Being an athlete is all about overcoming challenges; it’s what makes a victory, a new competition record, a personal best, or even a comeback such a beautiful thing. Without a challenge to overcome, the pursuit of athletic competition doesn’t mean much.
As an athlete, one unique challenge to overcome is optimizing a tendon’s performance abilities without sacrificing its health in the process. As with any other tissue in the body, tendons adapt to the specific demands imposed upon them (known as the SAID principle), and knowing about the unique mechanical properties they possess can help athletes train in a manner that optimizes their performance while simultaneously reducing their risk of injury.
As an athlete, one unique challenge to overcome is optimizing a tendon’s performance abilities without sacrificing its health in the process. Share on XSo, this article will provide an overview of the unique mechanical properties of tendons and how different training demands influence them. With this information dispensed, I’ll discuss the different general strategies that can be incorporated into an athlete’s training for optimizing tendon performance and tendon rehabilitation.
A Quick Rundown on Tendons and Injury Statistics
Most athletes and coaches know that a tendon is a structure that takes one end of a muscle and anchors it onto a bone, serving as a “middleman” bringing the bone along for the ride when the muscle contracts.
And for many individuals, that’s about the extent of their tendon knowledge. But with these structures being as critical (and often just as vulnerable) as they are for athletic performance, it’s worth knowing what can be done to ensure the tendon achieves optimal performance output while remaining at the lowest risk possible for sustaining injury.
Injury Statistics
Soft tissue injuries (injuries to muscles and tendons) account for upward of 50% of injuries in the United States every year and are notably higher in athletic populations.1,2 These injuries can be either sudden and traumatic (such as from tripping and falling or tearing a hamstring when sprinting) or chronic, occurring from a prolonged lack of adequate health (typically brought on by repeated overuse beyond what the tendon can cope with). It’s estimated that 50%–75% of all running injuries are due to overuse, with the Achilles tendon and knee tendons being most commonly affected.2
Regardless of how the injury arises, tendon pain and dysfunction, at best, will impede maximal athletic performance. At worst, it will sideline an athlete for the season—or even end their career.
Therefore, strengthening tendons (which optimizes performance and prevents injury) should be a top priority for strength coaches working with their athletes while ensuring optimal rehabilitative strategies are being carried out by rehabilitative specialists treating the athlete.
The basis for optimizing either of these aspects begins with a fundamental awareness of what determines a tendon’s overall strength and performance capability.
There are two critical factors that will determine the strength of a tendon:3,4
- The amount of collagen within the tendon. (Collagen is a structural protein that’s highly prevalent within the tendon that runs in a parallel fashion.)
- The number of crosslinks attaching to these parallel collagen molecules within the tendon. (The more crosslinks there are, the stiffer the tendon becomes.)
Regarding crosslinking: these crosslinks running across the parallel collagen molecules restrict the sliding of individual collagen fibrils alongside one another, which reduces the extent of deformation (lengthening) the tendon can undergo, ultimately making it stiffer.
Knowing these two factors will help you understand why rehabilitation-based training of a tendon will require a different training methodology than performance-based training (both will be discussed shortly).
On the surface, it would seem that training tendons to become stiffer is all it takes to optimize tendon health. This is true—but only up to a point. A stiff tendon will indeed become a strong tendon (strong is good), but the stiffer the tendon becomes, the greater the risk of injury to its attaching muscle.5 This is particularly true with athletes who compete in power-based sports requiring their tendons to withstand a repeated high rate of force development (RFD).
A stiff tendon will indeed become a strong tendon (strong is good), but the stiffer the tendon becomes, the greater the risk of injury to its attaching muscle. Share on XTo determine how to avoid incurring this type of injury, we first need to discuss the unique mechanical properties that tendons exhibit. As you read through, it may help to visualize how these properties apply to the Achilles tendon, the hamstrings tendons, the patellar tendon, and the hip flexor tendon, all of which are robust in nature and of paramount importance to track and field athletes.
Mechanical Properties of Tendons
As strength and conditioning specialists and rehabilitative specialists, we must be aware that tendons are more than a simplistic piece of connective tissue that anchors a muscle onto a bone; how we train and stimulate these structures has vast implications for performance potential and injury reduction in athletic populations.
These unique responses result from the tendon’s structure and molecular composition. When we understand these features, we can use them to the athlete’s advantage based on their needs and goals for performance or rehabilitation.
For the scope of this article, there are two critical characteristics of tendons to be aware of:
- The regional mechanical properties that exist between either end of the tendon.
- The viscoelastic properties that a tendon possesses.
A brief rundown on these unique properties will set the stage for how they can be utilized to the athlete’s benefit based on different training interventions.
