How We Heal: Tendon Injuries

The idea that pain should immediately drive us to a particular rehab protocol ignores what may be going on with the tissue. The site, presentation, cause, and degree of injury matter, as does your health, age, and training history. Rather than give a protocol for tendon injuries, we’re going to look at how tendons heal and help provide some context to tendon injuries.

How We Heal: Tendon Injuries

“The idea of the building, the intention of it, its design, are all immutable and are the essence of the building. The intention of the original builders is what survives. The wood of which the design is constructed decays and is replaced when necessary. To be overly concerned with the original materials, which are merely sentimental souvenirs of the past, is to fail to see the living building itself.” Douglas Adams, “Last Chance to See.”

Like Adam’s description of the Goldin Pavilion Temple above, our bodies are living structures whose individual parts are continually changing. We grow. Cells die. Age and experience leave their marks, often in the form of scars and scar tissues. No part of our bodies remains the same from birth to adulthood. We comprise living tissues that help carry out our intent, maintain homeostasis, and when they must, heal.

One of the challenges of being an active person is the inevitability of injuries. Not that our activities inevitably cause injuries; life does that all on its own. We are left, however, to figure out how to continue being active in spite of those injuries. Whether suffering a broken bone, a tweaked back, a muscle strain, or a torn ligament or tendon, most people want to get back to their training as soon as possible. But many people make the mistake of treating all soft tissue injuries alike. Treating a tendon injury the same way you treat a muscle injury will mostly likely prolong your pain. We’re going to look at different types of soft tissue injuries and discuss how the body heals from those injuries. The goal is not to prescribe a rehab protocol but to give context to different types of ailments.

In this article, we discuss how tendons heal. There is a lot of variability in tendon injuries and the healing process. The site, presentation, cause, and degree of injury matter—as does your health, age, and training history. Tendons are poorly vascularized, living tissue full of the cells that help them grow and repair when damaged, and they do not heal the way muscles do.

Read more: Rehabbing Adductor Tendonitis; Elbow Pain While Lifting Weights; Podcast Q&A: Elbow Tendonitis

Blood Supply

In general, the more vascularized a tissue is, the more quickly it will heal: muscles heal relatively quickly, tendons (having less blood flow) heal less quickly, and ligaments have less than most tendons. Cartilage is a non-vascular connective tissue and has limited repair capabilities.

Different tendons have different blood supplies as well. The flexors of the wrist, for example, are poorly vascularized (even for tendons). Their synovial sheath limits where blood vessels may enter. Serious injuries to those tendons will often require surgery because their natural healing is quite limited. Round and flat tendons, such as the Achilles and the rotator cuff (respectively), have different vascular supplies (Fenwick 2002). And some tendons have areas of decreased blood flow, “including the supraspinatus, biceps tendon, Achilles tendon, patella tendon, and posterior tibial tendon. These avascular zones are commonly associated with degeneration and rupture, although there is no direct evidence to suggest that hypovascularity is a primary cause of tendon rupture” (Fenwick 2002).

Tendons will look more like a muscle where it connects to muscle—at the myotendinous junction—and it will look more like bone where it connects to the bone—at the osteotendinous junction. Injuries at these junctions will heal differently from each other. So, as with real estate, the first step in understanding a tendon injury is location, location, location. The general healing process stays the same, but the nature and will cause the time for healing to vary and the timing for exercise intervention to vary with it.

What are Tendons?

Tendons are tough fibrous tissues that connect muscles to bones, transmit force, and stabilize joints. More like springs than ropes, tendons store and release energy, making them dynamic structures. But, unlike any material you’d find at a hardware store, tendons change according to the needs of the body. They have viscoelastic properties, meaning they act a little bit like liquids (deforming to do their jobs) and a little bit like rubber bands (storing and returning energy). As they deform under increasing stress, their ability to transmit force changes. These properties start with the mechanical makeup of the tendon.

Tendons are a bundle of bundles. The smallest unit is the collagen molecule, arranged into collagen fibrils and bundled together into collagen fibers. Collagen fibers run in the direction of force: “Where tension is exerted in all directions, the fiber bundles are interwoven without regular orientation, and the tissues are irregularly arranged. If tension is in only one direction, the fibers have an orderly parallel arrangement, i.e., are regularly arranged” (O’Brien 2005). Most tendons’ fibers are uniform, being specialized at transmitting tensile forces along only one axis. Collagen fibers are thricely bundled together and encased in an endotenon, which encompasses the fibers, blood vessels, lymphatics, and nerves. This bundle of bundles forms the tendon unit, which is all wrapped together in an epitenon, allowing smooth movement against other bodily structures.

