The Ties That Bind
Tendons & Ligaments & Sinew, Oh My!
Last week we talked about bones, bone repair, and the role that the Breath Runner Method can play in helping minimize damage to the bones and maximize bone health. To re-cap: when we run, we move bones. The faster those bones move, and the larger the range of movement for certain bones (primarily in the legs), the faster we can run. How does our body manage to keep all of these mostly rigid, bony ball and socket structures in place and moving appropriately? This is the role of our tendons and ligaments.
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As we have mentioned before, both tendons and ligaments are types of connective tissue in the body, but serve different functions. Tendons are tough, fibrous bands of tissue (not unlike a climber’s rope) that connect muscles to bones. Tendons transmit the force generated by muscles to the bones. When bones move in a coordinated fashion, movement occurs. Tendons are composed primarily of collagen, a protein that provides strength and elasticity.
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Ligaments, while also tough, fibrous bands of tissue, connect bones to other bones. They provide stability to joints by limiting their range of motion and preventing excessive movement. As with tendons, ligaments are also composed primarily of collagen. Imagine if we took a classroom skeleton, and at pretty much every place where two bones meet, we duct-taped them together. From the tiny bones at the tips of the fingers to the big ball and socket joints in the hip and shoulders. That’s pretty much the way ligaments behave, albeit with quite a lot more finesse, mobility, and utility.
What’s important for us as runners to understand is that while both tendons and ligaments are critically important for proper body movement, and they are extremely resilient, they can still get damaged. Tendons tend to get injured when there is an over-stretching of the fibers, commonly known as strains, while ligaments are more prone to things like sudden, extreme twisting movements, widely known as sprains. Of course, both are subject to acute damage from impact injuries, but we’re going to focus more on the chronic injuries, which occur slowly over time, progressively getting worse and worse. In worst-case scenarios, either acute or chronic, the fibers could get torn completely, requiring surgery to repair them.
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A leading researcher in tendon/ligament studies, Dr. Keith Baar of the University of California Davis, says, “[The] tendon has long been undervalued. Most textbooks describe only one concept of this tissue: tendons attach muscles to bones. This is akin to saying that Michelangelo was a painter. Both statements are true, but do not even begin to describe the importance of their subjects. In attaching a compliant tissue to a stiff one, tendon has a very difficult mechanical role: overcoming impedance mismatch. Impedance mismatch occurs when two mechanically different tissues are joined, resulting in strain concentrations where injury is most likely to occur.” He further explains that “Tendon mechanics are not uniform; rather they have regional differences in stiffness along their length, ranging from compliant at the proximal (muscle) end to stiff at the distal (bone) end.” This has HUGE implications for us as runners.
Let’s think about one of the most notorious injury-prone tendons for runners, the Achilles tendon, or as it’s formally known, the Calcaneal tendon. It’s the strongest and thickest tendon in the entire human musculoskeletal system. One of its unique features is that it’s one tendon for TWO muscles; the gastrocnemius and the soleus, which together are referred to as the triceps surae. The Achilles tendon attaches on the foot to the calcaneus (heel) bone. Achilles tendinopathy, which is a catch-all term describing degenerative changes of the tendon (ranging from mild inflammation to a literal shredding of the tendon fibers), is one of the most common sports injuries, and accounts for 8–15% of all running injuries. More on this in a moment.
When we run, the mechanical movement of the foot and leg bones are controlled by the muscles throughout almost the entire body (literally from the neck down). Yet if the muscles alone had to do all the work of moving bones, we wouldn’t get very far, and wouldn’t be very fast, if we could move at all. The tendons play a unique role in animal locomotion in that they provide for both the storage and release of elastic energy provided by the muscles. Dr. Thomas J. Roberts, of the University of Oregon, explains, “During activities that require little net mechanical power output, such as steady-speed running, tendons reduce muscular work by storing and recovering cyclic changes in the mechanical energy of the body.” In other words, we can run because we have built-in springs in our legs. This natural load and recoil action of the tendons, especially the Achilles tendon, gives us a higher level of energy efficiency, which minimizes fatigue of the muscles attached to the tendon. It is thought that fatigue minimization may be one of the primary evolutionary principles driving human gait selection.
The problem with all this loading and recoiling of the Achilles tendon for runners seems to be the rate at which it happens, especially in relation to our individual level of fitness. When we look at the tendon under a microscope, we will see individual collagen fibrils, with a host of other protein-based material, all bundled together in what is known as the ExtraCellular Matrix (ECM). As shown in the illustration above, these fibrils combine to form fascicles, which combine to form the tendon itself. Note that there are similar terms which has relevance to this discussion: fascia, the connective tissue that permeates every part of our body, and sinew, which is basically tendon tissue that runs within the muscle. Fascia and sinew fibers are integral to the make up of tendons, with fascia wrapping itself around each and every strand, unit, and the entire tendon itself, and sinew being the integration of tendon and muscle fibers.
Biological tissues such as tendons are viscoelastic, which means they possess both elastic and viscous (liquid-like) properties. Viscous materials (like water) when stressed, resist both shear flow and strain in a one-dimensional direction over time. Elastic materials strain when stretched, then return to their original state once the stress is removed. Think of a rope; when a load is attached, the rope will elongate in one direction. The bigger the load, the greater the stretch, but the amount of stretching gets resisted in a exponential fashion. At first, the rope stretches easily, but as it grows taut, it gets harder and harder to gain any length. Now release the load off of the rope suddenly, and the rope will recoil (possibly violently), until eventually returning to it’s original length. The difference between a tendon and a rope is the liquid component. When tendon fibers get stretched, the liquid which acts as a lubricant within the individual collagen fibrils and between the fascicles gets squeezed out in a process known as collagen denaturation. You could see this in action with a wet rope of natural fiber. Stretch it taut and you’ll see the water droplets form and drip off the rope.
Dr. Baar explains that this viscoelastic trait has several important consequences for tendons:
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