Regional Variation in Tendon Function: Stiffness vs. Compliance
Tendons exhibit regional mechanical properties, meaning how a tendon responds to a given stress or force will vary from one area of the tendon to another. The reason for this phenomenon is that one end of the tendon attaches to a very stiff tissue (i.e., a bone) while the other end attaches to a very compliant tissue (i.e., a muscle). In other words: when the tendon tugs on the bone, the bone has no give, but when it tugs on the muscle, the muscle will have some give.
This difference in the tendon’s attachment to two mechanically different tissues results in what’s known as impedance mismatch and is what gives tendons their unique regional variation.6,7
The point at which the muscle attaches to the tendon is known as the myotendinous junction (muscle-tendon junction), which is the most frequently injured region involving muscle strains and tendon injury.8
How an athlete trains and loads their tendons will influence either the stiffness or the compliance of the myotendinous junction of the tendon:
- It can be made stiffer, which will improve athletic performance.
- Or it can be made more compliant, which may reduce performance but will improve the tendon’s health.
The myotendinous junction is essentially a blending of the muscle tissue and the tendon tissue. Conceptually, it’s the same as interlacing your fingers of one hand with the fingers of your other hand (one hand being the muscle fibers and the other being tendon fibers). This blending and interlacing of tendon fibers with muscle fibers leads to an increased surface area for connection between these interfacing tissues. Additionally, it leads to greater shearing forces when they’re being pulled apart. (Shear is what happens when one surface slides across another.)6
The collagen fibers of the tendon closest to the muscle undergo a high rate of shear, which breaks the crosslinks attaching to the collagen molecules. This, in turn, leads to a less stiff (more compliant) tendon.
Why is this worth knowing? Because this shearing and crosslink-breaking phenomenon will form the foundational basis for why tendon rehabilitation movements must be done slowly (discussed later in the article).
Viscoelasticity of Tendons
Tendons have a relatively high water concentration, meaning the tendon itself will take on and exhibit the unique properties liquid can produce. One such unique property is viscoelasticity, which means the tendon can mechanically behave like an elastic material and also like a liquid.9,10
The easiest way to visualize the concept of such viscoelastic properties is by recognizing how water behaves based on the speed at which you enter a swimming pool. If you casually step into the pool, the water molecules will move and flow around you without resistance. Do a belly flop off the high-dive tower, however, and they will behave like a sheet of concrete upon impact, not doing much to move around your body.
So, the slower the movement, the more time the water molecules have to move around your body. The quicker the movement, the less time they have, therefore behaving more like a sheet.
This forms the basis for why fast tendon-loading exercise increases the stiffness of a tendon and how it improves athletic performance; the faster you load the tendon, the more it behaves like a sheet, and thus the stiffer it becomes.
Let’s use the next section to visualize why this stiffness is so imperative for athletic performance.
Training Tendons to Improve Athletic Performance
Athletes participating in velocity-based and power-based sports require stiff tendons, since a stiff tendon allows for a greater and more immediate force transfer between the body and the surface it’s in contact with.
Think of it this way: If you had a strap hooked up to a sled and had to pull it from point A to B as quickly as possible, would you rather that strap be a braided rope or a bungee cord?
You’d want the strap to be stiff! It’s the same in velocity-based athletes; stiff tendons allow for a more immediate transfer of force and, thus, propulsion. In other words, you don’t want your sprinting force to be absorbed by stretchy tendons when running; you want stiff tendons since they will strike the ground with less give and propel you down the track much faster.
So, if increasing tendon stiffness is the name of the game, implementing exercises that involve a high rate of force development is the best bet. The faster the tendon is loaded, the more it will stiffen up.
As such, depending on the unique needs of the athlete and their sport, periodized training regimens that use plyometric exercises should be a priority for strength and conditioning coaches. They should be programmed strategically and incorporated appropriately, emphasizing explosiveness and mimicking the specific demands of their sport.
The specific lower-body plyometric exercises that can be used are almost endless and can range from bodyweight movements (vertical jumps, broad jumps, sprints, etc.) to resistance-based movements (such as trap bar jumps, power cleans, explosive kettlebell swings, and so on). Again, it all comes down to the needs and abilities of the athlete.
Using plyometric-based exercises will increase tendon stiffness, which is good; but remember: it’s only good up to a certain point. Share on XSo, using plyometric-based exercises will increase tendon stiffness, which is good; but remember: it’s only good up to a certain point. The stiffer the tendon becomes, the more vulnerable it becomes, particularly when subjected to fast, explosive movements utilizing a high rate of force development from its attached muscle. Too much stiffness with inadequate tissue compliance leads to the tearing of tissues around the musculotendinous junction.5
Once this straining and tearing of the musculotendinous junction occurs, it’s no longer about performing fast movements; it now becomes a game of improving tissue health through slow loading.