The majority of collagen fibers are type I fibers. Type I collagen is the A-team of connective tissue and is also found in bones and skin. It is strong and suited to the tendon’s work. Type I collagen makes up 60% of the dry mass of the tendon and 95% of its total collagen (See Wang 2006):

  • 20% of the tendon is the cellular component: mostly tenocytes, which are specialized cells that secrete and build up the extracellular matrix (ECM). Very important for healing.
  • The other 80% of the tendon is the ECM, which is made of the following:
    • 70% is water.
    • 95% of the remaining 30% is Type I collagen. The rest is mostly types III and V collagens, proteoglycans, and glycoproteins.

At rest, the type I collagen fibers are not completely taught. They have some give or crimp to them—like a too-long rope stretched across a room. This crimping is important in the tendon’s function under tension.

Stress-Strain Curve

The amount of force and the length of time that force is applied to a tendon both affect if and how it may be damaged. Most discussions of tendon injuries look at a stress-strain curve, which plots the typical tolerance of a tendon to a certain amount of force (stress) and the resultant deformation of that tendon (strain).

As an aside, the stress-strain curve illustrates what makes tendons so incredibly useful. Tendons change their structural and mechanical properties in response to mechanical forces (Wang 2006). Not only does this mean that tendons change in response to physical training, but they also change shape under load as the demands for force transfer change.

Two things affect how tendons change under load. The first is their fiber pattern—the crimp to the collagen fibers. When a force first stretches out the tendon, the tendon changes shape easily, without much of the force being transferred to its attached bone. Think of our too-long rope across a room: if you open a door and pull on the rope, at first there is no tension along its length, only a straightening of the rope itself. The tensile force is lost to the shape of the rope. When the crimp is being pulled straight, this will usually be labeled as the “toe region” of the stress-strain curve.

The second important characteristic is the tendon’s viscoelastic properties. After the toe region, the tendon will respond linearly for a little while, showing proportional changes in stress and strain. As the strain rate increases, the tendon becomes less deformable. Becoming stiffer, it is more effective at moving loads but increasingly prone to microscopic failures—small tears. If the strain continues, the tendon will stop transferring force. The force will again be lost in the tendon—this time causing failure, first with macroscopic tears and eventually a rupture.

Damaged Tendons

Many tendon issues are what we usually identify as overuse injuries—tennis and golfer’s elbow being two of the most common. Called tendinopathy, an overuse injury is likely small, repetitive strains that cause microscopic failures and, in time, a breakdown of collagen fibers, impeding the tendon’s ability to endure further stress. This may lead to decreased integrity and an eventual acute macroscopic failure from activities that would not cause such a failure in healthy tendons. This is why it can be important to identify overuse symptoms early and get ahead of them.

Whether a person is experiencing a minor overuse injury or a significant tear (or is even healing from tendon surgery), the general tendon healing process is the same. Tendons heal in three phases: the inflammation phase, the repair phase, and the remodeling phase.

Inflammatory Stage: Starts Immediately (Days 0 to 7)

Any tissue injury kicks off some predictable (though complex) responses. The most notable being the inflammatory response:

Inflammation begins when injury, infection or some other insult exposes the tissue to pro-inflammatory substances. Such substances constitute a diverse range of biomolecules and toxins, including environmental irritants and antigens, bacterial and viral products, and inflammatory mediators produced by our own cells. These substances engage in a complex web of interactions with tissue and the immune system to trigger profound changes in the injured area, which manifest as the classic clinical signs of inflammation: tumor, dolor, rubor, and calor. That’s swelling, pain, redness, and heat for those of you who, like me, are a bit rusty on your Latin. – John Sullivan, “Stopping the Spread of Misinflammation.”

When the tendon is damaged, blood vessels in the tendon rupture. The bleeding leads the healing process, followed by fibrous clotting. The secretion of cytokines causes swelling and pain, which will inhibit movement. Two of the cytokines (PDGF and TGF-Beta) activate tenocytes (specialized cells that secrete collagen proteins) which will rebuild the extracellular matrix.

As we will see, movement and exercise can help during the repair phase, but they will not help much with the inflammatory phase. The inflammatory phase needs to run its course. Things like the R.I.C.E. method and NSAIDs may help alleviate pain and make the person more comfortable by treating the symptoms of inflammation. Even if we could, we don’t want to prevent or stop the inflammatory process for healing.

