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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Aerobic vs Anaerobic

To breathe, or not to breathe; that is the question

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There’s so much noise around the various parameters of run training that it’s easy to have the very basics lost in the confusion.  What pace?  Which zone?  What heart rate?  One of the very basic things which we think gets lost is the difference(s) between aerobic and anaerobic efforts.  Follow along as we de-tangle myth from legend.  We’ll try to keep things simple, but understand that while the What’s and How’s of these parameters may be relatively simple, the Why’s can get very complex.

Aerobic: from the Greek words “aero”, (air) + “bios” (life).  The word is often credited to scientist Louis Pasteur who, in 1863, defined it as, “able to live or living only in the presence of oxygen, requiring or using free oxygen from the air," (Note: he was referencing certain bacteria)

Anaerobic: Same as above, but adding in the Greek “an” (without).  Pasteur defined it as “capable of living without oxygen.” (Same note applies)

Without a doubt, we are aerobic beings.  We need oxygen to live.  So why is there so much discussion about anaerobic exercise?  If not breathing means dying, why would we want to go anaerobic?

The simple answer is that nature designed us this way.  There are VERY short periods of time when our muscles can contract without having to utilize oxygen in the process.  These are quick, powerful bursts of energy, meant to maximize force production.  So, for our running, the simplest way to think about this is:

Aerobic exercise: Training which primarily conditions the heart (which it is often referred to as “cardio”, short for cardiovascular), such as running or cycling. We can sustain this level of exercise for a prolonged period of time (many minutes up to hours), depending on the rate of exertion and the individual’s adaptation to the activity.

Anaerobic exercise: Strength and power, such as weight lifting or High Intensity Interval Training (HIIT).  This level of exercise can only be sustained for very short durations (seconds to minutes), depending on rate of exertion and the individual’s adaptation to the activity.

Let’s set some ground work by doing a shallow dive into the chemistry of all of this.  This can easily get confusing and overwhelming, but it’s important to understand the biological imperatives in order for us to make sense of the What’s and How’s of our training.

At its most basic, our muscles favor cellular aerobic respiration.  Here’s what that looks like from a scientist’s point of view:

C6H12O6 + 6O2 → 6CO2 + 6H2O + 38 ATP (energy)

Here’s what that means: our muscle cells take one molecule of glucose (C6H12O6, the most elemental form of sugar), and six molecules of oxygen (O2), in order to create the energy needed to move the muscle fibers.  This process is known as the Krebs Cycle.  Of course, there’s a whole lot more going on in this process; feel free to follow that hyperlink to dive into the details if you so choose.  A key to this process is the creation of the molecule called adenosine triphosphate, or ATP.  ATP is the body’s energy fuel source of choice.  It is a highly charged (ionized) molecule.  Due to its negative charge, ATP’s chemical bonds can store a large amount of energy, which can be liberated easily within the muscle cell’s mitochondria, the “power factory” where all this ensorcelled chemistry occurs.  In aerobic respiration, when each glucose molecule gets broken down, and when combined with those six oxygen molecules, the reaction creates 38 ATP molecules.

We’re keeping it simple, so we’re continuing on — the end result of this combustion is a release of six molecules of carbon dioxide (CO2) and six molecules of water (H2O), along with the energy (38 ATP) needed to actually do the work.  This is considered a very metabolically efficient process, since the end “waste” products are nothing more than carbon dioxide and water, which our bodies are well equipped to get rid of.  One of the nice things about this process is that while muscle glycogen (glycogen being two or more linked molecules of glucose) is relatively limited in quantity, our body has large reserves of glycogen stored away in the liver.  When we’re operating in the aerobic zone, our body can tap into these reserves, and use these to fuel the operation.  Additionally, it’s really easy for us to ingest something with a lot of carbohydrates (like a sports drink), which replenishes a proportion of the balance, allowing us to operate for long periods of time.  Something to keep in mind, however, is that it takes a relatively long time to pull glycogen out of the liver and get it into the muscles.

Of course, there is a LOT more going on in the muscles than this simple formula, and this is why it’s so easy to get swamped with seemingly contradictory information.  We’ll get into all of that in a moment.  First, let’s talk about anaerobic respiration, a.k.a, Glycolysis.  Here’s the chemical formula:

C6H12O6 + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 Pyruvate (C3H4O3) + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O + energy

So again we’re starting with one molecule of glucose, but this time we’re adding in two molecules of nicotinamide adenine dinucleotide (NAD+), an essential “coenzyme”, or an organic non-protein compound that binds with an enzyme to catalyze a reaction; two molecules of adenosine diphosphate (ADP), another essential organic compound found in living cells which has an essential role in the energy flow of cells; and two molecules of inorganic phosphate (Pi), which is required by the body for things like energy metabolism, signal transduction and pH buffering.  Notice that the only oxygen in this formula is that which is bound up within glucose.  The mitochondria crunches these components together into an explosive amalgamation which results in two molecules of pyruvate, a transport molecule which carries carbon atoms to and from the mitochondria; two molecules of reduced nicotinamide adenine dinucleotide (a positively charged version (NAD⁺) goes in, and an uncharged version (NADH) comes out); two charged hydrogen atoms (also known as hydrogen ions); two molecules of water; and two molecules of ATP.  There’s an additional two molecules of ATP created in this process (not shown), for a total of 4 ATP.

So simple, right?  Here’s the take-away: *IF* we keep our effort levels relatively low and controlled, we can utilize the aerobic process and create 38 ATP, and clean up is easy.  Once we start exercising at a level where the demands for energy in the muscles becomes so severe that they can no longer sit around and wait for the aerobic process to pull in all those energy-rich oxygen molecules and re-package them into clean little CO2 and water molecules, the mitochondria starts grabbing less energy-dense but more readily available organic compounds, smashes them together to break them into little firecrackers of energy.  This is a messier process (as you may have noticed from the above chemical formula), which only gives us 4 ATP.  Less bang for the buck, but the needs are exponentially higher, and the time it takes to get that energy is reduced, which is why this metabolic “short-cut” gets used.  The resultant waste products get dumped out into the bloodstream faster than the liver, kidneys, and other cells can clean them up, and the increasing number of hydrogen ions acidifies the blood plasma.

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