Let’s look at the following section to understand just how profound of an impact this slow loading will have on improving tendon health, either as an injury prevention strategy or as a rehabilitative strategy.
Training Tendons to Improve Athletic Health
Injuries are a part of sports, and while every precaution must be taken to avoid injury as best as possible, they will still happen. When tendon injury occurs, optimizing the rate and extent of recovery is essential for the athlete.
Prevention is better than any cure. As strength coaches, we must understand that slow tendon loading has a place at the table for injury prevention training; preventing tendons from stiffening to the point that borders on the edge of injury is critical. And as rehabilitation specialists, we must understand that the rate of movement and frequency at which we prescribe tendon loading exercises for an athlete can optimize the recovery process.
As S&C coaches, we must understand that slow tendon loading has a place at the table for injury prevention training; preventing tendons from stiffening to the edge of injury is critical. Share on XEliminating the Tendon’s Stress-Shielding Response
An unhealthy or injured tendon is a mixture of healthy and unhealthy collagen fibers; the healthier the collagen within the tendon, the healthier the tendon itself will be. Collagen will become stronger and healthier when given mechanical stimulation through loading exercises, but if this mechanical stimulation is performed relatively quickly, only the healthy fibers will be stimulated.10
This is due to a unique phenomenon known as stress shielding occurring within the tendon. It involves the tendon’s ability to “shield” the unhealthy, injured, or weaker collagen fibers when the tendon is subjected to load (i.e., exercise), whereby the healthy collagen protects the unhealthy fibers from contributing to the movement (i.e., from being stimulated).
One way to overcome this protective shielding is to utilize stress relaxation, which is another phenomenon unique to viscoelastic materials. When the tendon is subjected to holding a prolonged isometric contraction while under load, the healthy collagen fatigues, resulting in progressive relaxation of the loaded collagen fibers.11
This, in turn, leads to the reduction of stress shielding, allowing mechanical signaling to be imparted to the unhealthy or injured collagen fibers.
As such, performing mid-range isometric contractions for varying durations of time against moderate-to-heavy loads (based on the athlete’s abilities) two or three times per week has been shown to be effective in eliminating stress-shielding in tendons.11
To learn more about implementing stress relaxation, check out this case study by Dr. Keith Baar: Stress Relaxation and Targeted Nutrition to Treat Patellar Tendinopathy.
While it was once believed that eccentric-based exercises were most effective for tendon rehab, current literature suggests that the rate at which an exercise is performed is the determining factor for an exercise’s rehabilitative effectiveness and not the phase of the movement itself.10
This means that if performed at a slow rate of speed, the concentric phase of exercise can be just as effective as its eccentric counterpart since slow movement is responsible for greater shearing and subsequent crosslink breaking near the myotendinous junction.10
Keep in mind, however, that eccentric exercises can still be employed in the rehabilitation world and have plenty of merits.
Frequency and Intensity of Loading
The best medicine in the world will lose its effectiveness if not taken at appropriate intervals. The same can be said for tendon loading exercises; optimal outcomes are, in part, determined by optimal frequency. It’s not a perfect science here, mind you, as numerous variables can play into determining optimal exercise frequency for an athlete.
The best medicine in the world will lose its effectiveness if not taken at correct intervals. It’s the same for tendon loading exercises; optimal outcomes are partly determined by optimal frequency. Share on XNonetheless, some exciting research has shown that very short periods of tendon activity (loading) followed by long rest periods seem optimal for stimulating collagen production.5 Within this same body of research, it’s also been observed that the body needs 6–8 hours before the exercised tissues can sense another stimulus being provided (i.e., the tissues need this length of time before returning to exercise sensitivity).
As pointed out by Dr. Keith Baar (a leading expert in the world of tendon-based rehabilitation), tendon loading for optimal health can be performed with slow movement for 5–10 minutes against a predetermined load.5,10 The extent of this load will depend on many factors unique to the athlete and their state of rehabilitation.
Surprisingly, for injured athletes, the amplitude of the load used for performing the exercise is not important for the stimulation of collagen synthesis.5,12 It would, however, stand to reason that the load should be appropriately challenging for the exercise or movement session duration. This is outstanding news since it permits the athlete to begin rehabilitation as quickly as possible. It should go without saying that any loading exercises should be pain-free; however, mild discomfort (that doesn’t worsen) should be anticipated.
Here’s a visual recap of what all of this may look like for an athlete’s tendon rehabilitation:
Nutritional Interventions for Tendons
For patients and athletes looking to attack every angle of their tendon rehabilitation, nutritional interventions can be considered, as some promising work by Dr. Keith Baar, Gregory Shaw, and others have shown that vitamin C and gelatin supplementation can increase collagen synthesis in tendons that have undergone recent loading.11,13–16 There’s still much more the scientific community is looking into, but the preliminary research seems promising.