Repair Stage: Overlaps with the Inflammatory Stage (Days 3 to 60)

The repair stage has two parts to it: extrinsic and intrinsic. Each refers to where the tenocytes (the rebuilding blocks) come from.

During the extrinsic phase, tenocytes from outside of the tendon create type III collagen. Recall that we want type I collagen arranged in relatively uniform rows parallel to the axis of force that will cross the tendon. This type III collagen comes from quick-responding, less-mature tenocytes and acts as a buffer until type I collagen can fill the gaps. Type III collagen is not as strong as type I and is not arranged uniformly.

During the intrinsic phase, mature tenocytes from inside the endotenon begin synthesizing type I collagen. The A-team is back, and it follows the lines of mechanical stress along the tendon. New type I collagen synthesis sees the degradation of the early type III collagen. Even though we now have type I collagen, the repaired tissue will never reach its pre-injury strength, as we will see in the remodeling stage.

Remodeling Stage: Overlaps with the Repair Stage (Days 20 to 180)

The most variable stage in terms of time, process, and results. How well the tendon functions after this stage will depend on a person’s age, the tendon itself, the type of injury, activity level, and the pre-injury status of the tendon.

During the remodeling phase, the rate of new tissue synthesis decreases, and the repaired tissue undergoes some changes. The collagen fibers will form crosslinks and become stronger. The result is a stiffer, more fibrous tissue with a higher tensile strength than during the repair phase or early in the remodeling phase. Eventually, the repaired tissue becomes more fibrous and then more scar-like. By the end of the remodeling stage, the metabolism of tenocytes has decreased, as has the tendon vascularity in the injured region (Wang 2006). This scar tissue prevents the tendon from healing as strong as it was pre-injury, and it will not heal as well if a future injury occurs.

A popular field of study right now is scarless healing of tendons, which would allow surgical fixes that may result in tendons being as strong as they were pre-injury.

How Strength Training Can Help (or Not)

If we could pick the exact moment in the healing process to start exercising an injured tendon, it would be at the shift from extrinsic to intrinsic repair. When the intrinsic repair begins, type I collagen will follow the mechanical force being put on the tendon. The clinical theory is that by moving, stretching, and lightly loading the tendon, we can help the collagen fibers line up properly.

Movement and exercise also help minimize adhesions in scar tissue formation, which can improve movement, reduce pain, and improve the function of the tendon long-term (Wang 2006). Though not known for certain, “It is suggested that mechanical loading enhances tendon repair and remodeling by stimulating fibroblast activities (e.g., increased collagen synthesis)” (Wang 2006). The grain of salt is that much of our tendon knowledge comes from rat and canine studies.

The most important factor that aids tendon healing is not the load or the intensity of mechanical stress. The immediate goal should not be to strengthen the tendon as much as possible. Tendons are not hypertrophic the way muscles are and will not heal faster or better with a too-early increased load. The important part of tendon healing is the movement and use of the tendon. Any movement about the joint will cause tensile forces to travel along the tendon, helping with collagen synthesis and (potentially) lessening adhesions in the scar tissue.

Though time and the form of healing will vary greatly from issue to issue, the takeaway here should be to treat your tendons like tendons. If you suffer a tendon injury, you do not fix it with a titrating mechanical load or increased volume of stress to the tissue. In fact, you do not fix it at all. The tendon heals, and you help it out with light movement, stretching, and a return to a full range of motion. The overriding priority should be to avoid reinjuring the tissue during any healing phase while managing pain and restoring movement. If you must linearly progress something, make it a slow increase in the range of motion about the joint, with the goal of returning to and maintaining a full pre-injury range of motion.

References

O’Brien, Moria. 2005. “Anatomy of Tendons.” Tendon Injuries: 3–13. https://doi.org/10.1007/1-84628-050-8_1

Wang, James H. 2006. “Mechanobiology of Tendon.” J. Biomech. 39 (9): 1563–82. doi: 10.1016/j.jbiomech.2005.05.011.

Fenwick, Steven A., Brian L. Hazleman, and Graham P. Riley. 2002. “The Vasculature and its Role in the Damaged and Healing Tendon.” Arthritis Res. 4 (4): 252–260. https://doi.org/10.1186/ar416.

Helpful Videos

Tendon Anatomy (https://www.youtube.com/watch?v=44UbshvyQt4)

Physiology of Tendon Healing (https://www.youtube.com/watch?v=zSwwvit00mg)

Biomechanics: Tendon & Ligament Injury (https://www.youtube.com/watch?v=43I_rGIngFQ&list=PLfNRQ67UEa-HhAqdKAJzfNxU9X6-wRMv7&index=11)

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