas where scar tissue meets healthy muscle can also experience altered strain distribution, potentially contributing to vulnerability during demanding movement.
This is particularly relevant in a horse, where the hindquarters are responsible for generating and transmitting enormous forces during locomotion.
And this raises an important question: When we see a horse moving again after a muscle injury, are we seeing complete restoration of the original tissue, or are we seeing a body that has adapted to the tissue it has been left with?
These are not necessarily the same thing.
The presence of scar tissue does not automatically mean a horse will experience ongoing pain, dysfunction or reduced performance. Many horses recover remarkably well, particularly when muscle regeneration is successful and rehabilitation allows the developing tissue to adapt progressively to loading.
But significant fibrosis can have lasting consequences, and the external appearance of a healed injury tells us very little about the microscopic organisation of the tissue beneath the skin.
This is why rehabilitation matters.
Appropriate, progressive movement provides mechanical signals that influence collagen organisation, muscle regeneration and the restoration of functional strength. Too much loading before the tissue has developed sufficient strength can disrupt healing, while prolonged inactivity can contribute to weakness and poor functional recovery.
The challenge is finding the right balance for the individual horse and the stage of tissue healing.
Time alone does not guarantee that a muscle has regained its original capacity.
We also need to appreciate that muscle healing is not simply a matter of waiting for inflammation to disappear or for a visible tear to close. It is a complex biological process involving inflammation, cellular regeneration, collagen deposition, tissue remodelling and gradual adaptation to mechanical forces.
For me, understanding these processes changes the way we look at horses recovering from injury.
Rather than simply asking whether an injury has healed, perhaps we should also be asking what type of tissue has formed, how that tissue now behaves under load, and whether the horse has regained the functional capacity required for the work we are asking it to perform.
Because repair and regeneration are two very different biological outcomes.
And just because the body has successfully repaired an injury does not necessarily mean it has restored the original architecture of the tissue.
The body is incredibly intelligent in its ability to adapt and survive, but those adaptations deserve our understanding and respect.
The absence of an obvious injury is not always evidence of a complete return to normal function and there doesn’t need to be a scar on the skin to have significant muscle damage present.
These photos are from the latest dissection in Ireland hosted by Equi-ed and in conjunction with Equine Tendon and Equigate all superior equine products to help and manage our horse partners better, check out their pages.
The photo below shows semitendinosis with a depression to the right of the tail(first photo) next photo shows the scar tissue and tear in the aponeurosis of semitendinosis, last photo is a transverse cut through semitendinosis to show the extent of scar tissue.