Of course, nutritional intervention for tendons is a long-term strategy, but the preliminary research is rather exciting. The research is aimed at the positive effects of providing the body with an abundance of amino acids (glycine, in particular, since it is the predominant amino acid within collagen) and vitamin C, which acts as a cofactor for increasing collagen synthesis.
The details are beyond the scope of this article, but here’s a brief rundown of what’s been shown:
- Ingesting a slurry of 48 mg of vitamin C mixed with 15 grams of gelatin (which contains a high level of glycine) one hour prior to tendon loading maximizes the uptake of amino acids (including glycine) into the exercise-induced collagen.16
- This slurry is ingested approximately one hour before tendon loading as the bioavailability of glycine (the body’s ability to absorb what’s present) should peak around this time.
- Results have shown statistically significant increases in glycine uptake within the exercise-stimulated collagen fibrils.
It’s really neat stuff, and while I don’t have the space to go into specifics within this article, check out the references at the end of this article to read up more on the findings of these interventions.
Balancing Tendon Performance and Health
Being an athlete is an ongoing battle between optimizing performance and reducing the likelihood of injury. Regarding tendons, athletes, coaches, and rehabilitation specialists should be aware of the relationship between tendon performance and tendon health. As such, the athlete’s training regimen should incorporate the various loading parameters required to optimize tendon performance and tendon health.
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References
1. Calve S, Dennis RG, Kosnik PE, Baar K, Grosh K, and Arruda EM. “Engineering of functional tendon.” Tissue Engineering. 2004;10(5–6):755–761.
2. Järvinen M. “Epidemiology of tendon injuries in sports.” Clinical Sports Medicine. 1992;11(3):493–504.
3. Ellingson AJ, Pancheri NM, and Schiele NR. “Regulators of collagen crosslinking in developing and adult tendons.” European Cells & Materials. 2022;43:130–152.
4. Fessel G, Gerber C, and Snedeker JG. “Potential of collagen cross-linking therapies to mediate tendon mechanical properties.” Journal of Shoulder and Elbow Surgery. 2012;21(2):209–217.
5. Baar K. “Minimizing injury and maximizing return to play: Lessons from engineered ligaments.” Sports Medicine. 2017;47:5–11.
6. Paxton JZ and Baar K. “Tendon mechanics: the argument heats up.” Journal of Applied Physiology. Published online 2007.
7. Arruda EM, Calve S, Dennis RG, Mundy K, and Baar K. “Regional variations of tibialis anterior tendon mechanics is lost following denervation.” Journal of Applied Physiology. 2006;101(4):1113–1117.
8. Jakobsen JR and Krogsgaard MR. “The myotendinous junction—A vulnerable companion in sports. A narrative review.” Frontiers in Physiology. 2021;12:635561.
9. Ikoma K, Kido M, Nagae M, et al. “Effects of stress-shielding on the dynamic viscoelasticity and ordering of the collagen fibers in rabbit Achilles tendon.” Journal of Orthopaedic Research. 2013;31(11):1708–1712.
10. Keith Baar – “Physical Training, Performance and Injury Prevention”; 2018. Accessed February 11, 2023. https://www.youtube.com/watch?v=CgcR5J1dwcY
11. Baar K. “Stress relaxation and targeted nutrition to treat patellar tendinopathy.” International Journal of Sport Nutrition and Exercise Metabolism. 2019;29(4):453–457.
12. Paxton JZ, Hagerty P, Andrick JJ, and Baar K. “Optimizing an intermittent stretch paradigm using ERK1/2 phosphorylation results in increased collagen synthesis in engineered ligaments.” Tissue Engineering Part A. 2012;18(3–4):277–284.
13. Levine M and Violet PC. “Breaking down, starting up: can a vitamin C-enriched gelatin supplement before exercise increase collagen synthesis?” American Journal of Clinical Nutrition. 2017;105(1):5–7.
14. Lis DM and Baar K. “Effects of different vitamin C-enriched collagen derivatives on collagen synthesis.” International Journal of Sport Nutrition and Exercise Metabolism. 2019;29(5):526–531.
15. Paxton JZ, Grover LM, and Baar K. “Engineering an in vitro model of a functional ligament from bone to bone.” Tissue Engineering Part A. 2010;16(11):3515–3525.
16. Shaw G, Lee-Barthel A, Ross ML, Wang B, and Baar K. “Vitamin C-enriched gelatin supplementation before intermittent activity augments collagen synthesis.” American Journal of Clinical Nutrition. 2017;105(1):136–143.
